This was one of the shortest but absolute best videos I've seen of yours. Safety is number one, two, and three. Without proper safety knowledge, us techs can not do our job. Would love to see more videos like this. -Mr. Carlson's biggest fan.
A very interesting video Mr Carlson, it made me look up how houses are wired here in the UK. Once upon a time you either had a ground rod (some rural areas still do) or you had the house ground bonded to the sheath of the mains supply which went back to the substation before grounding out. The single phase domestic supply was one of the three phases of the national grid to live and the ground at the transformer for neutral so the earth and neutral were connected at that point. New builds and I think anything that is re-wired now has a system just like you describe with earth connected to neutral in the company fuse box at the distribution board. The other thought that comes to mind is that the earth connection on the secondary side of an isolation transformer seems to be absolutely pointless as it doesn't offer the safety that it appears to. The earth connection in an appliance is to provide a route to earth should anything fail internally and ground out the live causing the fuse to blow somewhere - either in the appliance, in the wall plug or at the distribution board. With more sophisticated systems with earth leakage or residual current detection that would trip out first. None of those things will happen if the live and neutral are isolated. But on the other hand you are unlikely to suffer much harm if you touch both the live and earth. When mine arrives I must test what potentials are there.
Just a beginner here, but I thought the correct way was to hook up your oscilloscope normally, and to the ground and use an isolation transformer (with no ground) on the device you are testing.
Hi Paul, wow I don't understand why that transformer is officially out in the market as certified isolation transformer. In Europe this would never ever get the declaration of confirmatory to be sold as isolation transformer. Following VDE/ EN norm the secondary side never is attached to protective ground/ earth. But anyway what ever norms are telling at all it doesn't make any sense...., or is there is something so much substantially different in Canada''s power distribution compared to Europa I don't know?? Anyway very interesting video ! Thank you. 73 Peter
+TRXBench There seems to be a lot of confusion about this even in europe. Mostly an "isolation transformer" has a earth connection on the secondary and a "safety transformer" has not, but has this clearly marked with a warning next to the outlet (and warns you not to hook up more than one device). But I have seen a "safety isolation transformer" (made by peaktech) that has a earth connection on the secondary, completely defeating its purpose (even false advertising imho). But I'd be very interested, if there are codes in Canada, that prohibit safety transformers without mains earth reference. After all these things are only for full galvanic separation (thus preventing any current flow out of the DUT into any earth around) during repairs under power. Nobody in their right mind would use them for anything else. But a well made and very important video nontheless. Thank you for it.
+Róbert Valdimarsson Hi Robert, yes very important issue and you are right a lot of confusion all over. I'm talking only about a galvanic protective separation transformer (Schutztrennung) according VDE 0570 Part 2-4, DIN EN 61558-2-4, EN 61558-2-4, IEC 61558-2-4
+TRXBench I agree fully, but politics and bureaucracy gets in the way of sense once again. If they want to include that "Earth" they need to re-label the transformer, and remove the word "Isolation" Merry Christmas to you and yours Peter!
+TRXBench Yes, you pretty much have to rely on the norm numbers here. I have a propper one here for up to 1500VA. I got it very cheaply second hand and quickly found out why: It does not have a inrush current limiter. So every time i switch it on, I have to make a trip to the switch box and reset the breaker :D But a limiter module is not too expensive. Still the best money I have spent so far, since it might very well save my life some day. Mery christmas and a happy new year to all of you too.
Good public service announcement. Learning the hard way of testing main voltage on an oscilloscope at the receptacle...50/50 chance of meeting your maker. Neutral is not always neutral when Jose wired your house. Maybe a good idea to backprobe to breaker box to verify mains wiring to test equipment? Make sure neutral and hot are not reversed on the electrical reseptacle . Neutral is actually the center tap on the pole transformer supplying the breaker box. Ground should have no continuity with neutral as it is wired to a copper earth pole.
Nice video, there are variac's that are also isolated, Philips made a bunch of them, I have the 12NC242252900005 (newer version: 12NC242252900007). Very affordable second hand and really nice quality also. So you saying "absolute no isolation" (14:19 and 15:47) it refers to your specific non-isolated variac.
Clip your scope ground on the top end of that isolation transformer & see the BOOM !!! You can back feed high voltage from a Unit Under Test via the scope ground clip & lift all other grounds & equipment in the shop . Rule one is know your equipment .
@@MrDoneboy This particular design of isolation transformer only isolates the secondary from mains. There is a big difference between a bench test isolation transformer and this. All these video prove is there's a whole lot of people who have no business playing with electricity doing just that.
As others have pointed out. It is important to remember that if you float your scope and then connect probe ground to a hot connection the entire case and terminals of the scope and anything else connected to it are now hot. You only need to touch any of those points and ground and you are fried. That is why you normally never float the scope but rather the DUT.
An alternative to be extra safe is to float both the device and the oscilloscope (using separate transformers!). Still you have to be mindful that your ground lead can energize your oscilloscope w.r.t. the device under test, so don't handle both at the same time in that case.
Well yeah that's pretty obvious from even basic electrical theory. It's more about parasitic components of circuits that actually become an issue sometimes!! But yes I do generally aggree with you of course!!
Me Carlson, I’ve seen this warning/explanation many times but what I haven’t seen is a proper fix for the problem. I can disconnect the “earth” lead on the output side of my isolation transformer but I don’t think that’s the proper fix. What is?
I have had Earth ground running through my isolation transformer for decades and never have had a problem. I never plug my test equipment into my isolation transformer only DUT. Many devices have components referenced chassis ground. Running the Earth ground all the way through to the DUT helps to eliminate noise while testing. That said, Mr Carlson is absolutely correct in his presentation and I am fully aware of how my bench is configured. Thanks Mr Carlson
Thanks, I now have a better understanding. But it seems like you didn't finish the topic on Isolation Transformers. It's good to be aware that the secondary might be linked to Earth. But then, how should we setup a proper isolation system? Should we break the secondary connection? I want to be able to use my scope to test the long list of devices I have.
Francois, The best way to ensure total isolation is to remove any ground connection at the output of your IT. Some manufacturers ground the metal container that boxes the IT as a matter of safety. Depending on the type of outlet socket this can be problematic as the mounting screws can become grounded and defeat the purpose. For example, in the UK and Ireland, all socket mountings are connected to the earth pin and when mounting them as I had to do , I literally had to cut the copper connection to isolate the output otherwise the mounting screws would have made the output grounded. These are small things that need to be considered whilst making sure your IT is suitable for bench work.
Thank you so much for the info about these isolation transformers. I did not know they would connect the separate grounds. I can not believe it is called an isolation transformer. You may have saved some lives with this video. I honestly would assume an isolation transformer is ISOLATED.
For starters you would normally use the isolation transformer to isolate the device under repair, not the scope.My isolation transformer the test plug is only connected to the secondary, the ground prong is completely open on the transformer.Mine is a Hammond manufacturing co, made in Canada.
I have a Hammond model 171B isolation transformer. The ground prong of the output receptacle is definitely connected to the input cord ground and the transformer frame on mine (I just checked). In addition, Hammond's own data sheets for the product show the output ground terminal connected: www.hammfg.com/electronics/transformers/line/171 (My unit is an older model with no circuit breaker, and with an outlet instead of a pigtail for the output).
Just a couple of comments: 1. You mentioned powering an oscilloscope from an Isolation Transformer (although, admittedly, you didn't actually suggest doing that). That should almost NEVER be done. (The only exception might be if you are actually repairing an oscilloscope, where the scope is the Device Under Test (DUT)) Only the DUT should be connected to an isolation transformer. 2. A standard Isolation Transformer can be converted to a "technician's isolation transformer" by internally disconnecting the transformer's secondary neutral from ground. This maintains the ground path all the way to the DUT (assuming that the DUT has a ground prong). It is much safer than using the cheater plug while also isolating the DUT from the mains. However, even with a "tech" Isolation Transformer, you still need to be careful not to do things like connect the secondary (output) to ground.
I won't name any names but there are 2 professional techs here on you tube that use 2 isolation transformers because it's safer. they plug both their scope and tv or whatever into an isolation transformer. They both work as professional techs servicing consumer electronics.
I believe your drawing of the Auto Trans was over simplified. Per the scale on the trans the output is variable from 0 to 140% of the input. Therefore the AC main should be someplace lower than the auto trans tap.
This is one of the most complex and complicated issues in electronics (for me). I've heard so many do's and don'ts and now I'm more confused than ever. So, HOW DO YOU HOOKUP YOUR WORKBENCH? Do you ONLY plug the DUT (Device Under Test) to the isolation transformer? I think that's what they recommend for working on old Ham radios that don't have a power transformer - that have a "live" or "hot" chassis. Or, do you ONLY plug your test instruments into the isolation transformer and have the DUT plugged into house power? Or, do you plug everything into the isolation transformer? I think I've read that that can be dangerous, too. In any case, I guess it's a good idea to have a GFCI (Ground Fault Circuit Interrupter) (like used in bathrooms) multiple outlet bar to have the fastest shutdown of power if an imbalance in circuit power is detected. Another question I have is - do I need (would it be useful to have) one of those Siglent SDS1202X-E scopes to troubleshoot a bunch of old Tektronix 465 oscilloscopes I have? I think I'm looking for an excuse to buy one. I just checked out your Tektronix TDS 340A scope, too. I can get one of those on eBay for about the same price as the Siglent. The advantage being that the face-plate is arranged in a more familiar pattern like other Tek scopes whereas the Siglent is somewhat confusing to my eye. My rationale is that I don't know how functional or accurate any of those old 465's are and the Siglent or the TDS 340A would be new(er) and better able to give me more accurate readings, to troubleshoot those old 465's, wouldn't it? Maybe? Or would I be better off spending my money on other things?
In my universal phase motor ECU, I've designed it so that the chassis is directly connected to the safety ground, thus meaning that I can effectively touch the chassis with both hands and not get a shock. It also means that any capacitors holding their charge should discharge to ground once the unit is unplugged.
I've seen many comments here about only the DUT being isolated from ground, however, when I was a student learning troubleshooting and repair techniques, the instructors at the vocational school cautioned us to never have the test equipment grounded. They sent us out to the rest of the world having been taught the very same thing Mr. Carlson showed in this video. Every piece of test equipment in that shop had all of the ground pins on the power cords removed. Every one of them. Meanwhile in the electronics theory class down the hall, they blew up a brand new 540 series Tektronix scope because of said grounding issue! I don't remember anything else about the incident so I don't know what they were measuring, how it was hooked up or what- just that they mentioned it to one of our instructors and then we were told about that precaution. After that the electronics instructors were removing the ground pins from the power cords or using the adaptors as shown here. Anyone care to explain why it is bad to float your test equipment? Not looking of arguments or heated discussion, just good explanations.
I was taking an analog electronics class at the local community college back in the 90's. My lab partner was in the air force and she said usually stationed in an AWACS. I was taking notes and she was hooking up the circuit. so she had the power supply on and somewhere in the circuit she said lets see positive goes to negative and before i could say no!, she connected the connection. there was a bang and smoke came out the back of the power supply. The instructor walked over expressionless. Picked up the power supply, unlocked a closet. He exchanged the blown power supply for a new one, locked the closet and w/o a word put the new power supply down on the bench in front of us.
I made this exact mistake 2 days ago on my oscope. After the bright flash I started thinking about it and then realized -of course the ground clip is grounded - duh. Then I started looking into prices of variacs that have an isolated transformer and then found this video. BTW, my Oscope does not measure correctly anymore - both the voltages and wave forms are incorrect now. i have not yet tried to replace the probe so hopefully that will be the only issue. Just wanted to say I made this exact mistake and your video helped me understand and verify why. Totally makes sense now. I did not realized that the barrel was connected to earth ground and was treating it like a multimeter. Thanks for the Video!
