Hi Eric! I, too, have watched many of these dim-bulb/current limiting device how-to videos. Yours is definitely the most comprehensive. Most just show how to build one. Some explain why. Yours actually demonstrates how and why. Plus, your video is the only one I've seen that demonstrates the optimal order of isolation transformer, variable AC auto-transformer, dim-bulb/current limiting. As an aside, I was fortunate to pick up a pristine Heathkit IP-5220 Isolated Variable AC Power Supply. Internally, it uses the same order as you've shown. All I need to do is to plug in my dim-bulb into the IP-5220 and it does everything you've shown in your final configuration. Great video! Thanks!
And that's why you always get multiple sources of information folks. I've seen Mr Carlson's video about a build like that but he didn't go into detail about the order of the components. Thank you for your very detailed and easy to understand video I'm quite surprised that Mr Carlson didn't arrange it that way but there might be a reasoning behind it.
Nice video Eric. One thing I want to add about the isolation transformer you generally buy that I see most people skip over in their videos. I have 1000w hospital grade Tripp Lite (I believe the hospital grade ones already have the neutral isolation), but to have a true isolation transformer you need ground isolation as well. The input voltage line’s ground is connected to the metal case of the transformer. Even if you disconnect the ground wires on the output receptacles, the stock receptacles (as do most) have their ground lug connected directly to the mounting tabs. So when that receptacle is screwed securely to the metal case of the Tripp Lite it is indeed still grounded to the main input ground. So you would either need to get plastic isolation tabs and use plastic screw, or what I did was replace the stock receptacles with special ones where the ground lugs are not connected to the metal mounting tabs. These receptacles are generally orange in color. So I have complete isolations from hot, neutral, and ground.
Hi Dan, yes correct about the ground isolation and the special type of orange colored outlet. It might be a little hard to see in the video but the outlets I am using are that special type (orange color) with the isolated ground.
Oh very good, I’m sure you knew about it, but thought I should mention it for those watching the video and not know they don’t truly have ground isolation without the new outlets.
Thank you. I got my "dim bulb fix" for the night... :) And that's the 2nd biggest transformer I've ever seen! Thank you for the (always) informative video(s).
Glad to know you enjoyed the video. The upcoming power supply repair video that is currently being made has a transformer in it that tops this one for size :)
Best Dim Bulb Isolated Transformer Video I've Seen So Far - I do my config a little different; Isolated Transformer --> Dim Bulb --> Variac --> DUT. Love that you showed all configs (except mine) and how things are affected. Clean And Good Information
Thanks for the compliment! The order you have the components works well too and is often used. It functions basically like the VARIAC->dim bulb->transformer setup I showed.
This is by far the best and most comprehensive discussion I've seen on this subject. I'm going the rearrange the components of my dim bulb test system to match your recommended setup. Isolation transformer to autotransformer to bulb to device under test. thanks!
Very good explananation. I liked how you showed each configuration. I just wish I saw this 1week ago. As the incandesant bulb ban just went into effect.
Thanks for the nice feedback. The phasing out of most incandescent bulbs has been in effect for quite a while. Personally I stocked up on bulbs for uses like this project but the bulbs are still available online if not at your local stores.
Very interesting video on this subject , you have explained it extremely well. Being a electronicien myself this is something I had learned when at CEGEP (school), and I used it all the time. Thanks
Hi Eric, interesting and informative video; thanks. I see the logic in arranging the items as you do on your bench and indeed i used to have mine configured in the same order but, quite frankly, I couldn't put up with the noise. Since moving house, I now have the space to move the 'lab', such that it is, from a very drafty, cold garage to a nice warm ground floor room in the house. Whilst setting up, I decided I needed to do something about the 'growling' noise emanating from my toroidal iso-transformer: it really is quite offensive and not the sort of thing one would want next to their ear for any extended period!! I it put the first in the chain originally since it is a 2kVA unit and my mechanical engineer's logic suggested it would be better to put the 50VA variable behind it. Anyway, long story short, when I started the new setup I reversed the positions and had the variac first in chain. Lo & behold, the horrendous 'growling' ceased. It does return to a degree when I give it full mains voltage but below about 90-95% full power it is almost unnoticeable now, though I do not have the bulbs in circuit yet. They have always been last in the chain. Any ideas as to what the cause may be? I surfed the 'net' for hours trying to find clues as to what aggitates the toroid enough to make it angry, though without noticeable success. It does seem voltage dependent but I am at a loss with this one. Any advice or suggestions would be much appreciated as I do not like being 'growled' at by 240 of His Majesty's finest volts. Keep up the good work. 👨🎓
Hi, my guess is the impedance or matching between the transformers is not ideal and may be setting up some distortion on the AC waveform making the transformer growl or hum. And some transformers are just more prone to making noises. Even at no load, there is some current between your isolation transformer and variac. A properly sized/rated capacitor across the AC line between transformers (power factor correction) could possibly help correct this but the noise is not damaging to the transformer unless it is so extreme the transformer is running hot (could be indication of other problems too).
@@EriksElectronicsWorkbench Thanks Eric, I'll get the maths hat on and give it some consideration. I guess all the loading it sees at the moment is inductive so there could be something in it. The toroid is double-insulated as it came from an out-door-functions company and has no earth reference at all. Having the variac first also means I can have the earth ground on its metal case connected.
@@grahambambrook313 It seems placing the bulb for current limiting last is the best option (as I show in the video) and the order placement of the transformer and variac is about a toss up with some advantages on each way. Arrange how it works best for you and the components you are using. Yes one transformer driving another is a very inductive load to the first transformer (or variac) although in my own set up I have not had any issues.
Thank you for the video, I found it most educational. I liked seeing the four different configurations. What size isolation transformer are you using? Do have a recommendation for as to a minimum and maximum size to look for? Thank you.
Thanks for watching! I have two isolation transformers on the bench one is 1000 VA and the other is 350 VA (about 1000 watt and 350 watt). The 1000VA transformer is connected to my VARIAC which is rated to 750 VA and also to the current limiting bulb, or I can use it direct without the VARIAC and bulb. For the vast majority of projects these two transformers work fine. Depending on what you work on would dictate the transformer capacity but for most projects I find the 1000 VA is more than enough and I'd recommend that size. The 350VA is fine for small radios and such but not for large power supplies and power amplifiers.
I would like to add some protection. The lamp switch should be a 2A circuit breaker. Because, in case the switch is on and something bad happens, it is good to have an automatic circuit breaker. 2A is sufficient for many projects. I would also add an RCCB on the primary of the transformer (I choose your last configuration). If something fails in the transformer, I hope not! This RCCB would act before the home RCCB. Makes sense? I would also add a wattmeter, a PF meter, and a needle ammeter at the end, just to take note of the consumption of the radio or amplifier. For me, the needle is the best indicator for debugging these old junk. Nice video, risk is on me, but thanks for sharing.
Adding a fuse or circuit breaker is fine but make sure it is sized no larger than what the variac or transformer can handle. In other words the circuit protection needs to be the weakest link in the circuit.
Putting the iso transformer first is best. In this configuration the variac does not have to supply excitation current to the iso transformer which depending on the size of the iso transformer can be mA to Amps.
Thanks for this great video. Definitely one of the best out there. Is there anything wrong with using 2 isolation transformers? One for the dut and the other for an oscilloscope?
Glad you found the video helpful and thanks for watching! Using an isolation transformer to float the oscilloscope is not a good idea for safety reasons. If the ground clip on the scope's probe is connected to an elevated voltage a shock hazard exists from any part of the scope's metal components (case, front panel, etc) to other parts of the circuit being tested and if you come between those points you will get a shock. The best way to measure circuits that a traditional scope can't measure because of grounding issues are using an isolated probe/differential probe or an isolated scope like the Fluke Scopemeter.
great video.. I like your variac, mine shows voltage not amps. this is good.. glad to know my setup ( the 4th one) is good.. and I need to check inside the ISO and fix that !
Hi Eric, i have seen many video's the last 3 years about this subject, because i'm building for almost 3 years (due health issues) a project with a isolation transformer , dim bulb limiter, variac and electronic circuit breaker (own design) in 1 chassis, but this video is one of the best and comprehensive explanation on the different configurations that i have seen so far. From the beginning i had already the last configuration in mind to use in my own project, but now i have seen what effect of the other configurations are. Regarding the isolation transformer itself, did you do some modifications to lower the "ghostvoltage" from the secundary side to ground ? Grtz from Belgium
Bjorn, thanks for the compliment and I'm glad the video is helpful for you. The voltage that can appear from isolated secondary to ground is why I say the isolation transformer greatly reduces the shock hazard.. but it does not 100% eliminate it in all cases. The ghost voltage will disappear if you use a lower impedance AC voltmeter to test it (such as 1000 ohms per volt) so generally the current level of the ghost voltage is extremely small unless the transformer has a serious problem.
I'm in the same boat. I even have the measurements for the drawn but I'm just now testing rotary encoders for the nema 23 which will 1 to 1 turn the wiper.
Nice video, the best out there! There's an exact calculation to know what wattage I should choose for each load under test? For example if I have a 70w load under test, and pick a 70w light bulb even if the unit its ok the light bulb will lit at full brightness , how bigger the bulb wattage should be to give the right visual effect that the DUT is working fine?
Thank you, glad the video was helpful! One way to choose a bulb wattage is to consider what is the maximum short circuit current your device can tolerate. For example 150 watt 120 volt bulb will allow 1.25 amps (I=P/V). You will always drop some voltage on the bulb but the point is to limit current initially and then go to full line voltage when you are sure it is safe. 100-150 watt bulb is typically what I find works well for most devices being tested.
@@EriksElectronicsWorkbench Thanks for answering, but to clear my head 💡, the idea behind the use of the incandescent bulb is to prevent the unit(if its shorted) to cause more damage to this device, and also give us the visual sign that the device is shorted(full brightness if its shorted, and low brightness if the device is working fine), if you use a 150watt bulb to test a 150watt device, in this case assuming the device under test is working fine, the bulb will lit at full brightness anyway (both will work with the same current), in this case the bulb will not give us a false sign? Sorry if I'm not being clear enough, English its not my native language. Thank you again bro!
