You explain the circuit very well but I have a suggestion to make. I would put the isolation transformer first and the variac second. Traffos are designed to accept a particular input voltage and there is a reason for this. If the input voltage is variable then the primary of the IT could struggle to produce a secondary voltage at low input voltages due to EMF. This will cause over heating in the isolation transformer primary and could in time cause " Brown out" . This is where over heating of the windings causes a failure . It's common in motors but that's a different matter. Variacs are designed to vary input voltages and prolonged use at low voltages will eventually cause a problem . Hope this comment is helpful.
I did more of less the same kind of setup but i added 4 bulbs, 2 60w and 2 40w that I can switch in parallel to protect the tested device. I also added a small resettable breaker (1 amp). I can disable the isolation transformer if I want with a switch. I call it the ultimate dim bulb tester :)
thank you so much! for saving lives of young people that are experimating with such possible lethal devices! i hope you can understand my poor english ;) i comme from Bavaria (Part of germany) so i have much difficult to follow your explnation...because german is more an "order" language than one to explain;) best regards, a
Best tutorial I've seen on variac/isolation transformer setup, and I've watched many in preparation for building my own. As far as I've seen you've left nothing out! I plan on using two identical, inexpensive 120/240 step up/step down transformers (1K - 3K VA) to isolate by connecting the low voltage coils together and using the high voltage coils as the input/output. Maybe $20 each. Seems like it will work. You are only limited by the size of the transformers, or the variac. My variac is 2KV, 220 V here in the Philippines - no ground! Again, excellent post!
Many thanks Brooks. There are a few errors in that video. But I have listed them at bottom. Also big thanks for letting me know the name of the red throw switch. I been trying to find that out for ages.
If you have time, check out "bigclivedotcom". He's a big Scot and very knowledgeable and entertaining. You'd enjoy. I'm looking forward to seeing your other posts!
don't piggy back small transformers. They just make a lot of heat. If you can't find the real deal find an industrial control transformer that is like 120/240 to 120/240 or 120/240 to 240/480. You can ue 240 to 240 for 120/120 but you have to maintain the current limits which means you have to buy a bigger transformer. I see 1 kVA ICTs all the time for under a hundred bucks. Used for 500 watts it wil run cool and quiet.
Always, always include a fuse at variac output. If you fuse at input (say 15A) and you output at 1/3 voltage, you can output 45A from variac and fuse does not blow (it is a transformer after all, low output voltage=higher current at same inpu). However, variac does not handle 45A at the 1/3 of the winding that has output. I have seen variacs burned because of a fuse only at input, actually I think I burned one tiny variac that way myself. After that I never used variacs without a fuse at output. I made my setup quite differently. My setup has isolation transformer to mains and variac at the output side. No need to worry wou the isolation transformer handles overvoltage, I have variac set to output slight overvoltage like 275V on maxinum instead of normal 240V. My setup also includes separate isolating transformer for measurement devices. Connecting a grounded scope lead to device under test makes it mains referenced and if you touch open device in wrong place, you get shocked. Having scope isolated too avoids that problem. (At the time I made the setup, it was the required one for any repair shop, requirement included two separate isolation transformers). Large transformers pull a huge spike from mains when powered on. I have also a slow start mechanism: resistor to limit inrush current and after a second or so contactor bypasses resistor. My lab setup is for 230V/16A which means very heavy transformers that take huge current spike at startup, especially with a transformer+variac combo. Without a slow start circuit, fuses are always blown with my setup when it powers up.
I have four fuses on it for each stage. Both input and output. Not in video or diagram though. I wrongly assumed everyone would understand to use fuses liberally. This also blows fuses regularly but that is okay. I've learned to turn up the voltage slowly. Been a great asset to me over last 3 years.
@Phil Allison I agree that training yourself to touch things is very bad because it is safe leads to bad habits. The fact you can do that doesn not imply you should train to do that. Yet, when working with open unprotected circuits it is safer to make them safe, in case accidents happen like when adjusting something and your hand hits something unwanted by accident. Isolation can be a safety addition like for example residual current braker.
Dear. Congratulations on the great class. The knowledge you have and detachment in sharing demonstrates your personality. Congratulations!!! Lots of light and harmony. Thank you very much.