Possible that the board foil to the input connector burnt and possibly you can fix that and with a little luck there will be no other damage. Good luck and God Bless!
So dumb question then. What is the point of an isolation transformer if it does not solve the problem we are trying to solve in the first place? I just bought an oscilloscope and was buying an isolation transformer and then ran across this video. Wow am I confused now. What if I just plugged the oscilloscope into the wall with the three pronged adapter not having the ground connected? Total confusing now. Can you post a video on best practices?
Mr. Carlson, I have a power filter for my equipment, an isolation Transformer ( fixed after watching your video👍) A variac. I also have a current limited lightbulb when needed . My question is ,...Where is the best location to place my isolation Transformer? At the beginning of it all. In the middle or at the very end???
I think this is a very under rated topic . I don't believe it has been addressed fully here which is understandable because this video highlights the difference between filter conditioning transformers and isolation transformer. We need to highlight links or create videos on practical techniques to isolate techs and equipment . More discussion is needed to increase safety when working on mains . There seems to be two topics a) human safety techniques while working on electrical equip and b) safely using test gear on high voltage without destroying equipment . Equipment requiring mentioning : isolation clothing , isolation transformers , differential probes.
Thank you Mr. Carlson. If I ever get an isolation transformer like yours I'll be sure to open it up and cut the ground connection to the secondary side.
I was tought to always use TWO isolation transformers with no ground connection: One for load and the other for measuring device. Floating load can be measured with earthed scope but when ground lead is connected it becomes mains referenced. If you touch the device, you get shocked because it is no longer isolated from mains earth. Having also the scope floated via a second isolation transformer (with no earth), you avoid the problem. With that setup it is safe to touch any single point of a powered device. That is also safer than just isolating the scope from ground because in case anything leaks from "live" to scope chassis, it is isolated "live" ie. will not shock you. Never ever float only the scope and connect it to a mains connected load. Earth lead connected to eg. live wire makes scope chassis live which is very dangerous. As a kid I also made the classic mistake of connecting scope earth to live wire with all the sound and light effects assosiated. :) Never repeated that mistake.
In Australia it is normal for the output socket on an isolation transformer to be NOT connected to earth, if you need an earth on what your working on, then that has to be provided separately. Anyway I would NOT use one to float a scope, or any other test gear. If you do, and connect the probes earth clip to any substantial voltage, then that will appear on the outer metalwork of your scope, as in the WHOLE chassis will be LIVE, very dangerous indeed. If you then go to connect a second lead to another input, you will be touching a live socket, and you will get a shock. If you want to look at the mains using a 'normal' earthed scope, then tie back the probes earth clip and just connect the probe tip, using the scope's earth as the return path. Also before doing so check to make sure you are not exceeding the voltage specs. In the US you only have 120V, in Australia it's 240 on the active/live pin, so often an X100 probe is a better choice. Some early digital scope's can only take 5V directly into their BNC, check. Another method for a mains look-see is to use an AC output power pack (wall wart in the US) with say a 12V output, this can be handled with pretty much in total safety, and give you an X10 display, same as an X10 probe. Toroidal transformers seem the best, some small & cheap EI types, especially the ones that run warm, can be running a bit close to saturation and can round off the tops of mains crud your trying to observe. Another problem with floating equipment, is that the chassis will probably be sitting at anything other than earth, this is usually due to stray capacitance in the power transformer, and is usually exacerbated by mains filters that can easily put the chassis at half mains, whilst there is little to no current seen here, the static charge contained in a largish chassis, and the stray capacitance, can cause a current surge/voltage spike when you hook the earth clip, or an X1 probe doing damage to your gear. I once connected a floating video camera into a video input on a computer capture card, the spike destroyed a few hundred dollars of equipment - it does happen, floating gear is bad.
Thanks for the reminder! Safety is #1. As a 45+ year HVAC/R service tech, I’ve been “poked” by 24v, 120v, 575v, and even lightning (100’ underground behind Niagara Falls)-and I wouldn’t wish the experience on my worst enemy. Is the methodology of using “balanced input” (where you use 2 probes, with their ground leads clipped together and not connected to anything else, and their signal probes connected to the AC line) a safe alternative?
i think im gonna always use a battery operated scope even if its wimpy. you can get some good longevity out of a stack of 18650 batteries and its clean too :)
My understanding was that you should pretty much never float your scope, either through an isolation transformer, or especially from ground. Keep the scope plugged into the wall, and pit the device under test on the isolation transformer.
PelDaddy that was my first thought, put the scope i the wall, and put the device under test into the iso.... if you have an older piece of equipment (like 98% or so of stuff on this channel) it probably only has a 2 prong plug anyway.... so the iso-ground issue isn't an issue....
Teb Tengri the reason is that with a floating earth scope the BNC connectors are still attached together and to the chassis of the scope. So if you would hook mains voltage to the ground clip of a probe, all the other probes/BNCs and the case would be live. Since you normally treat the ground clips etc. as safe to touch, and often clip it straight onto the nearest ground of a DUT, you make it very easy for yourself to mess up and ruin your day. If you want the convenience of not having to worry where you stick your probes, there is no substitute for a proper isolated probe.
Balanced isolation transformers, which are sometimes used for audiophile and home theater systems, usually have a center-tap on the secondary which is sometimes connected to the ground connection on the output and chassis. Thus the output is not always isolated.
I (and probably lots of other people) would love to see you demonstrate the USE of a variac. People often talk about powering up vintage gear with a variac, bringing up the AC power to the vintage gear slowly (how slowly?) I'd like to see a demonstration of this, along with an explanation of why it is being done (allowing capacitors to reform?) I have been told that doing this helps to keep from damaging the vintage gear. Could you demonstrate this process and shed some light on what precisely is going on?
So my kinda layman's understanding of it is the following from what I remember from videos on the subject. Basically, the reason people care to supply lower AC voltage is because failed old capacitors in Tube gear have the potential to basically short the power supply to some degree and cause excessive current through the rectification section, including of course the rectifier tube(s). With bad enough capacitors and enough current/time powered on, you can potentially damage/destroy the rectifier tube(s) and transformers of old, broken gear in need of Cap replacement. As far as I can tell as an amateur, the ONLY reason to bother with a Variac is for 'testing' old tube electronics with presumably bad Caps that will overload the supply components. This is IMO stupid and unnecessary as the caps should be presumed bad (shorted, kinda) on anything old enough for Tubes and this seems like a complete waste of time given the only people who'd be doing this would also be replacing the certain failure-prone Caps (paper, Electrolytic especially, etc) afterwards anyway. It seems to me like you could skip this entirely by checking resistance and/or continuity from the input to the power supply section to see if it'll be partially shorted instead of just running it at lower AC voltage to mitigate damage versus full 120v. Anyway, the theory/practice behind the Variac is that lower AC voltage will proportionately lower current through the device with faulty Caps and thus lower heat/damage to sensitive components. I don't see why you couldn't instead just limit current instead with full 120v line with an inline fuse or something (like lots of devices have at their input, what a coincidence!) if you're gonna be 'testing' equipment that you suspect is faulty and liable to be damaged plugged in. So yeah, basically it's that simple. Lower line voltage equals lower fault current which is very common as old Caps are notorious for failing internally (basically turning into resistors/jumpers) and is by far the biggest problem in Tube gear not working or working poorly. Any vintage tube equipment you intend on using needs replacement caps (again, certain types only 99% of the time) or they're liable to burn out the rectification and transformers. Not exactly catastrophic for the vast majority of cases but maybe a bigger deal because Tubes are scarce.
@@Demoralized88 Thanks for spending all that time and energy to reply. (I'm surprised that this guy hasn't bothered to answer my comment.) But I've been an analog circuit designer for decades. I know how Variacs work. My question had to do only with the apparently common practice of slowly (how slowly?) bringing up line voltage to (perhaps long-stored) vintage equipment to reform caps, etc. I'd like to see this actually being done by someone who knows why it's done and who will provide commentary during the process. All of my experience (50 years) as an electronics guy have been with new equipment or things that have been in daily use since they were new. I want to restore some cool vintage audio gear and vacuum tube based test equipment that I've bought on ebay, at ham fest's, etc. So, I welcome input from people who do this slow warm up thing. It makes sense to me in theory. A piece of gear could have some extreme fault(s) that could cause damage when power is applied. I imagine that in addition to the capacitor-related issues, there could be other things. I think that using a Variac would allow a guy to initially trace through circuitry without having it blow up in his face.
Hi I have just come across this video I found it interesting as I have just this week been setting up my work bench with isolation transformer . You did a very good demonstration of what the problem is . But I am somewhat concerned about your proposed solution . What seems to be being disregarded is while you make your measurement if you have removed the earth wire from the scope power plug and you have connected the ground probe clip to the hot side At this point the steel case of your scope has become live in relation to anything else that is in your area that is connected to ground if you touched this case while adjusting a knob you would likely get a shock . If you had a data lead from your scope to a pc then you would then be putting high voltage down that lead on the ground into other equipment which you may damage . In the event that you are using as a demonstration I would use a DVM to measure the voltage between the conductor and the ground clip and ensure that it is a very low value possibly a volt or lower before making the connection with the ground clip I would not need to remove the earth from the scope . If one gets used to doing these basic checks before making a connection then it can save a lot of damaged equipment . I am in the UK so all out power is 240 / ref to earth regards bill
Hi Mr. Carlson, thank you very much for this informative video! 1. Is it safe to disconnect the oscilloscope's earth wire at its plug, so that it only has a live an neutral connection? 2. Is it safe to disconnect the earth wire running from the ac mains supply to the isolation transformer's output plug, so that there is only a live and neutral connection available? 3. When do you need to use an isolation transformer? 4. Can you please do a video on how to connect an isolation transformer to a DUT, like a TV, amplifier etc. and also to instruments like oscilloscopes, signal generators etc. so that all of them can be used safely together to probe and repair a product. Thank you very much for your channel. I have learnt so much from your videos!
I fixed TV sets for 40+ years but never knew about this! Thanks for sharing this information. I always used the Sencore isolation variac with no problems but now in my home workshop I have a B&K TR-110 isolation transformer and it never occurred to me that I may be not be isolated like I assumed I was. I'll have to check this out. You may have just saved my scope.
I suggest you check whether your isolation transformer and\or variac (my "variac" is isolating) have fuses. One of mine did not have a fuse, so I had to install one. I blew it often since. The other even had two fuses, one on each primary lead, but they were 3.15A good to keep the office powered but too much to limit the damage to the DUT.
Modern isolation transformers often have the neutral and ground tied together at the outlet on the secondary side of the transformer. It is a clip that can usually be removed.
Mr Carlson's Lab Hello and thanks for your videos! I am new to repairing electronics can you please explain why it's so important to keep the ground for safety. I keep hearing this but don't understand why it makes it more safe? Why is it more safe for the BNC connectors to be grounded or the chassis of the isolation transformer to be grounded? I am sorry for asking such a noob question I just don't see anyone explaining this... Thank you
The earth connection provides an alternative path for the power should a fault arise. Since it is always connected it will protect even with the appliance turned off or more importantly beleived to be turned off. The permanent connection also continues to work if the active and neutral lines become reversed as can happen when untrained people wire up plugs. A well made appliance will often switch both active and neutral lines to also protect against reversed plugs. Many modern products do not have metal exterior chassis and will employ what is called double insulation where the usually plastic chassis acts as a second insulating medium. Virtually all power tools are made this way these days and will not require an earth connection.