@@Skillz-s2n if the bulb (in series with the device you are testing) and the device itself both have the same watt rating you will get half the line voltage on the bulb and half the voltage on the device. So the bulb would not be at full brilliance. If the bulb is a wattage much smaller than the wattage consumed by the test device then the bulb will glow near full brilliance and you will drop most of the line voltage on the bulb even though there is no fault, not exactly what you want. I leave a 150 watt bulb in my tester for most uses. Sometimes I reduce down to 100 watt or increase to 300 watt. I almost never go to 60 watt or less. I've never needed to go over 300 watt.
Great video. Im buiding one now using a small variac and a 1 to 1 pyramid transformer. I havent decided how to wire it yet but im going to use a motor to turn the variac which has the same 3 amp ouput as the transformer. I also have an old hammond boat anchor 1kva which is earth connected on both sides but i havent found a use for it. I have alot of recording gear and i may use it when i record to avoid hum when tracking guitar.
Hi thanks again for the video it’s very helpful I am going to start repairing av amplifiers and tube amplifiers and before I even start to repair amplifiers I want to get a setup like you have and be safe in doing so can you recommend All the equipment I would need to be able to put together on a bench permanently mounted like all cables and thicknesses and ground wires and the type of bulb holder and switch onto the bulb holder and a Schematic of the equipment on how it should be put together then I can safely work on the amplifiers thank you Mark
I'm glad the video was helpful. My bench uses 14 awg stranded copper for 15 amp general purpose outlets (they are fused at 15 amps as well) and the isolation outlets/VARIAC/transformer have some 16 awg wiring because they are fused at a much lower value than 15 amps. There are wiring charts online you can look up conductor size and current rating to use what is appropriate for your setup. Whatever an outlet's wiring gauge is for hot and neutral then the ground is also the same gauge. There is no ground on the isolation outlets. Heavier gauge wiring is used on some other specialized bench power circuits rated over 15 amps. The dim bulb bypass switch is rated 15 amps up to 240 volts AC. Don't use an automotive grade switch. The lamp holder is a porcelain socket made to mount on a flat surface. Schematics of the isolation transformer, VARIAC and bulb as I have them set up are in the video.
@@EriksElectronicsWorkbench thank you for getting back to me I will have a look at the schematics and source all parts needed and build one I will send photos to you and just check to make sure I have got all equipment in the right orientation and thanks again mark if in the future you need help with anything just let me know thanks mark
Can you explain how to reference a ground on an oscillscope since the bnc ring is connected to house ground itself. Thanks very much! I will be trouble shooting a NAtional NC183D
In situations where connecting the oscilloscope ground into the circuit poses a shock and/or short circuit hazard you need to use a differential oscilloscope probe or a battery powered oscilloscope that has floating ground connections. Never attempt to float the grounds on a traditional AC line powered scope by isolating it as this is dangerous and poses a shock hazard.
Great Video Thanks! I’m in the process of changing all the old electrolytic’s in my HI-FI pre and power tube amps ( previously working fine) and am a bit anxious about switch on. What still concerns me is if I have a setup like this and say have a capacitor that decides to go faulty, is it still going to go bang all of a sudden even if I bring the voltage up slowly in stages-as there probably is still enough current flow? Also my amp has voltage regulation with 85V reference tubes so I assume the fun might not begin until I have the Variac at 85V and above? Many thanks for any knowlegeable answers 👍 Martin UK.
Glad you found the video helpful. The current limiting from the bulb will not prevent all types of faulty components from going up in smoke. It is most useful for faults in main power supply components or other power handling circuits in a device like an amp's output stage. The bulb is a visual indication of a problem so you can quickly cut the power if needed as well as a current limiter to prevent more serious damage from occurring like a power transformer being destroyed by a shorted filter capacitor. It depends on the circuit design if the regulated voltage circuit will have voltage present at low line voltages or if it suddenly switches in at a particular line voltage. You can monitor that regulated portion with a voltmeter to understand how it works. Double and triple check your work before power up as the dim bulb won't prevent damage to electrolytic installed backwards.
Hi Erik. I've found a problem. When switching this large isolation transformer, I've noticed that sometimes the house circuit breaker trips. It seems that the inrush current plays an important role, especially when the power is turned on at a point other than when the mains is crossing 0V. I've been thinking about connecting an NTC in series and creating some kind of controller to power up the entire system. The controller should manage the NTC's temperature, include a bypass relay after a few seconds, control a contactor for switching on and off based on conditions, a crossing 0V detector, (arduino project)etc... Perhaps there is something already available in the market.
Yes if the transformer is switched on at the peak of the AC cycle you will have a very large inrush current and large transformers can have considerable inrush currents. You might try using a solid-state relay (SSR) that has zero crossing switching built in. Make sure to size the relay for the power draw of your transformer. I think the zero crossing SSR is the simplest solution.
Hi, using the isolation transformer before the variac makes the variac isolated as well as the device you are working on. Any additional isolation transformer would not serve a useful purpose unless you had some unusual voltage conversion requirements (step up / down) beyond what the variac could do.
Excellent explanation, thank you! I have been trying to find a definitive answer on this issue for a long time. I had arrived at the third set up myself, partly because I wanted to ground my variac, so it had to go before the isolation transformer, but I now believe your fourth set up is better. BTW, wouldn't the 'bulb always on' problem also apply to the first set up when using a low wattage bulb?
Glad that you found the video helpful. Yes in the first configuration a small wattage bulb would glow once the variac is increased because of the small current that flows in the transformers even without a load on the isolation transformer secondary.
Excellent video ! I'm getting ready to build my setup, just like you did yours. I was thinking of adding a receptacle between the variac and isolation transformer with full line voltage so as not to have to use the variac all the time? I want to add an Amp and Voltmeter after the variac with the bypass switch as you did. My transformer is a beast ! Big Hammond 115v primary 782VA and 115VCT Single phase. I was wondering if I could just bond the ground and neutral together on the primary side, but not ground the chassis ? The primary side has three terminals. H1, H2 & H4. H1 & H4 = 125V from my 125V main. Which is how I'm going to wire it. H4 & H2 = 125V ? H1 & H4 = 242V on the primary side ? Secondary is 125V X1 & X3, which is how I am going to wire it, center tap equals 62.5V each side out of phase. It's an older model. A single phase. I could not find a schematic on it anywhere ! I tested it and everything works fine. I was confused at the 242V on the primary side center terminal? And what would be the effects of running it at 125V line as opposed to the 115V rating? Would ot do or possibly harm the trany ? Just a little more heat ? I don't plan on running anything that long that would saturate the filaments. I'm planning on putting a 6A fuse between the line and the transformer with a 5A fuse between the variac and transformer. Any advice or information you could tell me, much appreciated, thanks again for a great video !
Glad you enjoyed the video and found it helpful. An extra receptacle between the isolation transformer and variac can be handy, nothing wrong with doing that. It won't have the bulb current limiting though unless you add another series connected bulb with that outlet (assuming you are following my setup). As to grounding, if this whole project is in a metal box you should use a 3 wire cord and ground the box to earth ground but do not tie the box to neutral. Then use "isolated ground" outlets (which are orange colored) so the outlet's ground socket is floating from the earth ground. Without seeing the transformer in person it is a bit hard to say how the winding taps are configured. But it sound like you have sorted them to the correct voltages. I would not worry about the 125 vs. 115 volt operation, that is within normal specs of line voltage. If the VA rating is given at 115 volts you could back off the maximum load a little running at 125 volts to keep things safe. Short duration use really won't pose a problem. Just remember on the grounding issue that the isolated line side can't have any reference to the outside world.
Sounds like your setup will be practical and work well. I have a second isolation transformer setup with no series bulb and no variac. It is handy for times when you only need isolation and nothing else. @@ekbanjosworld4926
Great video. I've always had my Isolation Transformer, Variac, and Dim Bulb in the 2nd configuration you've shown and I guess since I'm using a 300w bulb, I haven't run into the issue it potentially could have. However, you've convinced me to try out the 4th config and will soon when I clear out the work I have going on. One configuration you didn't touch on is: Isolation Transformer > Dim Bulb > Variac. Any thoughts on that config? Thanks for the tips!
Glad you liked the video, thanks for watching! Isolation Transformer > Dim Bulb > Variac would function like the 2nd set up, just switching the transformer and Variac positions. Advantage would be the bulb, associated wiring, and Variac are isolated which adds safety. It should work ok but might have a some bulb glow even with no load on the Variac due to the current that always flows in the transformers (as mentioned in the video).
@@EriksElectronicsWorkbench would love a video of you adding a digital meter with a torrid to the dim bulb I’m having a terrible time adding mine. It works but then the limiting function goes away and the mains won’t turn off with the mains switch.
@@johnc.4625 I will consider that as a future topic. From your description, if the current limiting is not functioning the bulb is being bypassed somehow. A voltmeter should only need to be connected across the load (would be the most useful location). Don't try to measure across the bulb or across the mains switch.
Using an isolation transformer is always a good idea. Generally bringing up voltage slowly and running solid state devices on low voltage is not recommend because the circuits behave differently than tube circuits under low voltage conditions. However there are times bringing up the voltage with a variac is useful during testing on solid state circuits so it's one of those use as needed situations. You can use the dim bulb portion (and a suitable rated bulb) for an initial power on just to see if there is something drastically wrong. I would not leave the dim bulb in during testing etc as the lower voltage will make readings and operation unpredictable.
Your setup do not have a Residual Current Device (RCD) after the isolation side to mitigate potentially serious electrical shock to an operator. This can happen when testing DUT, the operator connects one of the terminals after the isolation to Ground (for some reasons) and then accidentally touch the other terminal. The most probable situation is that you connect GND lead of an oscilloscope to the DUT; and even if you connect light bulb load in series with the DUT, you may still get a serious shock if you accidentally touch any point that is live in the DUT. Perhaps adding MCB after the isolation is a good idea to provide extra protection in case of short circuit or an overload especially when the light bulb load is accidentally removed or intentionally bypassed from the test circuit.
The addition of any extra safety devices is up to the end user and details are beyond the scope of the video. However installing a GFI outlet would pose an issue since the GFI outlet needs an earth ground reference. The purpose of the isolated supply is to have no reference on either line to ground. The current limiting provided by the bulb is in no way meant to be a safety feature to the user. There are inherent risks with this test setup, thus the disclaimer in the video.