I was looking for some isolation transformers for the bench, picked up two 50lb or so transformers, Model 9T91C10 Gen. Electric. Capacitive(Constant) Voltage Transformers, I can't find any info on them, other then the sticker. 60Hz 95-130v input, 120 output. The person used them for their guitar amps while gigging.
Great presentation as usual. Beginner here. I'm trying to get my bench up and running and really want to understand this concept fully. This video clarifies. But like so many others I'm left wondering what do I do with my oscilloscope? I have to attach my probe ground to the chassis constantly. What should I do? float my scope as well? I haven't seen a single explanation of how to proceed from this point. I use an isolation transformer. I make sure the object under test is floating and there's no ground to neutral. Then what? I have to buy a differential probe? What about a battery powered oscilloscope? Pretty sure we're not far from that being commonplace, outside of automotive scopes. Will that solve some issues? Please explain to me how to proceed and get my bench up to speed
Hey Paul, I saw your build of a line share/isolations project and it inspired me to build yet another piece of test equipment for my lab, Looks like it will go on the wall like yours did just to meet roon-issues. QUESTION: Would you comment on my plan to use the primary windings back-to-back from two MOTs? I would be the only tech in my lab but, looking ahead, I would enclose my build in plexi-glas for safety sake. I would like to know what you think. It's all limited to 120v even though MOTs make it all sound over-the-top.
Even though it's a floating output, still exercise caution. Never touch the two output wires of the secondary at the same time, as your body will complete the circuit!
I've struggled so much on this subject and decided to compile my own notes in what I call: ----------------------------------------------------------------------------------------------- OSCILLOSCOPE PROBES/GROUNDING CAUTIONS "FOR DUMMIES" ----------------------------------------------------------------------------------------------- Oscilloscopes ARE NOT multimeters. Oscilloscopes are from Mars, Multimeters from Venus. The common and the positive probes of the multimeter DO NOT work the same way as a probe in an oscilloscope. The alligator clip on the scope probe is connected, through the power cord, to the Ground. If you clip it to something that isn't at Ground potential, you get a large current, and things blow. Engineers built the oscilloscope like this, and it has to do with safety and noise performance. What should you do? It all depends on what you want. If you are planning to work with ground connected circuits/power electronics you must exercise caution. If you are an enthusiast working with very low voltages, you shouldn't be so concerned. FOR GROUND CONNECTED CIRCUITS (POWER ELECTRONICS) 1. Contrary to what some colleges still teach, do not use an isolation transformer to float the oscilloscope. This is reckless and dangerous. It's better to isolate the device under test and let the scope work as designed (see #3 below). A Google search of, "How to die by isolation transformer" will give more examples and information. 2. Do not use cheat plugs or remove the ground. 3. Focus on protecting the Unit Under Test (UUT), not the oscilloscope, and use an isolation transformer with the following criteria: a) Must have a current limited supply b) Make sure to internally disconnect the transformer's secondary neutral from the ground. Many transformers often have the neutral and ground tied together. 4. Do not hook up communication cables via RS232, USB, etc. to your test circuit, otherwise your UUT will no longer be floating. 5. If power electronics are a must, invest in a differential probe. It's expensive but will save your life and your equipment. FOR CIRCUITS NOT GROUND CONNECTED From input levels such as a few mV to few Volts up to 50 V DC, and potential 15-50V DC, probes/grounding shouldn't be much of a concern.
11:14 Paul: "This little adapter could save your life, but whatever you do don't use one, because legal niceties are so much more important than actual safety". Another issue Paul didn't mention is having two isolated outlets on the same transformer. This is dangerous,. For example imagine two hot-chassis radios , one in each outlet, and the phase of either one was reversed, there would be full mains voltage across the two metal chassis. So if that transformer was mine, I would open it up and remove the ground connection from the outlet, and disconnect the second outlet completely. I'd rather be safe than compliant.
I have a Really Old "Hammond" isolation transformer, model 171B60 that solves this problem. It has just a 2-wire male plug on the AC Line input, and a 2-wire female socket on the transformer output. Got this transformer from my uncle back in 1974, and I'm not sure how many years old it was then (I see the 'newer' Hammond 171B are 3-wire input, and 3-wire output)
Hi Mr Carlson: I have a Variac Transformer TDGC2kM 2000VA. I want to use on my bench for project , my question is can I disconnect the grounding wire so that it will be safety to my oscilloscope?
Great video. I work on a lot of tube guitar amps and I use a variac and isolation transformer. Do you recommend plugging the unit/amp under test AND the O-scope into the isolated (and earth/ground lifted) power source or only the unit / amp under test? I realize this could be a tricky question because when the scope is earth grounded and the probe clip is on the chassis, the chassis is once again grounded to earth as if the test unit was earthed at the receptacle, even though the mains AC voltage is "isolated." Thanks for any response you can offer. I'm in Mission and I refurbished a cabinet for a vintage amp you later did a video on the electronics rebuild.
As a note, in the UK our supply typically includes a separate earth that is handled way back at the generator, so earth and neutral have no connection within our premises.
Gary Richardson : Attaching one of the two wires of the mains generating alternator physically to the earth ensures that this wire is at zero potential (relative to the earth, which you are also touching) and thus safe to touch by you. You could not touch the other wire from the alternator or your body would complete the circuit to the earth, with risk of electrocution. If neither of the two wires from the alternator were attached to the earth, there would in effect be no continuously neutral or live wire, as both of the wires would take it in turns to be either live or neutral (for a duration which is relative to the alternation frequency of the alternator). It is safe to touch both the neutral wire and the frame of any metal cased and earthed machine (supply alternator also connected to the earth), simply because it is at the same potential as you, relative to the (elected) live wire, so if the live wire inside the machine should break and then touch the metal frame of the machine, the frame of the machine would then conduct the live current to the earth, rendering the metal frame essentially harmless to the touch. This is due to the very large voltage drop that the earth imparts to the circuit and hence machine case. It must be said that this condition would normally result in a fuse blowing due to the inevitably large current flow now taking place, assuming the earth wire is substantially large enough to carry the now elevated current flow.
+Scott Johnston On a Isolation transformers they are never exactly 1:1 so I always put the Variac into the wall first, then my Isolation transformer, turning the variac down to 118. if I use my Isolation transformer with out my Variac I get 135 volts which is to high for anything.
I run my isolation tranny into the wall, then my variac runs off the isolation transformer. I can get a maximum voltage of 148 out of that. Because my line is 123 with 3v added from the isolation transformer. Then the 140v range on the variac. Of course people can do it different but I do mine that way. I also run the variac through a dpdt switch that allows me to select the wall or isolation transformer as the voltage supply. I did that so if needed I'm not limited to 2.5amps from the isolation transformer.
Why will I use an I/T when for $15 I can purchase an RCD (residual current device) compulsory in new homes in Australia. I have 22 circuits in my house, and each circuit is connected with a RCD. They trip in 30 m/s and 15 mA imbalance. I can hold, with bare hands and bare feet while placing a metal pin into any of my power outlets and the RCD trips and I am safe. I bought a new $250 I/T for my 14 year old future electrical engineer,,, I am afraid for him to use it as it will not stop him from being electrocuted, only protect the scope and other equipment. Can someone please explain it to me, thanks.
Why is an isolation transformer safer than using current directly from the outlet? Doesn't the secondary side provide as much electrical energy? Also, an idea for a Tech Tips Tuesday for us noobs -- what is the difference between the neutral and the ground wire? I see outlets wired with the white neutral and green ground screw connected all of the time
There is no reference to ground from an isolation transformer. You could shove a fork in the hot socket of the transformer's secondary, put the fork in your mouth, and grab a copper pole and nothing would happen because the earth is no longer part of the circuit. If you did that with the normal AC mains you would die because the earth itself IS part of the circuit.
Can you explain why in an arcade cabinet running a monitor that requires an isolation transformer still had the earth ground connected on both sides of the transformer? I have other people telling me that you have to have earth ground connected on both sides. They tell me I should have my isolation transformer connected to earth ground when I am working on a monitor outside of the cabinet. I am 99% sure they are wrong.
I made my own box with an isolation transformer, some bulbs which can be individually engaged and disengaged and a volt/current meter. However, I have a question. The upside of the isolation transformer is that if you touch the live coming out of it, you don't get electrocuted. However if you touch both live and neutral, you do and the RCD won't cut power as it's located before the transformer. At least this is my understanding. Do we feel that the main advantage of having an isolation transformer is worth giving up the RCD protection? I am honestly asking, not questioning.
Anyone trying to use a so-called "isolation transformer" needs to not only think clearly about the relationship of "earth" ground to the two AC power leads, but also carefully check the transformer internal wiring before using it. For example, I just received from Jameco a Goldsun (China) model GRP-1200, which connected all (2 primary and 2 secondary) outlet "earths" to the so-designated lead in the incoming mains cable and to the GRP-1200 case, which is acceptable for some applications but not those requiring actual isolation (floating secondary). Worst of all, one of the primary outlets' "earth" connector was not correctly soldered, so it connected to nothing and thus would defeat the safety function of earthing (on the *primary* side!). The four "earth" connectors on the outlets were bussed via a bare wire, which was not mechanically bonded to the outlets but just laid alongside the connector and spot-soldered. Several of the internal wires were pressing against corners of various kinds, such as the transformer laminations, which may eventually abrade or arc through, leading to short circuits. I will be rewiring and retesting this unit before using it.
I break that primary ground loose and then I install a Heinneman breaker and a GFCI on the secondary side. If I want to get really fancy, I will isolate the whole transformer from the housing. I then tie the primary ground into the housing and I hook the secondary ground up to the isolated transformer frame, and bond the neutral to the core & frame but NOT the housing. I will have to use an orange isolated ground receptacle so the grounding plug prong is NOT strapped into the strap frame. I will also fuse the primary side.
@@MrCarlsonsLab That is why there is an isolated ground hospital outlet on a Heinemann breaker outlet and a GFCI outlet running out through a Schurter line filter. I have a sailboat inverter charger too. THAT SUCKER IS SUPER NOISY, unless it is running off the batteries, but at the dock on dock AC power it's beyond horrible for noise. Your radios are worthless on dock power through those.
Where does a current limiter come into play? I have a lightbulb current limiter plugged into a Variac and the Variac is connected to the wall. If I shorted out the oscilloscope (connected directly to mains) by placing the ground clip on something with voltage wouldn’t the lightbulb limiter cut the current flowing to the device I’m working on? Thanks for any info
Maybe I'm just woefully unqualified to do my job, but isn't the entire point of an isolation transformer to, you know, isolate the ground?? for example, in marine electrical applications we often use isolation transformers in order to break the grounding connection between different boats in a marina and thus prevent a galvanic cell from forming. This also moves the source of power and quite critically the neutral ground connection onboard the boat as opposed to on the dock, which eliminates the potential for leakage of AC stray current into the water. It would be my understanding that a transformer configured as you demonstrated couldn't be considered an isolation transformer at all, and it would be more accurate to describe it as a polarization transformer. What am I missing?
I've been researching and asking everyone I can and I'm so frustrated. I'm a new tech and I want to study ac to dc circuitry. I want a 12 to 24 ac output so I can experiment safely. No one has answered my question clearly if I can buy a transformer that does this safely. It's so frustrating. I know expensive lab equipment exists to do what I'm looking for but I don't have 2000usd when living in Canada to experiment on rectification circuits to transform ac to dc. Any help would be appreciated so much.
I don't understand much of this since I only have an old multimeter... but if I had an oscillioscope, I would plug it into the mains which is grounded tied to neutral in the mains box. Then when working on 120/240 volt boards for repair, I would connect the repair board to an isolating transformer,,, meaning a modified one,,, open the box and cut the ground wire to output side. now my scope should be safe since there is no feed back to the grid,,,BUT that wouldn't make me safe, I would still be handling 120/240 volt active and neutral,,, so the way I read it, and IT only protects your scope, which of course is fine,,, but only if the ground wire is removed from the output,,, then there can be no feed back to the grid,,, I have no practical experience in this matter,, I am a learner and a theoritical person who analyses issues,,, please tell me if I am right.
It's disputable whether the output socket of an isolating transformer should have an earth pin. Some say yes, some no. I am inclined to prefer no, but not everyone agrees. However, (In the UK anyway) there is a far worse gotcha to beware of, in that there are transformers sold for building site tools which used to be called 'isolating transforners' but which are actually autotransformers. I believe the practice of calling them this has now been banned, but you may still see some old hands in the building trade referring to them as isolating transformers. If one of these finds its way onto the electronics testbench it's an accident waiting to happen. On any unknown transformer, do a resistance check between the live and neutral output and all three input pins, especially the earth. If there is a low resistance to earth OR to the the input supply, it is NOT an isolating transformer, and could cause an accident on the bench.
I use a Variac to control the motor speed of a motorized impeller and it works well (zero speed to full speed). Would it make any difference how the motor start/run capacitor is wired into the Variac? One side of the capacitor could be wired to the input hot lead on the Variac, or it could also be wired to the output tap on the Variac. Not sure if it would make any difference. I am also thinking about using an isolation transformer in conjunction with the Variac. Thanks.
I am in Uk so 220-240v ac. I am just setting up my workbench. I have an isolation transformer and have modified it via a switch on the front panel so I can select the original wiring set up or my modified setup. The modified set up has no earth/ground to the casing, transformer or output socket ( which feeds my variac input). The Variac has an earth tag which is connected to ground on isolation transformer output socket, so on modified switch selection on isolation transformer it is not earthed. Any comments would be appreciated good or bad.
... UNLESS... it is an isolating variac... which does exist. Sorry, I have to muddy the waters when I have the chance... which also connects the ground backwards through your wonderful neutral line at the breaker box if you have bog-standard wiring systems like most of us have... which muddies the waters even further... Hell, where I live I can't trust that the water pipe in my lab doesn't somehow connect through to neutral at some point because of someone deciding that my explaining electronics and ground to them suddenly made them an expert in all things electrical... (glances upstairs)... anyway. to filter the waters back to something drinkable, ALWAYS CHECK EVERYTHING before doing anything... belt and suspenders. That's my mad-science tip of the day, and why I still have all my fingers.
If one wanted to check their house line with a scope, how would they connect the scope safely? Can the scope be connected to the wall on the 10X as long as it's also plugged into an isolation transformer? What about high voltages in old tube equipment? Extremely new guy here with only basic electronic knowledge and about to buy his first scope and I don't want to toast it right off. Siglent SDS1104X-E. A lot of scope for a newbie, but I wanted something I wouldn't need to upgrade for a long time.
Very informative video, thank you for posting it. I have an old Variac that has an input and output, each has only two prongs of the same width, and no ground pin. The diagram you drew of your Variac shows the "hot" side connected to the Variac transformer windings and the "neutral" going straight through. Since both prongs of my Variac are the same width, is there a potential functional problem or safety hazard if the prong connected to the Variac windings is plugged into the "neutral" of my wall outlet?
Ok Stupid Question Time....If the ground is meant to protect us from electrocution in an AC system, how would it make sense to essentially bond the neutral and the ground together? Why not simply have a separate grounding bus bar in the breaker box, (which has been present in boxes I've worked in; it's usually located in one of the lower corners of the box, and that's how I have always connected them up ) to allow complete isolation from the AC line voltage? Or am I just stupid? I say that because I always had it in my mind that since AC current is always changing polarity the ground insured that we couldn't become a conduit for a 'complete circuit. So It would seem to me that if the ground and the neutral are both tied to the same bus bar then we would defeat the whole purpose of a separate ground lug in our electrical sockets, and on our line cords.
Hey, 13:44 you have not explained why it’s dangerous, absolutely not recommended, to use a scope this way isolating the earth pin, ie it’s chassis, from the earth wire of your house.
I'm learning but confused. I thought the whole reason to use an isolation transformer was because the earth ground from the wall outlet DID NOT carry through to the ground outlet of the isolation transformer, or am I wrong?
what is the point of such a transformer that doesnt isolate the ground? because i thought isolation transformers lets you say float your function gen output from your scopes input so you are free you create your own gnd reference? at university lab this is what they told me..
+Adil Malim my isolating transformer has earth-free output and appears to have been made that way from the factory - why need an earth on an output with no earth reference? I think the only reason electricity distribution is earth-referenced is due to lightning and static protection as lightning is implicitly earth-referenced and an isolated grid struck by lightning could hit huge voltages w.r.t. ground and flash-over in all kinds of unfortunate places. Not so much of a problem on the bench.
+Adil Malim You want to (*Float* ) the device being tested *only*. All your other test equipment should be directly plug in to the wall plug, that is grounded.
+MrMac5150 yes i understand that, im assuming you use this isolation transformer for the Device under test so doesnt it make sense for it to not be mains earth referenced?
+Glebs Litvjaks In my mind I would say Transient Spikes Yes, Surges No, Noise NO and Yes depending where the noise is coming from, Noise on a AM radio caused by dirty insulators on the step down transformer, NO, the electric company will need to wash off the connectors to get rid of that kinda noise.
you should add a fuse to the wiper of the variac luckly mine opened at the neutral end so repairable ...shure made me feal silly or just stupid not to thenk of the current in the secondary to blow the pri. fuse on short and turned down variac....for a cheap isolation transformer, use two transformers back to back..i use industrial control transformers 120v to 480v connected to 480v to 120v. twice as big no nice box but i got em free.
Makes me wonder why you even need a third pin and 3-wire power cord in the US. If the ground pin is connected to neutral in the breaker panel anyway, why not connect ground to neutral in the appliance itself? Using a plug with a small and large pin to distinguish hot and neutral would suffice.
So after watching your video a few times, and as you expained so well, what's the purpose for an isolation transformer that doesn't actually isolate. Then you show how you can use the 3 outlet adapter to achieve true isolation but then go on to say this isn't a good idea. I'm confused....
Everything said is true but the logic is wrong. You should never power your test equipment using your isolation transformer. Dut should be powered with the isolation. Nothing can protect you from misuse.
I have used that short cut with both isolation trans formers and vairacts. OK then what is the proper way to protect your self? Isolate the whole bench from ground? ie suspended ground in the fuse box or junction box?
I knew about the problem with the ground, nice to know the reason why to. Good explanation. That said your demonstration with the bulb and the probe hovering so closely to exposed mains with your bare hands, made me shiver and that in a video about safety........ :)
+Bart Zuidgeest LOL, When I watched the video I chuckled, my hands look way closer to those open clips than they actually were. Ahh....the magic of video. Thanks for your comment Bart!
If the Isolation transformer had no ground on the output side there would be no issue with this transformer. You demonstrated that with the no ground plug adapter. Your transformer is not completely isolated with ground. If you look internally that transformer has ground from the input connected to the ground output or it is connected to the chassis, therefore not isolated. Some transformers are this way even though they may claim to be isolated. Pure isolation transformers usually have a 2 prong on the output and there is no connectivity to any of the the input and are completely isolated. Also, some variac's have isolation transformers built in and may claim to be isolated, are not completely isolated.
The only way that light bulb lit up at 8:28 is if there is a direct connection on the secondary between the neutral and ground. Current path is from secondary "hot" to bulb to scope ground to service panel to secondary ground and then through secondary coil to "hot" to bulb.
It's worth noting that some "isolation" transformers have the neutral of the secondary bonded to mains earth. Tripp Lite is the company that comes to mind here...
You clearly state that you do NOT recommend disconnecting ground on your isolation transformer (and that older transformers were constructed with no ground connection). Can someone elaborate on the specific risks created in this way? I am constructing an isolation transformer using a 500VA core in a wooden box for working on guitar amps, and I don't plan to have any ground connection at all (your video reinforces my faith in the insulating properties of wood). No flames please, I'm here to learn :-).
Hi if I understand your diagram correctly. By connecting my equipment under test in the isolation transformer. I'm safe. Because there is no low ohm connection between neutral or hot on secondary of the isolation transformer and mains. And your videos are great.
Thanks for the informative video. Here's a thought for you: You have an oscilloscope and a function generator plugged into a power strip and you routinely check your wave form by directly plugging one into the other with a bnc. we all do it, no problem... Now you change out your fancy new generator for an older tube based one that has a non polarized plug, the transformer is center tapped to chassis ground.You are about to plug it into the power strip... You''ve got one question to ask yourself punk. Are you feeling lucky? Well, are you? The greatest piece of safety equipment in the shop is resting on top of your shoulders. Be safe you all.
I'm wondering if this is a new thing. Our service entrance panel is new and the neutral bus and ground bus are isolated and not electrically connected. I'm thinking they are in older homes but I'm pretty certain this is not the case for newer construction or any upgrade work. This of course is not a reason to not to pay very close attention to this issue.
It seems like an isolation transformer like this kind of sabotages it's own point? What's the reason for connecting the earth if the purpose is to float the DUT?
This was one of the shortest but absolute best videos I've seen of yours. Safety is number one, two, and three. Without proper safety knowledge, us techs can not do our job. Would love to see more videos like this.
-Mr. Carlson's biggest fan.
¹¹¹1😅
A very interesting video Mr Carlson, it made me look up how houses are wired here in the UK. Once upon a time you either had a ground rod (some rural areas still do) or you had the house ground bonded to the sheath of the mains supply which went back to the substation before grounding out. The single phase domestic supply was one of the three phases of the national grid to live and the ground at the transformer for neutral so the earth and neutral were connected at that point. New builds and I think anything that is re-wired now has a system just like you describe with earth connected to neutral in the company fuse box at the distribution board.
The other thought that comes to mind is that the earth connection on the secondary side of an isolation transformer seems to be absolutely pointless as it doesn't offer the safety that it appears to. The earth connection in an appliance is to provide a route to earth should anything fail internally and ground out the live causing the fuse to blow somewhere - either in the appliance, in the wall plug or at the distribution board. With more sophisticated systems with earth leakage or residual current detection that would trip out first. None of those things will happen if the live and neutral are isolated. But on the other hand you are unlikely to suffer much harm if you touch both the live and earth. When mine arrives I must test what potentials are there.
Just a beginner here, but I thought the correct way was to hook up your oscilloscope normally, and to the ground and use an isolation transformer (with no ground) on the device you are testing.
Great safety video, and as always, very understandable. Hope you and your family have a fantastic Christmas and New Year. Regards Bob (UK)
+Bob Mellor
Thanks Bob! Merry Christmas to you and yours as well!