@@EriksElectronicsWorkbench Fair enough. I brought this up because most modern home electrical systems have protection devices in place to protect home users and electrical appliances from electrical hazards. So there are protections before the isolation, but after the isolation those safety features are eliminated. Regarding the GFCI, I think you are referring to the voltage operated ELCB, which is now obsolete because it does not provide much protection to the operator. And yes, that device senses the voltage in reference to the Earth. But most modern device is an RCD whereby it detects the imbalance between the current flowing in the live line and the current returning in the neutral line and there is no Earth line involved. Actually adding some electrical protections is pretty simple. Just hook up a small 6A or a 10A RCBO (which is basically MCB + RCD) just after the isolation transformer. It may also protect the transformers from smoking due to inadvertent short circuit or an overload (in situation whereby the light bulb load is accidentally removed or intentionally bypassed).
Thank you for this video! As a noob hobbyist, my primary concern is a) not killing myself and b) not blowing up my oscilloscope. So this is very helpful. One thing I did learn, and correct me if I am wrong, is that using a powered differential probe is safer than using this setup because you can then still use the ground for the Oscilloscope, while isolating the probe connections from the scope, which in theory should allow you to probe a dead short on the test device without electrocuting yourself or blowing up your scope, right? Or is that wrong?
Glad you found the video helpful. The isolation transformer's main purpose is for safety from the AC line which has a ground reference. Although the transformer can allow you to connect a grounded oscilloscope, that will essentially undo the isolation transformer safety by introducing a new path to ground on the transformer secondary. A dangerous shock hazard now exists. Differential probes are a good solution or using an isolated scope such as a Fluke Scopemeter will keep you and the equipment safe.
ISI DIM BULB TESTER VIDEO…I use the 4th and final version of the setup… I now own an oscilloscope and learning about ISO power. Question…does adding a GFCI or AFCI “after” the setup, add any safety? I can’t find any video demonstrating why the GFCI won’t work. Testers give false reading because most of us lift ground. The tester can’t send current to ground…so it won’t test correctly. What am I missing…or is the GFCI a redundant safety feature not needed? THANKS
A GFCI monitors the current flowing in the hot and neutral circuits. If there is the tiniest of an imbalance in current, meaning current is taking an alternate path to ground, the GFCI will trip. An isolation transformer's purpose is to isolate from ground so a GFCI won't work in this application because the transformer secondary has no reference to ground. You could install the GFCI on the isolation transformer's primary side (i.e. the outlet on your workbench).
When the bulb is shorted it is just an isolation transformer and variac, so use with caution because there is no current limiting. It is important to be able to bypass (short) the bulb so you can apply full line voltage (when appropriate) to the device when testing.
If you want to limit the inrush current surge a single component can do this, called the NTC Thermistor, you can find them here www.mouser.com/c/circuit-protection/thermistors/ntc-thermistors/ you have to choose the proper resistance and wattage rating among other things. Keep in mind some motors cannot be current limited as they will not start up correctly and will overheat.
For the 3rd configuration (variac-transformer-bulb). If it is CT transformer on secondary ... than i should use 2 bulb, one for positif and one for negatif, am i right?
It depends on how you are using the transformer. If you have individual loads on each side of the center tap and they each need different current limiting then yes a bulb on each secondary winding opposite the center tap would be useful. If the secondary windings are connected in series and a single load then just one bulb in series with the load. If the secondary windings are connected in parallel and one load then also just one bulb in series with the load. And last, you could put the bulb in series with the secondary center tap for common current limiting to either winding.
another question. Which is the best illumination? I always have filament bulb at 230VAC, but perhaps now are other options. The noise of the new LED bulbs is crazy...
You can't use an LED lamp. It must be an incandescent bulb. Normally the bulb voltage is the same as the maximum line voltage. You are not choosing the bulb's wattage for how brilliant you want the bulb but rather based on how much current you expect the load to draw and how much current limiting you want. A low wattage bulb like 25 watts often limits too much current, and drops too much voltage on the bulb leaving little voltage for the load. A very large wattage like 500 watts won't offer much current limiting in most situations so you have to choose a bulb wattage appropriate for the load. If the bulb is glowing brightly you have too low of wattage for the intended load, or if the bulb wattage has been selected correctly, the load has a fault which is what we are protecting against.
@@EriksElectronicsWorkbench Sorry, it was my fault. I changed the subject and the result to be more in line with the topic we were discussing. Of course, I know it should be a filament lamp as a series current limiter. I was referring to the illumination on your workbench and how people light up the area. LED lights generate a lot of spurious frequencies. In the past, I used a 220V 60W lamp, and that was it
@@soulrobotics my work bench and the surrounding work area is illuminated by fluorescent lights. Some traditional ballast and some electronic ballast. Electronic ballast are electrically noisy. I also have an incandescent bulb overhead for times when I need light but must have the work area as electrically quiet as possible. Some videos are filmed with extra LED lights only to help with the camera exposure. LED lights generally are extremely electrically noisy.
All of my workbench instruments are plugged into a normal grounded outlet. It is bad practice and dangerous to float or isolate test equipment. If you need to use an oscilloscope and isolate from ground the safest way is to use something like a Fluke Scopemeter or differential scope probe. Most bench voltmeters and DMM's have floating inputs and are not a problem when working on isolation powered equipment.
Someone please help me out here. I totally understand the dim bulb and the variac. They make total sense to me. Please help me understand the isolation transformer though. Now, I totally understand step-up, and step-down transformers, and so 1-to-1 also makes sense. I understand that the secondary side has to be COMPLETELY ISOLATED from the primary side; no connections between neutral, ground, etc. What I DO NOT understand, and what I can’t seem to wrap my head around is how this isolation makes me, my devices being serviced, or my tools (oscilloscopes for example) ANY SAFER… 120 volts is 120 volts, right? The secondary has 120 volts on it too, just like the primary, and if I grab one leg of that 120v with one hand, and the other leg of that 120v with the other hand, I AM going to shoot 120v through my heart, whether I am on the primary side, or the secondary side of the transformer. The secondary side isn’t magically turned into human-resistant voltage. It’s STILL DEADLY, so SOMEONE PLEASE explain to me how the isolation does any good. Ok, so I’m no longer connected to the breaker box neutral and earth ground (why can’t a 3-prong adapter/ground lift adapter plug do the same thing?), but I’m STILL dealing with that 120v on the secondary, so how am I any safer? This has boggled my mind for a while, and I know it shouldn’t…. but… it does… Thanks in advance for any insight…
The isolation transformer does not protect the user in any way if you come in contact across the secondary. There is always that risk of shock. 120 volts from the isolation transformer is just as dangerous as from the wall outlet. But without the isolation transformer just one hand bumping a live point could inflict a lethal shock so the risk is at least reduced some with the isolation transformer. The isolation transformer can also make testing with a traditional grounded oscilloscope possible in certain situations although a full understanding of the circuit is needed and a shock hazard can still exist from exposed metal parts of the oscilloscope. A ground lift adapter won't isolate the item you are working on as the transformer is able to do. Keep in mind that vacuum tube equipment and devices like high powered audio amplifiers have high voltage DC power supplies in them so the AC line (isolated) is not the only shock hazard. It is best to follow the one hand rule and only probe or work with one hand near the live circuit as to minimize a shock going from one hand through your body and to the other hand. As always be safe and fully understand the safety precautions before working on a device.
@@EriksElectronicsWorkbench I respectfully disagree. My understanding is that with no external reference the 120/240v exists across the load if you care to measure it there but there is no real live/hot/phase/whatever you want to call it, and no real neutral/common/wywtci. It's all arbitrary voltages that depend on where you measure it. Provided the device is properly isolated, if you should touch the notional "hot", you become the reference. You are grounded and the point that you touch becomes ground in the circuit. That's why it's so important not to have any other reference on the device. If you have, for instance, a grounded scope lead attached that will be a much better, as in lower impedence that you are and you will get a shock if you touch anywhere other than at that grounded scope. You can prove this safely. Imagine a 12v transformer with open output windings feeding a high resistance wire wound resistor. You could touch any part of the circuit with a voltmeter probe and the other probe to earth would give zero volts because the 12v has no other reference to ground. Now hard short that same point to ground and use the meter to measure any other point on the circuit to ground. You will get a voltage that varies between zero and 12v, depending on exactly where you put the test probe. The same thing happens with a mains isolating transformer. If there's no other earth reference you can touch any single point of the circuit.
@@electrogrim I think you misunderstood when I said "across the secondary". I mean across as in from one end of the transformer's secondary winding to the other end, you would be shocked. Voltage is always a two point reference. If you contact only one end of the secondary winding and everything is perfectly isolated then in theory you would not receive a shock.
@@EriksElectronicsWorkbench Would the amp's power transformer typically have grounded connections? I assume that would defeat any protection that the isolation transformer provides when working on components fed from the secondary of the power transformer.
@@tbonemckone If the device has for example a chassis grounded center tap on the secondary of it's own transformer that would not matter as far as the protection from the isolation transformer. The primary winding side would not normally have a connection to the chassis.
No you can't use a microwave oven transformer because that type of transformer is a step up turns ratio. They produce a very high voltage on the secondary (vary dangerous too!). An isolation transformer is a 1:1 ratio so your line voltage on the primary is the same as the secondary voltage.
Logically it's obvious, but "for science" it would have been informative if, during the setup with the lamp in the middle, you had shown the voltage level on the Input of the isolating transformer also.
There's a lot of old stock still out there. I have several set aside for this test set up on my bench. They will last nearly forever since they only glow dimly.
There are plenty still available, as specialty incandescents are still being made. I just bought a pair of 60-W yellow "bug bulbs" at my local Big Box store. These, plus rugged bulbs for refrigerators, etc, will be available for the duration.
I just bought a pack of 4 .... 100 watt incandescent bulbs off of ebay for 8 bux ...... There's a ton of bulbs out there , buffet tables use the food safe ones that have unbreakable glass and the kitchens use them also.... Reptile owners need the old style bulbs for their pet iguanas and others ......
I have a nice iso transformer but no variac. I have been looking at a vac SC-10T from ebay that is a isolated variac but have not bought it because it seems everybody has seperate devices. I would like to know if the advatages and disadvantages of using a seperate variac and transformer vs a "all in one". I have a small bench so that would be one advantage for the SC-10T.