Hi Paul, wow I don't understand why that transformer is officially out in the market as certified isolation transformer. In Europe this would never ever get the declaration of confirmatory to be sold as isolation transformer. Following VDE/ EN norm the secondary side never is attached to protective ground/ earth. But anyway what ever norms are telling at all it doesn't make any sense...., or is there is something so much substantially different in Canada''s power distribution compared to Europa I don't know?? Anyway very interesting video ! Thank you. 73 Peter
+TRXBench There seems to be a lot of confusion about this even in europe. Mostly an "isolation transformer" has a earth connection on the secondary and a "safety transformer" has not, but has this clearly marked with a warning next to the outlet (and warns you not to hook up more than one device). But I have seen a "safety isolation transformer" (made by peaktech) that has a earth connection on the secondary, completely defeating its purpose (even false advertising imho).
But I'd be very interested, if there are codes in Canada, that prohibit safety transformers without mains earth reference. After all these things are only for full galvanic separation (thus preventing any current flow out of the DUT into any earth around) during repairs under power. Nobody in their right mind would use them for anything else.
But a well made and very important video nontheless. Thank you for it.
+Róbert Valdimarsson Hi Robert, yes very important issue and you are right a lot of confusion all over. I'm talking only about a galvanic protective separation transformer (Schutztrennung) according VDE 0570 Part 2-4, DIN EN 61558-2-4, EN 61558-2-4, IEC 61558-2-4
+TRXBench
I agree fully, but politics and bureaucracy gets in the way of sense once again. If they want to include that "Earth" they need to re-label the transformer, and remove the word "Isolation"
Merry Christmas to you and yours Peter!
+TRXBench Yes, you pretty much have to rely on the norm numbers here. I have a propper one here for up to 1500VA. I got it very cheaply second hand and quickly found out why: It does not have a inrush current limiter. So every time i switch it on, I have to make a trip to the switch box and reset the breaker :D But a limiter module is not too expensive. Still the best money I have spent so far, since it might very well save my life some day.
Mery christmas and a happy new year to all of you too.
+Róbert Valdimarsson
Thanks Robert, Merry Christmas!
It depends on your type of variac.
My Grundig RT4 has full isolation + variac.
Good public service announcement. Learning the hard way of testing main voltage on an oscilloscope at the receptacle...50/50 chance of meeting your maker. Neutral is not always neutral when Jose wired your house. Maybe a good idea to backprobe to breaker box to verify mains wiring to test equipment? Make sure neutral and hot are not reversed on the electrical reseptacle . Neutral is actually the center tap on the pole transformer supplying the breaker box. Ground should have no continuity with neutral as it is wired to a copper earth pole.
What if Frank wired your house?
Nice video, there are variac's that are also isolated, Philips made a bunch of them, I have the 12NC242252900005
(newer version: 12NC242252900007). Very affordable second hand and really nice quality also.
So you saying "absolute no isolation" (14:19 and 15:47) it refers to your specific non-isolated variac.
Clip your scope ground on the top end of that isolation
transformer & see the BOOM !!!
You can back feed high voltage from a Unit Under Test via the
scope ground clip & lift all other grounds & equipment in the shop .
Rule one is know your equipment .
Another informative video, thanks and merry Christmas..
+Jim McDowell
Merry Christmas to you too Jim!
Great video on isolation transformers and variac safety! Very informative! :)
At 8:30, you connected the hot wire straight to ground; is that all this basically is?
Been there, done that, big surprise.
In my book, any isolation transformer that feeds ground through to the secondary side is NOT an isolation transformer.
It's still considered isolated, just not from ground.
You are exactly correct, Sir!
@@NunYa953 Actually... From the neutral bar, which is connected to ground, in the electrical panel!
@@MrDoneboy
This particular design of isolation transformer only isolates the secondary from mains. There is a big difference between a bench test isolation transformer and this. All these video prove is there's a whole lot of people who have no business playing with electricity doing just that.
@@NunYa953 It is your life, sir...you only have one!
As others have pointed out. It is important to remember that if you float your scope and then connect probe ground to a hot connection the entire case and terminals of the scope and anything else connected to it are now hot. You only need to touch any of those points and ground and you are fried. That is why you normally never float the scope but rather the DUT.
An alternative to be extra safe is to float both the device and the oscilloscope (using separate transformers!). Still you have to be mindful that your ground lead can energize your oscilloscope w.r.t. the device under test, so don't handle both at the same time in that case.
Well yeah that's pretty obvious from even basic electrical theory. It's more about parasitic components of circuits that actually become an issue sometimes!! But yes I do generally aggree with you of course!!
yep all metal on the scope
Me Carlson, I’ve seen this warning/explanation many times but what I haven’t seen is a proper fix for the problem. I can disconnect the “earth” lead on the output side of my isolation transformer but I don’t think that’s the proper fix. What is?
Use the adapter, contrary to Mr. Carlson it's still safe.
Thanks Paul. You explained it very well. People need to be careful with the ground of USB plugs from their laptops and devices also.
+Robert Calk Jr.
Thanks for your input Robert!
+Robert Calk Jr.
Good point about USB. Check out the USB port isolator at Adafruit (search for product #2107)
+Ken Cosi
Looks like a handy tool, thanks.
I love this comment. A USB isolator is less than 19 dollars on Amazon. No reason not to use one!
USBs being dangerous... really lol
I have had Earth ground running through my isolation transformer for decades and never have had a problem. I never plug my test equipment into my isolation transformer only DUT. Many devices have components referenced chassis ground. Running the Earth ground all the way through to the DUT helps to eliminate noise while testing. That said, Mr Carlson is absolutely correct in his presentation and I am fully aware of how my bench is configured. Thanks Mr Carlson
Thanks, I now have a better understanding. But it seems like you didn't finish the topic on Isolation Transformers. It's good to be aware that the secondary might be linked to Earth. But then, how should we setup a proper isolation system? Should we break the secondary connection? I want to be able to use my scope to test the long list of devices I have.
Francois, The best way to ensure total isolation is to remove any ground connection at the output of your IT. Some manufacturers ground the metal container that boxes the IT as a matter of safety. Depending on the type of outlet socket this can be problematic as the mounting screws can become grounded and defeat the purpose. For example, in the UK and Ireland, all socket mountings are connected to the earth pin and when mounting them as I had to do , I literally had to cut the copper connection to isolate the output otherwise the mounting screws would have made the output grounded. These are small things that need to be considered whilst making sure your IT is suitable for bench work.
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Thank you so much for the info about these isolation transformers. I did not know they would connect the separate grounds. I can not believe it is called an isolation transformer. You may have saved some lives with this video. I honestly would assume an isolation transformer is ISOLATED.
For starters you would normally use the isolation transformer to isolate the device under repair, not the scope.My isolation transformer the test plug is only connected to the secondary, the ground prong is completely open on the transformer.Mine is a Hammond manufacturing co, made in Canada.
I have a Hammond model 171B isolation transformer. The ground prong of the output receptacle is definitely connected to the input cord ground and the transformer frame on mine (I just checked). In addition, Hammond's own data sheets for the product show the output ground terminal connected: www.hammfg.com/electronics/transformers/line/171
(My unit is an older model with no circuit breaker, and with an outlet instead of a pigtail for the output).
Just a couple of comments:
1. You mentioned powering an oscilloscope from an Isolation Transformer (although, admittedly, you didn't actually suggest doing that). That should almost NEVER be done. (The only exception might be if you are actually repairing an oscilloscope, where the scope is the Device Under Test (DUT)) Only the DUT should be connected to an isolation transformer.
2. A standard Isolation Transformer can be converted to a "technician's isolation transformer" by internally disconnecting the transformer's secondary neutral from ground. This maintains the ground path all the way to the DUT (assuming that the DUT has a ground prong). It is much safer than using the cheater plug while also isolating the DUT from the mains. However, even with a "tech" Isolation Transformer, you still need to be careful not to do things like connect the secondary (output) to ground.
+Louis Wilen
Thanks for your comment Louis!
yes , you are so right here .....
sometimes silly and basic things goes so under the radar , right ??
I won't name any names but there are 2 professional techs here on you tube that use 2 isolation transformers because it's safer. they plug both their scope and tv or whatever into an isolation transformer. They both work as professional techs servicing consumer electronics.
Robert Slackware Yep. At least a very low Ω path.
I believe your drawing of the Auto Trans was over simplified. Per the scale on the trans the output is variable from 0 to 140% of the input. Therefore the AC main should be someplace lower than the auto trans tap.
This is one of the most complex and complicated issues in electronics (for me). I've heard so many do's and don'ts and now I'm more confused than ever. So, HOW DO YOU HOOKUP YOUR WORKBENCH? Do you ONLY plug the DUT (Device Under Test) to the isolation transformer? I think that's what they recommend for working on old Ham radios that don't have a power transformer - that have a "live" or "hot" chassis. Or, do you ONLY plug your test instruments into the isolation transformer and have the DUT plugged into house power? Or, do you plug everything into the isolation transformer? I think I've read that that can be dangerous, too. In any case, I guess it's a good idea to have a GFCI (Ground Fault Circuit Interrupter) (like used in bathrooms) multiple outlet bar to have the fastest shutdown of power if an imbalance in circuit power is detected. Another question I have is - do I need (would it be useful to have) one of those Siglent SDS1202X-E scopes to troubleshoot a bunch of old Tektronix 465 oscilloscopes I have? I think I'm looking for an excuse to buy one. I just checked out your Tektronix TDS 340A scope, too. I can get one of those on eBay for about the same price as the Siglent. The advantage being that the face-plate is arranged in a more familiar pattern like other Tek scopes whereas the Siglent is somewhat confusing to my eye. My rationale is that I don't know how functional or accurate any of those old 465's are and the Siglent or the TDS 340A would be new(er) and better able to give me more accurate readings, to troubleshoot those old 465's, wouldn't it? Maybe? Or would I be better off spending my money on other things?
In my universal phase motor ECU, I've designed it so that the chassis is directly connected to the safety ground, thus meaning that I can effectively touch the chassis with both hands and not get a shock. It also means that any capacitors holding their charge should discharge to ground once the unit is unplugged.
I've seen many comments here about only the DUT being isolated from ground, however, when I was a student learning troubleshooting and repair techniques, the instructors at the vocational school cautioned us to never have the test equipment grounded. They sent us out to the rest of the world having been taught the very same thing Mr. Carlson showed in this video. Every piece of test equipment in that shop had all of the ground pins on the power cords removed. Every one of them. Meanwhile in the electronics theory class down the hall, they blew up a brand new 540 series Tektronix scope because of said grounding issue! I don't remember anything else about the incident so I don't know what they were measuring, how it was hooked up or what- just that they mentioned it to one of our instructors and then we were told about that precaution. After that the electronics instructors were removing the ground pins from the power cords or using the adaptors as shown here.
Anyone care to explain why it is bad to float your test equipment? Not looking of arguments or heated discussion, just good explanations.
I was taking an analog electronics class at the local community college back in the 90's. My lab partner was in the air force and she said usually stationed in an AWACS. I was taking notes and she was hooking up the circuit. so she had the power supply on and somewhere in the circuit she said lets see positive goes to negative and before i could say no!, she connected the connection. there was a bang and smoke came out the back of the power supply. The instructor walked over expressionless. Picked up the power supply, unlocked a closet. He exchanged the blown power supply for a new one, locked the closet and w/o a word put the new power supply down on the bench in front of us.
I made this exact mistake 2 days ago on my oscope. After the bright flash I started thinking about it and then realized -of course the ground clip is grounded - duh. Then I started looking into prices of variacs that have an isolated transformer and then found this video. BTW, my Oscope does not measure correctly anymore - both the voltages and wave forms are incorrect now. i have not yet tried to replace the probe so hopefully that will be the only issue. Just wanted to say I made this exact mistake and your video helped me understand and verify why. Totally makes sense now. I did not realized that the barrel was connected to earth ground and was treating it like a multimeter. Thanks for the Video!
oh mate. how did you get on? did you fix the scope?