The SC-10T that I found on eBay does indicate on its schematic that it is both an isolation transformer and variac. (but for safety you should verify the isolation with an ohm meter if you buy it) If you connect an incandescent bulb in series on the output side you will essentially have the same arrangement as I use: isolation transformer->variac->bulb. From the provided advertising the all in one SC-10T should function the same as having separate components and a bit more space saving.
so I assume your secondary stage of the iso transformer has no ground reference , therefore the variac will have no ground reference and neither will the dim bulb and device under test. so besides placing two ground probes on the dut which can cause voltage potential if not in the same place, are their any other vunerabiltys , can yo be shocked by touching the metal casing of the Variac as it is not grounded or something of that sort. thank you.
Yes correct that the transformer secondary does not have a ground reference which is what the "isolation" is referring to. Since the transformer's secondary does not reference either side to ground, if a fault occurs, for example, in the variac making some part of it live to one of the isolated AC lines you would not be shocked because no return path exists from the user through ground and back to the transformer's secondary. This is also why you should not use a ground referenced oscilloscope on the isolation transformer's secondary (on DUT). If you introduce a ground path on any part of the transformer's secondary then yes a shock hazard will exist.
Could you please clarify what you mean about the ground referenced oscilloscope? Are you suggesting having the oscilloscope connected to an isolated supply and not a grounded mains?
@@msouthwe1 AC mains powered oscilloscopes have the probe ground tied to chassis ground. You should not float an AC mains powered oscilloscope on an isolation transformer as there can still be a shock hazard if you contact a metal part of the scope and part of the circuit you are testing. In situations where the scope ground can't be tied to circuit ground potential the only safe way is to use an isolated ground scope like a Fluke Scopemeter or a differential scope probe.
Great video. One question about your Tripp Lite. Wouldn't what you were talking about with that unit also be a problem with its third prong ground side also. I guess if you are only plugging in a two prong vintage item that is ok. And most three prong items have that third prong isolated from the neutral parts of a device - it's in case a device energizes (shorts to its case etc.) itself etc. I bring it up because I also just watched a video about a modern variac with a built in isolation BUT third prong ground was NOT. This would concern me about total isolation. ruclips.net/video/8o7HtGTvBvA/видео.htmlsi=aqySuOViPgvYFf94 Edited 10 minutes later. He does explain that the company recommends using a cheater plug when plugging in a three prong cord to maintain needed isolation if one wants that on this unit. I guess one needs to be very careful about reading words on any product with verification of its meaning in practice!
Glad you liked the video. The Tripp Lite transformer does have a 3-wire plug but the ground is ok as that earth ground goes to the metal case the transformer is in and is a good safety feature. On the secondary side nothing is connected to earth ground (and modification to neutral was required as shown in video) so both AC lines are floating with respect to earth ground. No connection is made to the ground on the transformer outlets. Unless the VARIAC is a special isolated type it does not offer isolation and lifting the ground reference on a VARIAC will not create isolation protection. It has to be used with an isolation transformer. Always remember the AC neutral at the wall outlet is earth ground connected.
@@EriksElectronicsWorkbench The one referenced in the video apparently had its own built in isolation transformer. He shows all that in the first three minutes. I was thinking of the schematics you showed at 13:00 with third prong. I took that to mean they were connected through the metal box.
@@wdmm94 that schematic was showing how the "isolation" transformer was factory built but I modified it. No earth ground is brought to the isolated outlets on the bench. If the VARIAC has an isolation transformer built in you can ground it's metal case for safety since it is line powered but the outlets (secondary side) ground should not be used. You can test with an ohm meter to see if there is any connection from secondary neutral to ground if there is a safety concern. Check out my "ghost voltage" tech talk video for a little more on grounding and isolation issues. ruclips.net/video/WONMMZsLHDE/видео.html
ERIKS, Why is the variac meter measuring "In rush current spikes", is the variac transformer or isolation transformer causing the In rush current spike? Transformers shouldn't cause in rush current spikes. The Test Set up Config#2 with the Current Limiting Light Bulb in the middle will light up at very low line voltage which means that the isolation transformers primary has "leakage current"?
Wayne, transformers will have an inrush current when they are switched on at the peak of the AC sine wave. The 1st configuration has the high current spikes at low settings of the VARIAC because of the arrangement of the components and how they interact. All transformers have some current flow in the primary with no load/no current on the secondary. This is normal. The no load current flow is *not* from leakage current.. i.e. breakdown of the insulation. Technically the no load current between two transformers can be brought to near zero with a capacitor in parallel between them but this is really not needed on the size of transformers and currents we are dealing with here.
@@EriksElectronicsWorkbench When does a transformers have current flow in the primary with NO load on the secondary? The primary insulation is breaking down means there is "leakage current" flowing from the Top of the primary coil to the bottom of the primary coil. The Neutral wire on the bottom of the primary coil should be zero volts/zero amps. If it measures microamps uA or mA that is leakage current from the transformers eddy currents or transformers insulation. Try making a video lesson on how to test various transformers 1.) leakage current, .) in rush current, 3.) the transformers "isolation tests" how to test a transformers isolation
@@waynegram8907 An ideal transformer would have no internal loss and would not draw current if there is no secondary load but such is not the case in actual use. Transformers draw a small amount of primary current with no secondary load in order to establish the flux in the transformer core. Power up a transformer and leave it with no load on the secondary and you will find it becomes warm to the touch after a while which shows there is some small loss in the core. Yes, I will consider making a video on transformers, testing and so on.
@@EriksElectronicsWorkbench Yes the small loss in the core is Eddy Currents which are from the magnetic field flux magnetizing the core material which the Eddy currents creates heat. When testing various transformers Losses in the video lesson would help out. When applying a Squarewaveform 20hz to 20khz to an audio transformer you will see ringing and harmonics because of the transformers wafers laminations materials and core wire insulation material used which will vibrate and resonance causing the ringing, hysteresis, harmonics to go over in the video lesson.
I do not understand why the current meter of the variac is not shown in any of those schematics. It would have been nice to see whether it was in the primary or secondary circuit of that variac because it makes some difference in certain situations, I suppose. (And same thing with fuses. I suppose it makes some difference if the fuse is in primary circuit or secondary of the variac.)
The current meter is on the variac output (i.e. the movable tap), the fuse is on the line cord into the variac. The variac does not have a primary and secondary such as the terms are used with transformers as it is all one winding with no isolation.
@@EriksElectronicsWorkbench Yes I know that primary/secondary thing. But if you short the output, you can burn the wiper or that "secondary" part of the winding at quite low output voltages even if that input fuse does not blow yet, can't you, if you have no fused output.
@@jutukka I have several variacs in finished enclosures with fuses, outlets, etc built by different companies. Some have a fuse only on the ac line in and some have a fuse only on the output tap. None have fuses on both the input and output however I think for complete protection both fuses would be a good idea. With the current limiting bulb in my video there is no danger of excessive current flow but I suppose without the bulb you could damage the variac in certain situations with a short circuit and inadequate fuses.
@@EriksElectronicsWorkbench I have only one Variac and it is cheapest Chinese 230VAC/1000VA/3.3A/0-300VAC I could find. It has a traditional 5A 6x30mm glass fuse in input, and a 5A circuit breaker in output, and in addition a 100°C thermal fuse. LCD voltmeter. It cost less than €80, postage included. 😁 Works fine but an amp meter would have made it perfect. P.S. If you buy a cheap chinese stuff, be prepared that the smell of rubber and plastic parts and paint can be quite, well, amazing. For example those black rubber feet of my variac smelled so bad that if an insect flew near them, it probably would drop dead. I had to remove them. And when I put those rubber feet into a air-tight Minigrip bag, the smell came out through the plastic bag! 😭. Well, those rubber feet were easy to remove throw away but the top and bottom rubber insulator plates of the bolted toroid are smelly as well. 😝 And make sure that safety ground lugs really are in galvanic contact with the chassis metal or screw, not only thick layer of paint, as was happening in my variac.
Having the variac feeding the IT will cause problems . At low variac voltages the primary of the IT will struggle to generate secondary voltage due to back EMF. Could you do a vid on the difference between a tech IT and a medical IT.
The isolation transformer will work fine at low voltages but the slight mismatch between the variac and isolation transformer means you might have to set the variac output a little higher to compensate for the loss. But that setup would still allow setting any voltage out of the isolation transformer from zero upwards. I don't have a medical grade isolation transformer but typically medical grade means it has strict safety measures for ground faults, i.e a fault that could harm a person since the transformer could power a device directly attached to a person.
Best video on YT for a dim bulb tester hands down.
thank you!!
Hi Eric! I, too, have watched many of these dim-bulb/current limiting device how-to videos. Yours is definitely the most comprehensive. Most just show how to build one. Some explain why. Yours actually demonstrates how and why. Plus, your video is the only one I've seen that demonstrates the optimal order of isolation transformer, variable AC auto-transformer, dim-bulb/current limiting. As an aside, I was fortunate to pick up a pristine Heathkit IP-5220 Isolated Variable AC Power Supply. Internally, it uses the same order as you've shown. All I need to do is to plug in my dim-bulb into the IP-5220 and it does everything you've shown in your final configuration. Great video! Thanks!
Steven, thanks for the compliment and positive feedback. Your IP-5220 is a great piece of gear to have and will prove very handy on the bench.
And that's why you always get multiple sources of information folks.
I've seen Mr Carlson's video about a build like that but he didn't go into detail about the order of the components.
Thank you for your very detailed and easy to understand video
I'm quite surprised that Mr Carlson didn't arrange it that way but there might be a reasoning behind it.
Nice video Eric. One thing I want to add about the isolation transformer you generally buy that I see most people skip over in their videos. I have 1000w hospital grade Tripp Lite (I believe the hospital grade ones already have the neutral isolation), but to have a true isolation transformer you need ground isolation as well. The input voltage line’s ground is connected to the metal case of the transformer. Even if you disconnect the ground wires on the output receptacles, the stock receptacles (as do most) have their ground lug connected directly to the mounting tabs. So when that receptacle is screwed securely to the metal case of the Tripp Lite it is indeed still grounded to the main input ground. So you would either need to get plastic isolation tabs and use plastic screw, or what I did was replace the stock receptacles with special ones where the ground lugs are not connected to the metal mounting tabs. These receptacles are generally orange in color. So I have complete isolations from hot, neutral, and ground.
Hi Dan, yes correct about the ground isolation and the special type of orange colored outlet. It might be a little hard to see in the video but the outlets I am using are that special type (orange color) with the isolated ground.
Oh very good, I’m sure you knew about it, but thought I should mention it for those watching the video and not know they don’t truly have ground isolation without the new outlets.
EXCELLENT EXCELLENT video.
Yes, you certainly have the absolutely BEST isolation transformer/dim bulb/variac video done.
Great job Sir. Thank you.👍
Thanks for the compliment! Glad you enjoyed the video.
Thank you. I got my "dim bulb fix" for the night... :) And that's the 2nd biggest transformer I've ever seen! Thank you for the (always) informative video(s).
Glad to know you enjoyed the video. The upcoming power supply repair video that is currently being made has a transformer in it that tops this one for size :)
The most comprehensive video l have found on the subject. Great job!
Glad to know you found the video helpful!
Thank you, this was the most lucid presentation about how to arrange the major components, and the pros/cons of each arrangement.
Glad to know you found the video informative, thanks for watching!
Hi Erik I qualified as an electrician now technician that was a great explanation of how to configure the tester 👍
Best presentation of this topic!
Thanks for the compliment! Glad you found the video helpful.
Absolutely excellent vid. I now know the optimal setup. Thank you.
Thanks! and glad to know the video was helpful for you.
Best Dim Bulb Isolated Transformer Video I've Seen So Far - I do my config a little different; Isolated Transformer --> Dim Bulb --> Variac --> DUT. Love that you showed all configs (except mine) and how things are affected. Clean And Good Information
Thanks for the compliment! The order you have the components works well too and is often used. It functions basically like the VARIAC->dim bulb->transformer setup I showed.
This is by far the best and most comprehensive discussion I've seen on this subject. I'm going the rearrange the components of my dim bulb test system to match your recommended setup. Isolation transformer to autotransformer to bulb to device under test. thanks!
Thanks for watching and I'm glad the video was helpful for you.
Just discovered your channel. This ans. All my questions about how to set up a safe work bench.
I'm glad the video was helpful for you, thanks for watching!!
This was an absolute master class, thank you for sharing.
I appreciated the compliment! Glad the video was helpful for you.
Very good explananation. I liked how you showed each configuration. I just wish I saw this 1week ago. As the incandesant bulb ban just went into effect.
Thanks for the nice feedback. The phasing out of most incandescent bulbs has been in effect for quite a while. Personally I stocked up on bulbs for uses like this project but the bulbs are still available online if not at your local stores.
excellent work mr Erik, thanks
Thanks for watching, glad you enjoyed the video.
Very interesting video on this subject , you have explained it extremely well. Being a electronicien myself this is something I had learned when at CEGEP (school), and I used it all the time. Thanks
Thanks for the compliment and for taking time to watch the video. Glad you found it interesting.
Great video Erik, really clear.
Thank you for watching, glad you enjoyed the video.
Great job on your explanation.
thank you!
Hi Eric, interesting and informative video; thanks. I see the logic in arranging the items as you do on your bench and indeed i used to have mine configured in the same order but, quite frankly, I couldn't put up with the noise. Since moving house, I now have the space to move the 'lab', such that it is, from a very drafty, cold garage to a nice warm ground floor room in the house. Whilst setting up, I decided I needed to do something about the 'growling' noise emanating from my toroidal iso-transformer: it really is quite offensive and not the sort of thing one would want next to their ear for any extended period!!
I it put the first in the chain originally since it is a 2kVA unit and my mechanical engineer's logic suggested it would be better to put the 50VA variable behind it. Anyway, long story short, when I started the new setup I reversed the positions and had the variac first in chain. Lo & behold, the horrendous 'growling' ceased. It does return to a degree when I give it full mains voltage but below about 90-95% full power it is almost unnoticeable now, though I do not have the bulbs in circuit yet. They have always been last in the chain.
Any ideas as to what the cause may be? I surfed the 'net' for hours trying to find clues as to what aggitates the toroid enough to make it angry, though without noticeable success. It does seem voltage dependent but I am at a loss with this one. Any advice or suggestions would be much appreciated as I do not like being 'growled' at by 240 of His Majesty's finest volts.
Keep up the good work. 👨🎓
Hi, my guess is the impedance or matching between the transformers is not ideal and may be setting up some distortion on the AC waveform making the transformer growl or hum. And some transformers are just more prone to making noises. Even at no load, there is some current between your isolation transformer and variac. A properly sized/rated capacitor across the AC line between transformers (power factor correction) could possibly help correct this but the noise is not damaging to the transformer unless it is so extreme the transformer is running hot (could be indication of other problems too).
@@EriksElectronicsWorkbench
Thanks Eric, I'll get the maths hat on and give it some consideration. I guess all the loading it sees at the moment is inductive so there could be something in it. The toroid is double-insulated as it came from an out-door-functions company and has no earth reference at all. Having the variac first also means I can have the earth ground on its metal case connected.
@@grahambambrook313 It seems placing the bulb for current limiting last is the best option (as I show in the video) and the order placement of the transformer and variac is about a toss up with some advantages on each way. Arrange how it works best for you and the components you are using. Yes one transformer driving another is a very inductive load to the first transformer (or variac) although in my own set up I have not had any issues.
@@EriksElectronicsWorkbench 👍
Thank you for the video, I found it most educational. I liked seeing the four different configurations. What size isolation transformer are you using? Do have a recommendation for as to a minimum and maximum size to look for?
Thank you.
Thanks for watching! I have two isolation transformers on the bench one is 1000 VA and the other is 350 VA (about 1000 watt and 350 watt). The 1000VA transformer is connected to my VARIAC which is rated to 750 VA and also to the current limiting bulb, or I can use it direct without the VARIAC and bulb. For the vast majority of projects these two transformers work fine. Depending on what you work on would dictate the transformer capacity but for most projects I find the 1000 VA is more than enough and I'd recommend that size. The 350VA is fine for small radios and such but not for large power supplies and power amplifiers.
I would like to add some protection. The lamp switch should be a 2A circuit breaker. Because, in case the switch is on and something bad happens, it is good to have an automatic circuit breaker. 2A is sufficient for many projects. I would also add an RCCB on the primary of the transformer (I choose your last configuration). If something fails in the transformer, I hope not! This RCCB would act before the home RCCB. Makes sense?
I would also add a wattmeter, a PF meter, and a needle ammeter at the end, just to take note of the consumption of the radio or amplifier. For me, the needle is the best indicator for debugging these old junk. Nice video, risk is on me, but thanks for sharing.
Adding a fuse or circuit breaker is fine but make sure it is sized no larger than what the variac or transformer can handle. In other words the circuit protection needs to be the weakest link in the circuit.
Very well done. Thank you
Thanks for watching!
Putting the iso transformer first is best. In this configuration the variac does not have to supply excitation current to the iso transformer which depending on the size of the iso transformer can be mA to Amps.
Thanks for this great video. Definitely one of the best out there. Is there anything wrong with using 2 isolation transformers? One for the dut and the other for an oscilloscope?
Glad you found the video helpful and thanks for watching! Using an isolation transformer to float the oscilloscope is not a good idea for safety reasons. If the ground clip on the scope's probe is connected to an elevated voltage a shock hazard exists from any part of the scope's metal components (case, front panel, etc) to other parts of the circuit being tested and if you come between those points you will get a shock. The best way to measure circuits that a traditional scope can't measure because of grounding issues are using an isolated probe/differential probe or an isolated scope like the Fluke Scopemeter.
great video.. I like your variac, mine shows voltage not amps. this is good.. glad to know my setup ( the 4th one) is good.. and I need to check inside the ISO and fix that !
Glad you liked the video and thanks for watching!
Hi Eric, i have seen many video's the last 3 years about this subject, because i'm building for almost 3 years (due health issues) a project with a isolation transformer , dim bulb limiter, variac and electronic circuit breaker (own design) in 1 chassis, but this video is one of the best and comprehensive explanation on the different configurations that i have seen so far.
From the beginning i had already the last configuration in mind to use in my own project, but now i have seen what effect of the other configurations are.
Regarding the isolation transformer itself, did you do some modifications to lower the "ghostvoltage" from the secundary side to ground ? Grtz from Belgium
Bjorn, thanks for the compliment and I'm glad the video is helpful for you. The voltage that can appear from isolated secondary to ground is why I say the isolation transformer greatly reduces the shock hazard.. but it does not 100% eliminate it in all cases. The ghost voltage will disappear if you use a lower impedance AC voltmeter to test it (such as 1000 ohms per volt) so generally the current level of the ghost voltage is extremely small unless the transformer has a serious problem.
I'm in the same boat. I even have the measurements for the drawn but I'm just now testing rotary encoders for the nema 23 which will 1 to 1 turn the wiper.
Nice video, the best out there!
There's an exact calculation to know what wattage I should choose for each load under test?
For example if I have a 70w load under test, and pick a 70w light bulb even if the unit its ok the light bulb will lit at full brightness , how bigger the bulb wattage should be to give the right visual effect that the DUT is working fine?
Thank you, glad the video was helpful! One way to choose a bulb wattage is to consider what is the maximum short circuit current your device can tolerate. For example 150 watt 120 volt bulb will allow 1.25 amps (I=P/V). You will always drop some voltage on the bulb but the point is to limit current initially and then go to full line voltage when you are sure it is safe. 100-150 watt bulb is typically what I find works well for most devices being tested.
@@EriksElectronicsWorkbench Thanks for answering, but to clear my head 💡, the idea behind the use of the incandescent bulb is to prevent the unit(if its shorted) to cause more damage to this device, and also give us the visual sign that the device is shorted(full brightness if its shorted, and low brightness if the device is working fine), if you use a 150watt bulb to test a 150watt device, in this case assuming the device under test is working fine, the bulb will lit at full brightness anyway (both will work with the same current), in this case the bulb will not give us a false sign?
Sorry if I'm not being clear enough, English its not my native language.
Thank you again bro!