Did you get your Oscope fix yet.
@@carl112466 Dude is probably dead because he didn't know about current limiters.
Possible that the board foil to the input connector burnt and possibly you can fix that and with a little luck there will be no other damage. Good luck and God Bless!
Two of Ed
So dumb question then. What is the point of an isolation transformer if it does not solve the problem we are trying to solve in the first place? I just bought an oscilloscope and was buying an isolation transformer and then ran across this video. Wow am I confused now. What if I just plugged the oscilloscope into the wall with the three pronged adapter not having the ground connected? Total confusing now. Can you post a video on best practices?
Mr. Carlson,
I have a power filter for my equipment, an isolation Transformer ( fixed after watching your video👍) A variac. I also have a current limited lightbulb when needed .
My question is ,...Where is the best location to place my isolation Transformer?
At the beginning of it all. In the middle or at the very end???
I think this is a very under rated topic . I don't believe it has been addressed fully here which is understandable because this video highlights the difference between filter conditioning transformers and isolation transformer. We need to highlight links or create videos on practical techniques to isolate techs and equipment . More discussion is needed to increase safety when working on mains . There seems to be two topics a) human safety techniques while working on electrical equip and b) safely using test gear on high voltage without destroying equipment . Equipment requiring mentioning : isolation clothing , isolation transformers , differential probes.
I agree this is a huge topic that seems to catch out even the best of them.
Thank you Mr. Carlson. If I ever get an isolation transformer like yours I'll be sure to open it up and cut the ground connection to the secondary side.
I was tought to always use TWO isolation transformers with no ground connection: One for load and the other for measuring device. Floating load can be measured with earthed scope but when ground lead is connected it becomes mains referenced. If you touch the device, you get shocked because it is no longer isolated from mains earth. Having also the scope floated via a second isolation transformer (with no earth), you avoid the problem. With that setup it is safe to touch any single point of a powered device. That is also safer than just isolating the scope from ground because in case anything leaks from "live" to scope chassis, it is isolated "live" ie. will not shock you.
Never ever float only the scope and connect it to a mains connected load. Earth lead connected to eg. live wire makes scope chassis live which is very dangerous.
As a kid I also made the classic mistake of connecting scope earth to live wire with all the sound and light effects assosiated. :) Never repeated that mistake.
your lab looks cool, like inside the deck of a ww II old battle submarine
In Australia it is normal for the output socket on an isolation transformer to be NOT connected to earth, if you need an earth on what your working on, then that has to be provided separately. Anyway I would NOT use one to float a scope, or any other test gear. If you do, and connect the probes earth clip to any substantial voltage, then that will appear on the outer metalwork of your scope, as in the WHOLE chassis will be LIVE, very dangerous indeed.
If you then go to connect a second lead to another input, you will be touching a live socket, and you will get a shock.
If you want to look at the mains using a 'normal' earthed scope, then tie back the probes earth clip and just connect the probe tip, using the scope's earth as the return path. Also before doing so check to make sure you are not exceeding the voltage specs. In the US you only have 120V, in Australia it's 240 on the active/live pin, so often an X100 probe is a better choice. Some early digital scope's can only take 5V directly into their BNC, check.
Another method for a mains look-see is to use an AC output power pack (wall wart in the US) with say a 12V output, this can be handled with pretty much in total safety, and give you an X10 display, same as an X10 probe.
Toroidal transformers seem the best, some small & cheap EI types, especially the ones that run warm, can be running a bit close to saturation and can round off the tops of mains crud your trying to observe.
Another problem with floating equipment, is that the chassis will probably be sitting at anything other than earth, this is usually due to stray capacitance in the power transformer, and is usually exacerbated by mains filters that can easily put the chassis at half mains, whilst there is little to no current seen here, the static charge contained in a largish chassis, and the stray capacitance, can cause a current surge/voltage spike when you hook the earth clip, or an X1 probe doing damage to your gear. I once connected a floating video camera into a video input on a computer capture card, the spike destroyed a few hundred dollars of equipment - it does happen, floating gear is bad.
Very informative, thank you!
Thanks for the reminder! Safety is #1. As a 45+ year HVAC/R service tech, I’ve been “poked” by 24v, 120v, 575v, and even lightning (100’ underground behind Niagara Falls)-and I wouldn’t wish the experience on my worst enemy.
Is the methodology of using “balanced input” (where you use 2 probes, with their ground leads clipped together and not connected to anything else, and their signal probes connected to the AC line) a safe alternative?
i think im gonna always use a battery operated scope even if its wimpy. you can get some good longevity out of a stack of 18650 batteries and its clean too :)
My understanding was that you should pretty much never float your scope, either through an isolation transformer, or especially from ground. Keep the scope plugged into the wall, and pit the device under test on the isolation transformer.
+PelDaddy
Thanks for your comment!
PelDaddy that was my first thought, put the scope i the wall, and put the device under test into the iso....
if you have an older piece of equipment (like 98% or so of stuff on this channel) it probably only has a 2 prong plug anyway.... so the iso-ground issue isn't an issue....
I agree but there are 2 techs here on you tube that float their scopes thinking it makes things safer.
What's the reasoning not to float the scope?
Teb Tengri the reason is that with a floating earth scope the BNC connectors are still attached together and to the chassis of the scope. So if you would hook mains voltage to the ground clip of a probe, all the other probes/BNCs and the case would be live. Since you normally treat the ground clips etc. as safe to touch, and often clip it straight onto the nearest ground of a DUT, you make it very easy for yourself to mess up and ruin your day. If you want the convenience of not having to worry where you stick your probes, there is no substitute for a proper isolated probe.
Balanced isolation transformers, which are sometimes used for audiophile and home theater systems, usually have a center-tap on the secondary which is sometimes connected to the ground connection on the output and chassis. Thus the output is not always isolated.
Thank you Paul, I did not know this you quite possibly saved me from an injury
I (and probably lots of other people) would love to see you demonstrate the USE of a variac. People often talk about powering up vintage gear with a variac, bringing up the AC power to the vintage gear slowly (how slowly?) I'd like to see a demonstration of this, along with an explanation of why it is being done (allowing capacitors to reform?) I have been told that doing this helps to keep from damaging the vintage gear. Could you demonstrate this process and shed some light on what precisely is going on?
So my kinda layman's understanding of it is the following from what I remember from videos on the subject. Basically, the reason people care to supply lower AC voltage is because failed old capacitors in Tube gear have the potential to basically short the power supply to some degree and cause excessive current through the rectification section, including of course the rectifier tube(s). With bad enough capacitors and enough current/time powered on, you can potentially damage/destroy the rectifier tube(s) and transformers of old, broken gear in need of Cap replacement.
As far as I can tell as an amateur, the ONLY reason to bother with a Variac is for 'testing' old tube electronics with presumably bad Caps that will overload the supply components. This is IMO stupid and unnecessary as the caps should be presumed bad (shorted, kinda) on anything old enough for Tubes and this seems like a complete waste of time given the only people who'd be doing this would also be replacing the certain failure-prone Caps (paper, Electrolytic especially, etc) afterwards anyway. It seems to me like you could skip this entirely by checking resistance and/or continuity from the input to the power supply section to see if it'll be partially shorted instead of just running it at lower AC voltage to mitigate damage versus full 120v. Anyway, the theory/practice behind the Variac is that lower AC voltage will proportionately lower current through the device with faulty Caps and thus lower heat/damage to sensitive components. I don't see why you couldn't instead just limit current instead with full 120v line with an inline fuse or something (like lots of devices have at their input, what a coincidence!) if you're gonna be 'testing' equipment that you suspect is faulty and liable to be damaged plugged in.
So yeah, basically it's that simple. Lower line voltage equals lower fault current which is very common as old Caps are notorious for failing internally (basically turning into resistors/jumpers) and is by far the biggest problem in Tube gear not working or working poorly. Any vintage tube equipment you intend on using needs replacement caps (again, certain types only 99% of the time) or they're liable to burn out the rectification and transformers. Not exactly catastrophic for the vast majority of cases but maybe a bigger deal because Tubes are scarce.
@@Demoralized88 Thanks for spending all that time and energy to reply. (I'm surprised that this guy hasn't bothered to answer my comment.) But I've been an analog circuit designer for decades. I know how Variacs work. My question had to do only with the apparently common practice of slowly (how slowly?) bringing up line voltage to (perhaps long-stored) vintage equipment to reform caps, etc. I'd like to see this actually being done by someone who knows why it's done and who will provide commentary during the process.
All of my experience (50 years) as an electronics guy have been with new equipment or things that have been in daily use since they were new. I want to restore some cool vintage audio gear and vacuum tube based test equipment that I've bought on ebay, at ham fest's, etc. So, I welcome input from people who do this slow warm up thing. It makes sense to me in theory. A piece of gear could have some extreme fault(s) that could cause damage when power is applied. I imagine that in addition to the capacitor-related issues, there could be other things. I think that using a Variac would allow a guy to initially trace through circuitry without having it blow up in his face.
Hi I have just come across this video I found it interesting as I have just this week been setting up my work bench with isolation transformer . You did a very good demonstration of what the problem is . But I am somewhat concerned about your proposed solution . What seems to be being disregarded is while you make your measurement if you have removed the earth wire from the scope power plug and you have connected the ground probe clip to the hot side At this point the steel case of your scope has become live in relation to anything else that is in your area that is connected to ground if you touched this case while adjusting a knob you would likely get a shock . If you had a data lead from your scope to a pc then you would then be putting high voltage down that lead on the ground into other equipment which you may damage . In the event that you are using as a demonstration I would use a DVM to measure the voltage between the conductor and the ground clip and ensure that it is a very low value possibly a volt or lower before making the connection with the ground clip I would not need to remove the earth from the scope . If one gets used to doing these basic checks before making a connection then it can save a lot of damaged equipment .
I am in the UK so all out power is 240 / ref to earth regards bill
Hi Mr. Carlson, thank you very much for this informative video!
1. Is it safe to disconnect the oscilloscope's earth wire at its plug, so that it only has a live an neutral connection?
2. Is it safe to disconnect the earth wire running from the ac mains supply to the isolation transformer's output plug, so that there is only a live and neutral connection available?
3. When do you need to use an isolation transformer?
4. Can you please do a video on how to connect an isolation transformer to a DUT, like a TV, amplifier etc. and also to instruments like oscilloscopes, signal generators etc. so that all of them can be used safely together to probe and repair a product.
Thank you very much for your channel. I have learnt so much from your videos!
I fixed TV sets for 40+ years but never knew about this! Thanks for sharing this information. I always used the Sencore isolation variac with no problems but now in my home workshop I have a B&K TR-110 isolation transformer and it never occurred to me that I may be not be isolated like I assumed I was. I'll have to check this out. You may have just saved my scope.
And potentially, your life.
I suggest you check whether your isolation transformer and\or variac (my "variac" is isolating) have fuses. One of mine did not have a fuse, so I had to install one. I blew it often since. The other even had two fuses, one on each primary lead, but they were 3.15A good to keep the office powered but too much to limit the damage to the DUT.
that was supper de wb8idy
Modern isolation transformers often have the neutral and ground tied together at the outlet on the secondary side of the transformer. It is a clip that can usually be removed.
+theflinx
Thanks for your comment!