@@Skillz-s2n if the bulb (in series with the device you are testing) and the device itself both have the same watt rating you will get half the line voltage on the bulb and half the voltage on the device. So the bulb would not be at full brilliance. If the bulb is a wattage much smaller than the wattage consumed by the test device then the bulb will glow near full brilliance and you will drop most of the line voltage on the bulb even though there is no fault, not exactly what you want. I leave a 150 watt bulb in my tester for most uses. Sometimes I reduce down to 100 watt or increase to 300 watt. I almost never go to 60 watt or less. I've never needed to go over 300 watt.
@@EriksElectronicsWorkbench Thanks for the explanation!
excellent sir, thank you
Glad the video was helpful, thanks for watching!
Great video. What VA rating would br ideal to work on antique radios using non-polarized cords. Pretty=much limited to 5-tube am radios..
Glad you enjoyed the video. 300 VA would be a small size that would be good for 5-tube radios.
Great video. Im buiding one now using a small variac and a 1 to 1 pyramid transformer. I havent decided how to wire it yet but im going to use a motor to turn the variac which has the same 3 amp ouput as the transformer. I also have an old hammond boat anchor 1kva which is earth connected on both sides but i havent found a use for it. I have alot of recording gear and i may use it when i record to avoid hum when tracking guitar.
Hi thanks again for the video it’s very helpful I am going to start repairing av amplifiers and tube amplifiers and before I even start to repair amplifiers I want to get a setup like you have and be safe in doing so can you recommend All the equipment I would need to be able to put together on a bench permanently mounted like all cables and thicknesses and ground wires and the type of bulb holder and switch onto the bulb holder and a Schematic of the equipment on how it should be put together then I can safely work on the amplifiers thank you Mark
I'm glad the video was helpful. My bench uses 14 awg stranded copper for 15 amp general purpose outlets (they are fused at 15 amps as well) and the isolation outlets/VARIAC/transformer have some 16 awg wiring because they are fused at a much lower value than 15 amps. There are wiring charts online you can look up conductor size and current rating to use what is appropriate for your setup. Whatever an outlet's wiring gauge is for hot and neutral then the ground is also the same gauge. There is no ground on the isolation outlets. Heavier gauge wiring is used on some other specialized bench power circuits rated over 15 amps. The dim bulb bypass switch is rated 15 amps up to 240 volts AC. Don't use an automotive grade switch. The lamp holder is a porcelain socket made to mount on a flat surface. Schematics of the isolation transformer, VARIAC and bulb as I have them set up are in the video.
@@EriksElectronicsWorkbench thank you for getting back to me I will have a look at the schematics and source all parts needed and build one I will send photos to you and just check to make sure I have got all equipment in the right orientation and thanks again mark if in the future you need help with anything just let me know thanks mark
Can you explain how to reference a ground on an oscillscope since the bnc ring is connected to house ground itself. Thanks very much! I will be trouble shooting a NAtional NC183D
In situations where connecting the oscilloscope ground into the circuit poses a shock and/or short circuit hazard you need to use a differential oscilloscope probe or a battery powered oscilloscope that has floating ground connections. Never attempt to float the grounds on a traditional AC line powered scope by isolating it as this is dangerous and poses a shock hazard.
Great Video Thanks! I’m in the process of changing all the old electrolytic’s in my HI-FI pre and power tube amps ( previously working fine) and am a bit anxious about switch on. What still concerns me is if I have a setup like this and say have a capacitor that decides to go faulty, is it still going to go bang all of a sudden even if I bring the voltage up slowly in stages-as there probably is still enough current flow? Also my amp has voltage regulation with 85V reference tubes so I assume the fun might not begin until I have the Variac at 85V and above? Many thanks for any knowlegeable answers 👍
Martin UK.
Glad you found the video helpful. The current limiting from the bulb will not prevent all types of faulty components from going up in smoke. It is most useful for faults in main power supply components or other power handling circuits in a device like an amp's output stage. The bulb is a visual indication of a problem so you can quickly cut the power if needed as well as a current limiter to prevent more serious damage from occurring like a power transformer being destroyed by a shorted filter capacitor. It depends on the circuit design if the regulated voltage circuit will have voltage present at low line voltages or if it suddenly switches in at a particular line voltage. You can monitor that regulated portion with a voltmeter to understand how it works. Double and triple check your work before power up as the dim bulb won't prevent damage to electrolytic installed backwards.
Hi Erik. I've found a problem. When switching this large isolation transformer, I've noticed that sometimes the house circuit breaker trips. It seems that the inrush current plays an important role, especially when the power is turned on at a point other than when the mains is crossing 0V. I've been thinking about connecting an NTC in series and creating some kind of controller to power up the entire system. The controller should manage the NTC's temperature, include a bypass relay after a few seconds, control a contactor for switching on and off based on conditions, a crossing 0V detector, (arduino project)etc... Perhaps there is something already available in the market.
Yes if the transformer is switched on at the peak of the AC cycle you will have a very large inrush current and large transformers can have considerable inrush currents. You might try using a solid-state relay (SSR) that has zero crossing switching built in. Make sure to size the relay for the power draw of your transformer. I think the zero crossing SSR is the simplest solution.
very very good video thank you
Thanks for watching!
would it make sense to have a isolation transfer before and after the variac?
Hi, using the isolation transformer before the variac makes the variac isolated as well as the device you are working on. Any additional isolation transformer would not serve a useful purpose unless you had some unusual voltage conversion requirements (step up / down) beyond what the variac could do.
Excellent explanation, thank you! I have been trying to find a definitive answer on this issue for a long time. I had arrived at the third set up myself, partly because I wanted to ground my variac, so it had to go before the isolation transformer, but I now believe your fourth set up is better. BTW, wouldn't the 'bulb always on' problem also apply to the first set up when using a low wattage bulb?
Glad that you found the video helpful. Yes in the first configuration a small wattage bulb would glow once the variac is increased because of the small current that flows in the transformers even without a load on the isolation transformer secondary.
Excellent video ! I'm getting ready to build my setup, just like you did yours. I was thinking of adding a receptacle between the variac and isolation transformer with full line voltage so as not to have to use the variac all the time? I want to add an Amp and Voltmeter after the variac with the bypass switch as you did.
My transformer is a beast ! Big Hammond 115v primary 782VA and 115VCT Single phase. I was wondering if I could just bond the ground and neutral together on the primary side, but not ground the chassis ? The primary side has three terminals. H1, H2 & H4. H1 & H4 = 125V from my 125V main. Which is how I'm going to wire it. H4 & H2 = 125V ? H1 & H4 = 242V on the primary side ? Secondary is 125V X1 & X3, which is how I am going to wire it, center tap equals 62.5V each side out of phase.
It's an older model. A single phase. I could not find a schematic on it anywhere ! I tested it and everything works fine.
I was confused at the 242V on the primary side center terminal? And what would be the effects of running it at 125V line as opposed to the 115V rating? Would ot do or possibly harm the trany ? Just a little more heat ? I don't plan on running anything that long that would saturate the filaments.
I'm planning on putting a 6A fuse between the line and the transformer with a 5A fuse between the variac and transformer. Any advice or information you could tell me, much appreciated, thanks again for a great video !
Glad you enjoyed the video and found it helpful. An extra receptacle between the isolation transformer and variac can be handy, nothing wrong with doing that. It won't have the bulb current limiting though unless you add another series connected bulb with that outlet (assuming you are following my setup). As to grounding, if this whole project is in a metal box you should use a 3 wire cord and ground the box to earth ground but do not tie the box to neutral. Then use "isolated ground" outlets (which are orange colored) so the outlet's ground socket is floating from the earth ground. Without seeing the transformer in person it is a bit hard to say how the winding taps are configured. But it sound like you have sorted them to the correct voltages. I would not worry about the 125 vs. 115 volt operation, that is within normal specs of line voltage. If the VA rating is given at 115 volts you could back off the maximum load a little running at 125 volts to keep things safe. Short duration use really won't pose a problem. Just remember on the grounding issue that the isolated line side can't have any reference to the outside world.
Thanks for your advice ! I'm not intending to use the DBT with the first transformer receptacle. Excellent video !
Sounds like your setup will be practical and work well. I have a second isolation transformer setup with no series bulb and no variac. It is handy for times when you only need isolation and nothing else. @@ekbanjosworld4926
Great video. I've always had my Isolation Transformer, Variac, and Dim Bulb in the 2nd configuration you've shown and I guess since I'm using a 300w bulb, I haven't run into the issue it potentially could have. However, you've convinced me to try out the 4th config and will soon when I clear out the work I have going on. One configuration you didn't touch on is: Isolation Transformer > Dim Bulb > Variac. Any thoughts on that config? Thanks for the tips!
Glad you liked the video, thanks for watching! Isolation Transformer > Dim Bulb > Variac would function like the 2nd set up, just switching the transformer and Variac positions. Advantage would be the bulb, associated wiring, and Variac are isolated which adds safety. It should work ok but might have a some bulb glow even with no load on the Variac due to the current that always flows in the transformers (as mentioned in the video).
Thanks!
You're welcome and the donation is appreciated. Thanks :)
@@EriksElectronicsWorkbench would love a video of you adding a digital meter with a torrid to the dim bulb I’m having a terrible time adding mine. It works but then the limiting function goes away and the mains won’t turn off with the mains switch.
@@johnc.4625 I will consider that as a future topic. From your description, if the current limiting is not functioning the bulb is being bypassed somehow. A voltmeter should only need to be connected across the load (would be the most useful location). Don't try to measure across the bulb or across the mains switch.
Hi Eric,
Any advantage to using this bulb tester on vintage transistor receivers/amplifiers and tape decks.
Using an isolation transformer is always a good idea. Generally bringing up voltage slowly and running solid state devices on low voltage is not recommend because the circuits behave differently than tube circuits under low voltage conditions. However there are times bringing up the voltage with a variac is useful during testing on solid state circuits so it's one of those use as needed situations. You can use the dim bulb portion (and a suitable rated bulb) for an initial power on just to see if there is something drastically wrong. I would not leave the dim bulb in during testing etc as the lower voltage will make readings and operation unpredictable.
Your setup do not have a Residual Current Device (RCD) after the isolation side to mitigate potentially serious electrical shock to an operator. This can happen when testing DUT, the operator connects one of the terminals after the isolation to Ground (for some reasons) and then accidentally touch the other terminal. The most probable situation is that you connect GND lead of an oscilloscope to the DUT; and even if you connect light bulb load in series with the DUT, you may still get a serious shock if you accidentally touch any point that is live in the DUT. Perhaps adding MCB after the isolation is a good idea to provide extra protection in case of short circuit or an overload especially when the light bulb load is accidentally removed or intentionally bypassed from the test circuit.
The addition of any extra safety devices is up to the end user and details are beyond the scope of the video. However installing a GFI outlet would pose an issue since the GFI outlet needs an earth ground reference. The purpose of the isolated supply is to have no reference on either line to ground. The current limiting provided by the bulb is in no way meant to be a safety feature to the user. There are inherent risks with this test setup, thus the disclaimer in the video.
@@EriksElectronicsWorkbench Fair enough. I brought this up because most modern home electrical systems have protection devices in place to protect home users and electrical appliances from electrical hazards. So there are protections before the isolation, but after the isolation those safety features are eliminated. Regarding the GFCI, I think you are referring to the voltage operated ELCB, which is now obsolete because it does not provide much protection to the operator. And yes, that device senses the voltage in reference to the Earth. But most modern device is an RCD whereby it detects the imbalance between the current flowing in the live line and the current returning in the neutral line and there is no Earth line involved. Actually adding some electrical protections is pretty simple. Just hook up a small 6A or a 10A RCBO (which is basically MCB + RCD) just after the isolation transformer. It may also protect the transformers from smoking due to inadvertent short circuit or an overload (in situation whereby the light bulb load is accidentally removed or intentionally bypassed).
Thank you for this video! As a noob hobbyist, my primary concern is a) not killing myself and b) not blowing up my oscilloscope. So this is very helpful. One thing I did learn, and correct me if I am wrong, is that using a powered differential probe is safer than using this setup because you can then still use the ground for the Oscilloscope, while isolating the probe connections from the scope, which in theory should allow you to probe a dead short on the test device without electrocuting yourself or blowing up your scope, right? Or is that wrong?
Glad you found the video helpful. The isolation transformer's main purpose is for safety from the AC line which has a ground reference. Although the transformer can allow you to connect a grounded oscilloscope, that will essentially undo the isolation transformer safety by introducing a new path to ground on the transformer secondary. A dangerous shock hazard now exists. Differential probes are a good solution or using an isolated scope such as a Fluke Scopemeter will keep you and the equipment safe.
@@EriksElectronicsWorkbench Thanks!
ISI DIM BULB TESTER VIDEO…I use the 4th and final version of the setup…
I now own an oscilloscope and learning about ISO power. Question…does adding a GFCI or AFCI “after” the setup, add any safety? I can’t find any video demonstrating why the GFCI won’t work. Testers give false reading because most of us lift ground. The tester can’t send current to ground…so it won’t test correctly. What am I missing…or is the GFCI a redundant safety feature not needed?
THANKS
A GFCI monitors the current flowing in the hot and neutral circuits. If there is the tiniest of an imbalance in current, meaning current is taking an alternate path to ground, the GFCI will trip. An isolation transformer's purpose is to isolate from ground so a GFCI won't work in this application because the transformer secondary has no reference to ground. You could install the GFCI on the isolation transformer's primary side (i.e. the outlet on your workbench).
I’d like to see what the current does on the DUT when you short out the bulb?
When the bulb is shorted it is just an isolation transformer and variac, so use with caution because there is no current limiting. It is important to be able to bypass (short) the bulb so you can apply full line voltage (when appropriate) to the device when testing.
Is there a simple analog soft start circuit?😅
plan to use for angle grinder
If you want to limit the inrush current surge a single component can do this, called the NTC Thermistor, you can find them here www.mouser.com/c/circuit-protection/thermistors/ntc-thermistors/ you have to choose the proper resistance and wattage rating among other things. Keep in mind some motors cannot be current limited as they will not start up correctly and will overheat.
For the 3rd configuration (variac-transformer-bulb). If it is CT transformer on secondary ... than i should use 2 bulb, one for positif and one for negatif, am i right?
It depends on how you are using the transformer. If you have individual loads on each side of the center tap and they each need different current limiting then yes a bulb on each secondary winding opposite the center tap would be useful. If the secondary windings are connected in series and a single load then just one bulb in series with the load. If the secondary windings are connected in parallel and one load then also just one bulb in series with the load. And last, you could put the bulb in series with the secondary center tap for common current limiting to either winding.
another question. Which is the best illumination? I always have filament bulb at 230VAC, but perhaps now are other options. The noise of the new LED bulbs is crazy...
You can't use an LED lamp. It must be an incandescent bulb. Normally the bulb voltage is the same as the maximum line voltage. You are not choosing the bulb's wattage for how brilliant you want the bulb but rather based on how much current you expect the load to draw and how much current limiting you want. A low wattage bulb like 25 watts often limits too much current, and drops too much voltage on the bulb leaving little voltage for the load. A very large wattage like 500 watts won't offer much current limiting in most situations so you have to choose a bulb wattage appropriate for the load. If the bulb is glowing brightly you have too low of wattage for the intended load, or if the bulb wattage has been selected correctly, the load has a fault which is what we are protecting against.
@@EriksElectronicsWorkbench Sorry, it was my fault. I changed the subject and the result to be more in line with the topic we were discussing. Of course, I know it should be a filament lamp as a series current limiter. I was referring to the illumination on your workbench and how people light up the area. LED lights generate a lot of spurious frequencies. In the past, I used a 220V 60W lamp, and that was it
@@soulrobotics my work bench and the surrounding work area is illuminated by fluorescent lights. Some traditional ballast and some electronic ballast. Electronic ballast are electrically noisy. I also have an incandescent bulb overhead for times when I need light but must have the work area as electrically quiet as possible. Some videos are filmed with extra LED lights only to help with the camera exposure. LED lights generally are extremely electrically noisy.
a question: how you source your oscilloscope, RF generator, AC voltmeter, etc? with or without ground?.
All of my workbench instruments are plugged into a normal grounded outlet. It is bad practice and dangerous to float or isolate test equipment. If you need to use an oscilloscope and isolate from ground the safest way is to use something like a Fluke Scopemeter or differential scope probe. Most bench voltmeters and DMM's have floating inputs and are not a problem when working on isolation powered equipment.
Someone please help me out here. I totally understand the dim bulb and the variac. They make total sense to me. Please help me understand the isolation transformer though. Now, I totally understand step-up, and step-down transformers, and so 1-to-1 also makes sense. I understand that the secondary side has to be COMPLETELY ISOLATED from the primary side; no connections between neutral, ground, etc. What I DO NOT understand, and what I can’t seem to wrap my head around is how this isolation makes me, my devices being serviced, or my tools (oscilloscopes for example) ANY SAFER… 120 volts is 120 volts, right? The secondary has 120 volts on it too, just like the primary, and if I grab one leg of that 120v with one hand, and the other leg of that 120v with the other hand, I AM going to shoot 120v through my heart, whether I am on the primary side, or the secondary side of the transformer. The secondary side isn’t magically turned into human-resistant voltage. It’s STILL DEADLY, so SOMEONE PLEASE explain to me how the isolation does any good. Ok, so I’m no longer connected to the breaker box neutral and earth ground (why can’t a 3-prong adapter/ground lift adapter plug do the same thing?), but I’m STILL dealing with that 120v on the secondary, so how am I any safer? This has boggled my mind for a while, and I know it shouldn’t…. but… it does… Thanks in advance for any insight…
The isolation transformer does not protect the user in any way if you come in contact across the secondary. There is always that risk of shock. 120 volts from the isolation transformer is just as dangerous as from the wall outlet. But without the isolation transformer just one hand bumping a live point could inflict a lethal shock so the risk is at least reduced some with the isolation transformer. The isolation transformer can also make testing with a traditional grounded oscilloscope possible in certain situations although a full understanding of the circuit is needed and a shock hazard can still exist from exposed metal parts of the oscilloscope. A ground lift adapter won't isolate the item you are working on as the transformer is able to do. Keep in mind that vacuum tube equipment and devices like high powered audio amplifiers have high voltage DC power supplies in them so the AC line (isolated) is not the only shock hazard. It is best to follow the one hand rule and only probe or work with one hand near the live circuit as to minimize a shock going from one hand through your body and to the other hand. As always be safe and fully understand the safety precautions before working on a device.
@@EriksElectronicsWorkbench I respectfully disagree. My understanding is that with no external reference the 120/240v exists across the load if you care to measure it there but there is no real live/hot/phase/whatever you want to call it, and no real neutral/common/wywtci. It's all arbitrary voltages that depend on where you measure it. Provided the device is properly isolated, if you should touch the notional "hot", you become the reference. You are grounded and the point that you touch becomes ground in the circuit. That's why it's so important not to have any other reference on the device. If you have, for instance, a grounded scope lead attached that will be a much better, as in lower impedence that you are and you will get a shock if you touch anywhere other than at that grounded scope. You can prove this safely. Imagine a 12v transformer with open output windings feeding a high resistance wire wound resistor. You could touch any part of the circuit with a voltmeter probe and the other probe to earth would give zero volts because the 12v has no other reference to ground. Now hard short that same point to ground and use the meter to measure any other point on the circuit to ground. You will get a voltage that varies between zero and 12v, depending on exactly where you put the test probe. The same thing happens with a mains isolating transformer. If there's no other earth reference you can touch any single point of the circuit.
@@electrogrim I think you misunderstood when I said "across the secondary". I mean across as in from one end of the transformer's secondary winding to the other end, you would be shocked. Voltage is always a two point reference. If you contact only one end of the secondary winding and everything is perfectly isolated then in theory you would not receive a shock.
@@EriksElectronicsWorkbench Would the amp's power transformer typically have grounded connections? I assume that would defeat any protection that the isolation transformer provides when working on components fed from the secondary of the power transformer.
@@tbonemckone If the device has for example a chassis grounded center tap on the secondary of it's own transformer that would not matter as far as the protection from the isolation transformer. The primary winding side would not normally have a connection to the chassis.
Are you closings the other channel Mr. Carlson lab
Not the same guy lol
rofl
Do microwave ovens have a suitable tranformer to use as an isolation transformer?
Cheers
PeaceFromOz
No you can't use a microwave oven transformer because that type of transformer is a step up turns ratio. They produce a very high voltage on the secondary (vary dangerous too!). An isolation transformer is a 1:1 ratio so your line voltage on the primary is the same as the secondary voltage.