Mr Carlson's Lab Hello and thanks for your videos! I am new to repairing electronics can you please explain why it's so important to keep the ground for safety. I keep hearing this but don't understand why it makes it more safe? Why is it more safe for the BNC connectors to be grounded or the chassis of the isolation transformer to be grounded? I am sorry for asking such a noob question I just don't see anyone explaining this... Thank you
The earth connection provides an alternative path for the power should a fault arise. Since it is always connected it will protect even with the appliance turned off or more importantly beleived to be turned off. The permanent connection also continues to work if the active and neutral lines become reversed as can happen when untrained people wire up plugs. A well made appliance will often switch both active and neutral lines to also protect against reversed plugs.
Many modern products do not have metal exterior chassis and will employ what is called double insulation where the usually plastic chassis acts as a second insulating medium. Virtually all power tools are made this way these days and will not require an earth connection.
I was looking for some isolation transformers for the bench, picked up two 50lb or so transformers, Model 9T91C10 Gen. Electric. Capacitive(Constant) Voltage Transformers, I can't find any info on them, other then the sticker. 60Hz 95-130v input, 120 output. The person used them for their guitar amps while gigging.
Great presentation as usual. Beginner here. I'm trying to get my bench up and running and really want to understand this concept fully. This video clarifies. But like so many others I'm left wondering what do I do with my oscilloscope? I have to attach my probe ground to the chassis constantly. What should I do? float my scope as well? I haven't seen a single explanation of how to proceed from this point. I use an isolation transformer. I make sure the object under test is floating and there's no ground to neutral. Then what? I have to buy a differential probe? What about a battery powered oscilloscope? Pretty sure we're not far from that being commonplace, outside of automotive scopes. Will that solve some issues?
Please explain to me how to proceed and get my bench up to speed
Hey Paul, I saw your build of a line share/isolations project and it inspired me to build yet another piece of test equipment for my lab, Looks like it will go on the wall like yours did just to meet roon-issues. QUESTION: Would you comment on my plan to use the primary windings back-to-back from two MOTs? I would be the only tech in my lab but, looking ahead, I would enclose my build in plexi-glas for safety sake. I would like to know what you think. It's all limited to 120v even though MOTs make it all sound over-the-top.
Thank you! Public safety announcement.
Even though it's a floating output, still exercise caution. Never touch the two output wires of the secondary at the same time, as your body will complete the circuit!
I've struggled so much on this subject and decided to compile my own notes in what I call:
-----------------------------------------------------------------------------------------------
OSCILLOSCOPE PROBES/GROUNDING CAUTIONS "FOR DUMMIES"
-----------------------------------------------------------------------------------------------
Oscilloscopes ARE NOT multimeters. Oscilloscopes are from Mars, Multimeters from Venus. The common and the positive probes of the multimeter DO NOT work the same way as a probe in an oscilloscope. The alligator clip on the scope probe is connected, through the power cord, to the Ground. If you clip it to something that isn't at Ground potential, you get a large current, and things blow. Engineers built the oscilloscope like this, and it has to do with safety and noise performance.
What should you do? It all depends on what you want. If you are planning to work with ground connected circuits/power electronics you must exercise caution. If you are an enthusiast working with very low voltages, you shouldn't be so concerned.
FOR GROUND CONNECTED CIRCUITS (POWER ELECTRONICS)
1. Contrary to what some colleges still teach, do not use an isolation transformer to float the oscilloscope. This is reckless and dangerous. It's better to isolate the device under test and let the scope work as designed (see #3 below). A Google search of, "How to die by isolation transformer" will give more examples and information.
2. Do not use cheat plugs or remove the ground.
3. Focus on protecting the Unit Under Test (UUT), not the oscilloscope, and use an isolation transformer with the following criteria:
a) Must have a current limited supply
b) Make sure to internally disconnect the transformer's secondary neutral from the ground. Many transformers often have the neutral and ground tied together.
4. Do not hook up communication cables via RS232, USB, etc. to your test circuit, otherwise your UUT will no longer be floating.
5. If power electronics are a must, invest in a differential probe. It's expensive but will save your life and your equipment.
FOR CIRCUITS NOT GROUND CONNECTED
From input levels such as a few mV to few Volts up to 50 V DC, and potential 15-50V DC, probes/grounding shouldn't be much of a concern.
11:14 Paul: "This little adapter could save your life, but whatever you do don't use one, because legal niceties are so much more important than actual safety".
Another issue Paul didn't mention is having two isolated outlets on the same transformer. This is dangerous,. For example imagine two hot-chassis radios , one in each outlet, and the phase of either one was reversed, there would be full mains voltage across the two metal chassis.
So if that transformer was mine, I would open it up and remove the ground connection from the outlet, and disconnect the second outlet completely.
I'd rather be safe than compliant.
Best demonstration of potential ground loop hazards re scopes etc on
You Tube, clear and simple.
I have a Really Old "Hammond" isolation transformer, model 171B60 that solves this problem. It has just a 2-wire male plug on the AC Line input, and a 2-wire female socket on the transformer output. Got this transformer from my uncle back in 1974, and I'm not sure how many years old it was then (I see the 'newer' Hammond 171B are 3-wire input, and 3-wire output)
Hi Mr Carlson: I have a Variac Transformer TDGC2kM 2000VA. I want to use on my bench for project , my question is can I disconnect the grounding wire so that it will be safety to my oscilloscope?
Great video. I work on a lot of tube guitar amps and I use a variac and isolation transformer. Do you recommend plugging the unit/amp under test AND the O-scope into the isolated (and earth/ground lifted) power source or only the unit / amp under test? I realize this could be a tricky question because when the scope is earth grounded and the probe clip is on the chassis, the chassis is once again grounded to earth as if the test unit was earthed at the receptacle, even though the mains AC voltage is "isolated." Thanks for any response you can offer. I'm in Mission and I refurbished a cabinet for a vintage amp you later did a video on the electronics rebuild.
As a note, in the UK our supply typically includes a separate earth that is handled way back at the generator, so earth and neutral have no connection within our premises.
Gary Richardson : Attaching one of the two wires of the mains generating alternator physically to the earth ensures that this wire is at zero potential (relative to the earth, which you are also touching) and thus safe to touch by you. You could not touch the other wire from the alternator or your body would complete the circuit to the earth, with risk of electrocution. If neither of the two wires from the alternator were attached to the earth, there would in effect be no continuously neutral or live wire, as both of the wires would take it in turns to be either live or neutral (for a duration which is relative to the alternation frequency of the alternator).
It is safe to touch both the neutral wire and the frame of any metal cased and earthed machine (supply alternator also connected to the earth), simply because it is at the same potential as you, relative to the (elected) live wire, so if the live wire inside the machine should break and then touch the metal frame of the machine, the frame of the machine would then conduct the live current to the earth, rendering the metal frame essentially harmless to the touch. This is due to the very large voltage drop that the earth imparts to the circuit and hence machine case.
It must be said that this condition would normally result in a fuse blowing due to the inevitably large current flow now taking place, assuming the earth wire is substantially large enough to carry the now elevated current flow.
Lots to learn here, important stuff. Your doing a fine job.
Should I always use a isolation transformer with a variac? If so, which comes first? Wall- isolation-variac?
+Scott Johnston On a Isolation transformers they are never exactly 1:1 so I always put
the Variac into the wall first, then my Isolation transformer, turning the
variac down to 118. if I use my Isolation transformer with out my Variac I get 135 volts which
is to high for anything.
+Scott Johnston
I use a Tenma 72-1097 variable isolation transformer.
I run my isolation tranny into the wall, then my variac runs off the isolation transformer. I can get a maximum voltage of 148 out of that. Because my line is 123 with 3v added from the isolation transformer. Then the 140v range on the variac. Of course people can do it different but I do mine that way. I also run the variac through a dpdt switch that allows me to select the wall or isolation transformer as the voltage supply. I did that so if needed I'm not limited to 2.5amps from the isolation transformer.
Why will I use an I/T when for $15 I can purchase an RCD (residual current device) compulsory in new homes in Australia. I have 22 circuits in my house, and each circuit is connected with a RCD. They trip in 30 m/s and 15 mA imbalance. I can hold, with bare hands and bare feet while placing a metal pin into any of my power outlets and the RCD trips and I am safe.
I bought a new $250 I/T for my 14 year old future electrical engineer,,, I am afraid for him to use it as it will not stop him from being electrocuted, only protect the scope and other equipment. Can someone please explain it to me, thanks.
Why is an isolation transformer safer than using current directly from the outlet? Doesn't the secondary side provide as much electrical energy? Also, an idea for a Tech Tips Tuesday for us noobs -- what is the difference between the neutral and the ground wire? I see outlets wired with the white neutral and green ground screw connected all of the time
There is no reference to ground from an isolation transformer. You could shove a fork in the hot socket of the transformer's secondary, put the fork in your mouth, and grab a copper pole and nothing would happen because the earth is no longer part of the circuit. If you did that with the normal AC mains you would die because the earth itself IS part of the circuit.
Can you explain why in an arcade cabinet running a monitor that requires an isolation transformer still had the earth ground connected on both sides of the transformer? I have other people telling me that you have to have earth ground connected on both sides. They tell me I should have my isolation transformer connected to earth ground when I am working on a monitor outside of the cabinet. I am 99% sure they are wrong.
I made my own box with an isolation transformer, some bulbs which can be individually engaged and disengaged and a volt/current meter.
However, I have a question. The upside of the isolation transformer is that if you touch the live coming out of it, you don't get electrocuted.
However if you touch both live and neutral, you do and the RCD won't cut power as it's located before the transformer. At least this is my understanding.
Do we feel that the main advantage of having an isolation transformer is worth giving up the RCD protection? I am honestly asking, not questioning.
Anyone trying to use a so-called "isolation transformer" needs to not only think clearly about the relationship of "earth" ground to the two AC power leads, but also carefully check the transformer internal wiring before using it. For example, I just received from Jameco a Goldsun (China) model GRP-1200, which connected all (2 primary and 2 secondary) outlet "earths" to the so-designated lead in the incoming mains cable and to the GRP-1200 case, which is acceptable for some applications but not those requiring actual isolation (floating secondary). Worst of all, one of the primary outlets' "earth" connector was not correctly soldered, so it connected to nothing and thus would defeat the safety function of earthing (on the *primary* side!). The four "earth" connectors on the outlets were bussed via a bare wire, which was not mechanically bonded to the outlets but just laid alongside the connector and spot-soldered. Several of the internal wires were pressing against corners of various kinds, such as the transformer laminations, which may eventually abrade or arc through, leading to short circuits. I will be rewiring and retesting this unit before using it.
I break that primary ground loose and then I install a Heinneman breaker and a GFCI on the secondary side. If I want to get really fancy, I will isolate the whole transformer from the housing. I then tie the primary ground into the housing and I hook the secondary ground up to the isolated transformer frame, and bond the neutral to the core & frame but NOT the housing. I will have to use an orange isolated ground receptacle so the grounding plug prong is NOT strapped into the strap frame. I will also fuse the primary side.
Remember GFCI receptacles often create noise in radio receivers.
@@MrCarlsonsLab That is why there is an isolated ground hospital outlet on a Heinemann breaker outlet and a GFCI outlet running out through a Schurter line filter. I have a sailboat inverter charger too. THAT SUCKER IS SUPER NOISY, unless it is running off the batteries, but at the dock on dock AC power it's beyond horrible for noise. Your radios are worthless on dock power through those.
The dock power is also all running off of multiple GFCI breakers, and those GFI send coils are inducing tons of extra noise into the AC.