@@EriksElectronicsWorkbench thank you .
Logically it's obvious, but "for science" it would have been informative if, during the setup with the lamp in the middle, you had shown the voltage level on the Input of the isolating transformer also.
Good luck finding a suitable incandescent bulb. I was told they aren't being made anymore.
There's a lot of old stock still out there. I have several set aside for this test set up on my bench. They will last nearly forever since they only glow dimly.
There are plenty still available, as specialty incandescents are still being made. I just bought a pair of 60-W yellow "bug bulbs" at my local Big Box store. These, plus rugged bulbs for refrigerators, etc, will be available for the duration.
I just bought a pack of 4 .... 100 watt incandescent bulbs off of ebay for 8 bux ...... There's a ton of bulbs out there , buffet tables use the food safe ones that have unbreakable glass and the kitchens use them also.... Reptile owners need the old style bulbs for their pet iguanas and others ......
Banned manufacturing in the USA, but not some other countries to my knowledge.
TLDR:
Isolation Transformer > Variac > Light bulb
I have a nice iso transformer but no variac. I have been looking at a vac
SC-10T from ebay that is a isolated variac but have not bought it because it seems everybody has seperate devices. I would like to know if the advatages and disadvantages of using a seperate variac and transformer vs a "all in one". I have a small bench so that would be one advantage for the
SC-10T.
The SC-10T that I found on eBay does indicate on its schematic that it is both an isolation transformer and variac. (but for safety you should verify the isolation with an ohm meter if you buy it)
If you connect an incandescent bulb in series on the output side you will essentially have the same arrangement as I use: isolation transformer->variac->bulb. From the provided advertising the all in one SC-10T should function the same as having separate components and a bit more space saving.
@@EriksElectronicsWorkbench Thanks for the quick response. I am still watching your video and double checking my transformer for isolation.
There's a video of the SC-10T taken apart. Once you disconnect the ground on its output, which is connected from factory, it's truly isolated.
👍
so I assume your secondary stage of the iso transformer has no ground reference , therefore the variac will have no ground reference and neither will the dim bulb and device under test. so besides placing two ground probes on the dut which can cause voltage potential if not in the same place, are their any other vunerabiltys , can yo be shocked by touching the metal casing of the Variac as it is not grounded or something of that sort. thank you.
Yes correct that the transformer secondary does not have a ground reference which is what the "isolation" is referring to. Since the transformer's secondary does not reference either side to ground, if a fault occurs, for example, in the variac making some part of it live to one of the isolated AC lines you would not be shocked because no return path exists from the user through ground and back to the transformer's secondary. This is also why you should not use a ground referenced oscilloscope on the isolation transformer's secondary (on DUT). If you introduce a ground path on any part of the transformer's secondary then yes a shock hazard will exist.
@@EriksElectronicsWorkbench thank you for the detailed response I really appreciate that.
@@johnc.4625 Happy to help with questions and thanks for the donation. :)
Could you please clarify what you mean about the ground referenced oscilloscope? Are you suggesting having the oscilloscope connected to an isolated supply and not a grounded mains?
@@msouthwe1 AC mains powered oscilloscopes have the probe ground tied to chassis ground. You should not float an AC mains powered oscilloscope on an isolation transformer as there can still be a shock hazard if you contact a metal part of the scope and part of the circuit you are testing. In situations where the scope ground can't be tied to circuit ground potential the only safe way is to use an isolated ground scope like a Fluke Scopemeter or a differential scope probe.
Great video. One question about your Tripp Lite. Wouldn't what you were talking about with that unit also be a problem with its third prong ground side also. I guess if you are only plugging in a two prong vintage item that is ok. And most three prong items have that third prong isolated from the neutral parts of a device - it's in case a device energizes (shorts to its case etc.) itself etc. I bring it up because I also just watched a video about a modern variac with a built in isolation BUT third prong ground was NOT. This would concern me about total isolation. ruclips.net/video/8o7HtGTvBvA/видео.htmlsi=aqySuOViPgvYFf94
Edited 10 minutes later. He does explain that the company recommends using a cheater plug when plugging in a three prong cord to maintain needed isolation if one wants that on this unit. I guess one needs to be very careful about reading words on any product with verification of its meaning in practice!
Glad you liked the video. The Tripp Lite transformer does have a 3-wire plug but the ground is ok as that earth ground goes to the metal case the transformer is in and is a good safety feature. On the secondary side nothing is connected to earth ground (and modification to neutral was required as shown in video) so both AC lines are floating with respect to earth ground. No connection is made to the ground on the transformer outlets. Unless the VARIAC is a special isolated type it does not offer isolation and lifting the ground reference on a VARIAC will not create isolation protection. It has to be used with an isolation transformer. Always remember the AC neutral at the wall outlet is earth ground connected.
@@EriksElectronicsWorkbench The one referenced in the video apparently had its own built in isolation transformer. He shows all that in the first three minutes. I was thinking of the schematics you showed at 13:00 with third prong. I took that to mean they were connected through the metal box.
@@wdmm94 that schematic was showing how the "isolation" transformer was factory built but I modified it. No earth ground is brought to the isolated outlets on the bench. If the VARIAC has an isolation transformer built in you can ground it's metal case for safety since it is line powered but the outlets (secondary side) ground should not be used. You can test with an ohm meter to see if there is any connection from secondary neutral to ground if there is a safety concern. Check out my "ghost voltage" tech talk video for a little more on grounding and isolation issues. ruclips.net/video/WONMMZsLHDE/видео.html
ERIKS, Why is the variac meter measuring "In rush current spikes", is the variac transformer or isolation transformer causing the In rush current spike? Transformers shouldn't cause in rush current spikes. The Test Set up Config#2 with the Current Limiting Light Bulb in the middle will light up at very low line voltage which means that the isolation transformers primary has "leakage current"?
Wayne, transformers will have an inrush current when they are switched on at the peak of the AC sine wave. The 1st configuration has the high current spikes at low settings of the VARIAC because of the arrangement of the components and how they interact. All transformers have some current flow in the primary with no load/no current on the secondary. This is normal. The no load current flow is *not* from leakage current.. i.e. breakdown of the insulation. Technically the no load current between two transformers can be brought to near zero with a capacitor in parallel between them but this is really not needed on the size of transformers and currents we are dealing with here.
@@EriksElectronicsWorkbench When does a transformers have current flow in the primary with NO load on the secondary? The primary insulation is breaking down means there is "leakage current" flowing from the Top of the primary coil to the bottom of the primary coil. The Neutral wire on the bottom of the primary coil should be zero volts/zero amps. If it measures microamps uA or mA that is leakage current from the transformers eddy currents or transformers insulation. Try making a video lesson on how to test various transformers 1.) leakage current, .) in rush current, 3.) the transformers "isolation tests" how to test a transformers isolation
@@waynegram8907 An ideal transformer would have no internal loss and would not draw current if there is no secondary load but such is not the case in actual use. Transformers draw a small amount of primary current with no secondary load in order to establish the flux in the transformer core. Power up a transformer and leave it with no load on the secondary and you will find it becomes warm to the touch after a while which shows there is some small loss in the core. Yes, I will consider making a video on transformers, testing and so on.
@@EriksElectronicsWorkbench Yes the small loss in the core is Eddy Currents which are from the magnetic field flux magnetizing the core material which the Eddy currents creates heat. When testing various transformers Losses in the video lesson would help out. When applying a Squarewaveform 20hz to 20khz to an audio transformer you will see ringing and harmonics because of the transformers wafers laminations materials and core wire insulation material used which will vibrate and resonance causing the ringing, hysteresis, harmonics to go over in the video lesson.
I do not understand why the current meter of the variac is not shown in any of those schematics. It would have been nice to see whether it was in the primary or secondary circuit of that variac because it makes some difference in certain situations, I suppose. (And same thing with fuses. I suppose it makes some difference if the fuse is in primary circuit or secondary of the variac.)
The current meter is on the variac output (i.e. the movable tap), the fuse is on the line cord into the variac. The variac does not have a primary and secondary such as the terms are used with transformers as it is all one winding with no isolation.
@@EriksElectronicsWorkbench Yes I know that primary/secondary thing. But if you short the output, you can burn the wiper or that "secondary" part of the winding at quite low output voltages even if that input fuse does not blow yet, can't you, if you have no fused output.
@@jutukka I have several variacs in finished enclosures with fuses, outlets, etc built by different companies. Some have a fuse only on the ac line in and some have a fuse only on the output tap. None have fuses on both the input and output however I think for complete protection both fuses would be a good idea. With the current limiting bulb in my video there is no danger of excessive current flow but I suppose without the bulb you could damage the variac in certain situations with a short circuit and inadequate fuses.
@@EriksElectronicsWorkbench I have only one Variac and it is cheapest Chinese 230VAC/1000VA/3.3A/0-300VAC I could find. It has a traditional 5A 6x30mm glass fuse in input, and a 5A circuit breaker in output, and in addition a 100°C thermal fuse. LCD voltmeter. It cost less than €80, postage included. 😁
Works fine but an amp meter would have made it perfect.
P.S. If you buy a cheap chinese stuff, be prepared that the smell of rubber and plastic parts and paint can be quite, well, amazing. For example those black rubber feet of my variac smelled so bad that if an insect flew near them, it probably would drop dead. I had to remove them. And when I put those rubber feet into a air-tight Minigrip bag, the smell came out through the plastic bag! 😭. Well, those rubber feet were easy to remove throw away but the top and bottom rubber insulator plates of the bolted toroid are smelly as well. 😝
And make sure that safety ground lugs really are in galvanic contact with the chassis metal or screw, not only thick layer of paint, as was happening in my variac.
Nice Chanel
Thank you!
Having the variac feeding the IT will cause problems . At low variac voltages the primary of the IT will struggle to generate secondary voltage due to back EMF. Could you do a vid on the difference between a tech IT and a medical IT.
The isolation transformer will work fine at low voltages but the slight mismatch between the variac and isolation transformer means you might have to set the variac output a little higher to compensate for the loss. But that setup would still allow setting any voltage out of the isolation transformer from zero upwards.
I don't have a medical grade isolation transformer but typically medical grade means it has strict safety measures for ground faults, i.e a fault that could harm a person since the transformer could power a device directly attached to a person.