Where does a current limiter come into play? I have a lightbulb current limiter plugged into a Variac and the Variac is connected to the wall. If I shorted out the oscilloscope (connected directly to mains) by placing the ground clip on something with voltage wouldn’t the lightbulb limiter cut the current flowing to the device I’m working on? Thanks for any info
Maybe I'm just woefully unqualified to do my job, but isn't the entire point of an isolation transformer to, you know, isolate the ground??
for example, in marine electrical applications we often use isolation transformers in order to break the grounding connection between different boats in a marina and thus prevent a galvanic cell from forming. This also moves the source of power and quite critically the neutral ground connection onboard the boat as opposed to on the dock, which eliminates the potential for leakage of AC stray current into the water.
It would be my understanding that a transformer configured as you demonstrated couldn't be considered an isolation transformer at all, and it would be more accurate to describe it as a polarization transformer.
What am I missing?
I've been researching and asking everyone I can and I'm so frustrated. I'm a new tech and I want to study ac to dc circuitry. I want a 12 to 24 ac output so I can experiment safely. No one has answered my question clearly if I can buy a transformer that does this safely. It's so frustrating. I know expensive lab equipment exists to do what I'm looking for but I don't have 2000usd when living in Canada to experiment on rectification circuits to transform ac to dc. Any help would be appreciated so much.
I don't understand much of this since I only have an old multimeter... but if I had an oscillioscope, I would plug it into the mains which is grounded tied to neutral in the mains box. Then when working on 120/240 volt boards for repair, I would connect the repair board to an isolating transformer,,, meaning a modified one,,, open the box and cut the ground wire to output side. now my scope should be safe since there is no feed back to the grid,,,BUT that wouldn't make me safe, I would still be handling 120/240 volt active and neutral,,, so the way I read it, and IT only protects your scope, which of course is fine,,, but only if the ground wire is removed from the output,,, then there can be no feed back to the grid,,, I have no practical experience in this matter,, I am a learner and a theoritical person who analyses issues,,, please tell me if I am right.
It's disputable whether the output socket of an isolating transformer should have an earth pin. Some say yes, some no. I am inclined to prefer no, but not everyone agrees.
However, (In the UK anyway) there is a far worse gotcha to beware of, in that there are transformers sold for building site tools which used to be called 'isolating transforners' but which are actually autotransformers. I believe the practice of calling them this has now been banned, but you may still see some old hands in the building trade referring to them as isolating transformers.
If one of these finds its way onto the electronics testbench it's an accident waiting to happen.
On any unknown transformer, do a resistance check between the live and neutral output and all three input pins, especially the earth. If there is a low resistance to earth OR to the the input supply, it is NOT an isolating transformer, and could cause an accident on the bench.
I use a Variac to control the motor speed of a motorized impeller and it works well (zero speed to full speed).
Would it make any difference how the motor start/run capacitor is wired into the Variac? One side of the capacitor could be wired to the input hot lead on the Variac, or it could also be wired to the output tap on the Variac. Not sure if it would make any difference.
I am also thinking about using an isolation transformer in conjunction with the Variac. Thanks.
I am in Uk so 220-240v ac. I am just setting up my workbench. I have an isolation transformer and have modified it via a switch on the front panel so I can select the original wiring set up or my modified setup. The modified set up has no earth/ground to the casing, transformer or output socket ( which feeds my variac input). The Variac has an earth tag which is connected to ground on isolation transformer output socket, so on modified switch selection on isolation transformer it is not earthed.
Any comments would be appreciated good or bad.
... UNLESS... it is an isolating variac... which does exist. Sorry, I have to muddy the waters when I have the chance... which also connects the ground backwards through your wonderful neutral line at the breaker box if you have bog-standard wiring systems like most of us have... which muddies the waters even further... Hell, where I live I can't trust that the water pipe in my lab doesn't somehow connect through to neutral at some point because of someone deciding that my explaining electronics and ground to them suddenly made them an expert in all things electrical... (glances upstairs)... anyway. to filter the waters back to something drinkable, ALWAYS CHECK EVERYTHING before doing anything... belt and suspenders.
That's my mad-science tip of the day, and why I still have all my fingers.
If one wanted to check their house line with a scope, how would they connect the scope safely? Can the scope be connected to the wall on the 10X as long as it's also plugged into an isolation transformer? What about high voltages in old tube equipment?
Extremely new guy here with only basic electronic knowledge and about to buy his first scope and I don't want to toast it right off. Siglent SDS1104X-E. A lot of scope for a newbie, but I wanted something I wouldn't need to upgrade for a long time.
Very informative video, thank you for posting it. I have an old Variac that has an input and output, each has only two prongs of the same width, and no ground pin. The diagram you drew of your Variac shows the "hot" side connected to the Variac transformer windings and the "neutral" going straight through. Since both prongs of my Variac are the same width, is there a potential functional problem or safety hazard if the prong connected to the Variac windings is plugged into the
"neutral" of my wall outlet?
Ok Stupid Question Time....If the ground is meant to protect us from electrocution in an AC system, how would it make sense to essentially bond the neutral and the ground together? Why not simply have a separate grounding bus bar in the breaker box, (which has been present in boxes I've worked in; it's usually located in one of the lower corners of the box, and that's how I have always connected them up ) to allow complete isolation from the AC line voltage? Or am I just stupid? I say that because I always had it in my mind that since AC current is always changing polarity the ground insured that we couldn't become a conduit for a 'complete circuit. So It would seem to me that if the ground and the neutral are both tied to the same bus bar then we would defeat the whole purpose of a separate ground lug in our electrical sockets, and on our line cords.
Hey, 13:44 you have not explained why it’s dangerous, absolutely not recommended, to use a scope this way isolating the earth pin, ie it’s chassis, from the earth wire of your house.
I'm learning but confused. I thought the whole reason to use an isolation transformer was because the earth ground from the wall outlet DID NOT carry through to the ground outlet of the isolation transformer, or am I wrong?
what is the point of such a transformer that doesnt isolate the ground? because i thought isolation transformers lets you say float your function gen output from your scopes input so you are free you create your own gnd reference? at university lab this is what they told me..
+Adil Malim my isolating transformer has earth-free output and appears to have been made that way from the factory - why need an earth on an output with no earth reference? I think the only reason electricity distribution is earth-referenced is due to lightning and static protection as lightning is implicitly earth-referenced and an isolated grid struck by lightning could hit huge voltages w.r.t. ground and flash-over in all kinds of unfortunate places. Not so much of a problem on the bench.
+Adil Malim You want to (*Float* ) the device being tested *only*. All your other test equipment
should be directly plug in to the wall plug, that is grounded.
+MrMac5150 yes i understand that, im assuming you use this isolation transformer for the Device under test so doesnt it make sense for it to not be mains earth referenced?
+Glebs Litvjaks In my mind I would say Transient Spikes Yes, Surges No, Noise NO and Yes depending where
the noise is coming from, Noise on a AM radio caused by dirty insulators on the step down
transformer, NO, the electric company will need to wash off the connectors to get rid of that
kinda noise.
Glebs Litvjaks
Your welcome, we have all run into noise problems, it is just tracking them down, takes a little work.
Would it make sense to have safety capacitors (Class X, Class Y) anywhere in the isolation transformer path?
you should add a fuse to the wiper of the variac luckly mine opened at the neutral end so repairable ...shure made me feal silly or just stupid not to thenk of the current in the secondary to blow the pri. fuse on short and turned down variac....for a cheap isolation transformer, use two transformers back to back..i use industrial control transformers 120v to 480v connected to 480v to 120v. twice as big no nice box but i got em free.
Excellent explanation for safety.
Makes me wonder why you even need a third pin and 3-wire power cord in the US.
If the ground pin is connected to neutral in the breaker panel anyway, why not connect ground to neutral in the appliance itself?
Using a plug with a small and large pin to distinguish hot and neutral would suffice.
So after watching your video a few times, and as you expained so well, what's the purpose for an isolation transformer that doesn't actually isolate.
Then you show how you can use the 3 outlet adapter to achieve true isolation but then go on to say this isn't a good idea. I'm confused....
Everything said is true but the logic is wrong. You should never power your test equipment using your isolation transformer. Dut should be powered with the isolation. Nothing can protect you from misuse.
I have used that short cut with both isolation trans formers and vairacts. OK then what is the proper way to protect your self? Isolate the whole bench from ground? ie suspended ground in the fuse box or junction box?
I knew about the problem with the ground, nice to know the reason why to. Good explanation. That said your demonstration with the bulb and the probe hovering so closely to exposed mains with your bare hands, made me shiver and that in a video about safety........ :)
+Bart Zuidgeest
It made me shiver too.
+Bart Zuidgeest
LOL, When I watched the video I chuckled, my hands look way closer to those open clips than they actually were. Ahh....the magic of video. Thanks for your comment Bart!
“However” .. if you happen reverse energize when you’re testing, with your gear ungrounded, you created a “hot” chassis test gear …
Thank you! RWB
What if I just break off the earth pin on the oscilloscope plug ?wouldn’t that make my oscilloscope isolated ?
If the Isolation transformer had no ground on the output side there would be no issue with this transformer. You demonstrated that with the no ground plug adapter. Your transformer is not completely isolated with ground. If you look internally that transformer has ground from the input connected to the ground output or it is connected to the chassis, therefore not isolated. Some transformers are this way even though they may claim to be isolated. Pure isolation transformers usually have a 2 prong on the output and there is no connectivity to any of the the input and are completely isolated. Also, some variac's have isolation transformers built in and may claim to be isolated, are not completely isolated.
Ok, I get all that, but how would you scope your power outlet?
Firstly, in my entire life I never ever saw this kind of laboratory instruments😂. I think you stole it from space station. ✌️
The only way that light bulb lit up at 8:28 is if there is a direct connection on the secondary between the neutral and ground. Current path is from secondary "hot" to bulb to scope ground to service panel to secondary ground and then through secondary coil to "hot" to bulb.
It's worth noting that some "isolation" transformers have the neutral of the secondary bonded to mains earth. Tripp Lite is the company that comes to mind here...
You clearly state that you do NOT recommend disconnecting ground on your isolation transformer (and that older transformers were constructed with no ground connection). Can someone elaborate on the specific risks created in this way? I am constructing an isolation transformer using a 500VA core in a wooden box for working on guitar amps, and I don't plan to have any ground connection at all (your video reinforces my faith in the insulating properties of wood). No flames please, I'm here to learn :-).
A . 5 a fuse on your probe will open immediately. You can go " LA vida loco" and use a 10amp. Just saying there are ways to mitigate ill effects.
Hi if I understand your diagram correctly. By connecting my equipment under test in the isolation transformer. I'm safe. Because there is no low ohm connection between neutral or hot on secondary of the isolation transformer and mains. And your videos are great.
Thanks for the informative video.
Here's a thought for you:
You have an oscilloscope and a function generator plugged into a power strip and you routinely check your wave form by directly plugging one into the other with a bnc. we all do it, no problem...
Now you change out your fancy new generator for an older tube based one that has a non polarized plug, the transformer is center tapped to chassis ground.You are about to plug it into the power strip...
You''ve got one question to ask yourself punk. Are you feeling lucky? Well, are you?
The greatest piece of safety equipment in the shop is resting on top of your shoulders.
Be safe you all.
+Slim Hermer
LOL! Thanks for your comment.
I'm wondering if this is a new thing. Our service entrance panel is new and the neutral bus and ground bus are isolated and not electrically connected. I'm thinking they are in older homes but I'm pretty certain this is not the case for newer construction or any upgrade work. This of course is not a reason to not to pay very close attention to this issue.
It seems like an isolation transformer like this kind of sabotages it's own point? What's the reason for connecting the earth if the purpose is to float the DUT?