Nice job, looks good, Ive had a variac hanging around for a while now that ive thought about doing this with. Isolation transformers seem a bit expensive though, so was looking at ways of rigging up a 1:1 arrangement using the coils of primary coils of 2 separate 240v transformers. One day hopefully I will get round to it.
Its good for what it is but people have forgotten about magnetic amplifiers you can do all this with a normal pot at low voltages and control HV via a tiny transformer
Yes your right. I do actually have power switch on the hot side. But the diagram shows live and neutral written down the wrong way around. Too late to correct it now. Well spotted! I have updated description. Thanks for letting me know.
I used a simple and cheap digital power meter. It measure current, voltage and power on an LCD display. Con etiology is made with a magnetic loop and ferrite core. You will never use the 50 and 300 milliampere meters.
perfect. Im in the process of making one of these but with the option to limit the current so I don't blow anything up! I've just started playing with some old valve radios. thanks for making this video.
Hiya Tech tins! Thanks for your video, I’ve been planning to make an isolating to myself with the 2 primary windings of identical microwave ovens but I’ve not made much progress.😢. Nevertheless, quick query: on your diagrams around 9:20 you show an oscilloscope connected to the DUT. Usually the oscilloscope probe ground is connected to the oscilloscope earth point, hence connecting the earthed probe to the DUT effectively grounds that side of your isolation tx. Then if you touch any live part while in contact with any earthed device you have a circuit through the isolated side of the tx and you get a shock. It feels like I’m missing something or that when a device is connected that is earthed, then you give up the safety benefit. Appreciate your thoughts!
Awesome video! I have aquired all the ingredients, ie variac, dim bulb assy, except for iso trans. I plan to search eBay for 1:1 120v trans. Is that the way you would proceed? Thanks!
Nice video--- thanks! I just picked up a nice Superior Electric Company model 116U variac I got for free off Craigslist. Now I'm looking around for an isolation transformer and meters and I'll be building something similar to your equipment. Probably fewer meters though--- an ammeter and voltmeter after the isolation transformer I'm thinking.
What I have there is three meters for current on a simple three way switch and one meter for voltage, so if your measuring low current you can switch between them to get a more accurate reading (ie 50mA, 300mA and 3A scales). But you could install just one active digital meter and that would give you all resolutions you need plus voltage. Here is an example for ya www.amazon.com/DROK-80-300V-Voltmeter-Multimeter-Transformer/dp/B01MRZAFAF I prefer analogue so no option but multiple meters for current. It is fun to build one of these. Enjoy your build and thanks for your post
I don't have any meters lying around, so I'm considering using a cheap multimeter that can be switched from one function to another. Thanks for your reply!
Since you are honooring me by replying to my posts, I'll ask another question: Based on the video, the most important ways to protect yourself and your equipment from the hazards of powering up unknown or experimental equipment are: 1: The hazards of being shocked by being connected to high voltages 2 The hazards of sort circuits or unplanned high current loads through unknown defects in experimental or unknown equipment. #1 is best corrected with an isloation transformer #2 Is commonly remedied by two suggested methods ---a variac auto transformer and gradually running up the input voltage to identify possible problems AND installing a current limiting device such as an incandescent light bulb in series with the 120 VAC voltage input. However, it seems to me that recommend both a variac and the current limiting light bulb are two solutions to the same problem. Wouldn't it be satisfactory to have the isolation transformer and the current limiting light together, but not bothering with a variac? The variac might indeed be nice to have, and I am lucky to have found one just this week. But it seems to duplicate most of the protection that the current limiting light bulb provides.
Depends what projects your working on. Current limiting bulb for AC will alert you of a direct short and brighten up taking all the power instead of the shorted current catching the wires on fire (a direct short on 240Volts AC will either cause the power cable to catch fire or burn out your valves/components) So its very good to have one if your working on old AC valve equipment. But it does not give you anything else. (Think of it like slow safety fuse that does not blow out but uses all the available power up safely) I need the variac for setting different amplitudes of the 50hz AC voltage on different projects (not just valve radios). Very useful bit of kit. I use it all the time. But the isolation transformer (like the light bulb) is only for safety. Think of it like this, the light bulb stops wires from catching fire, whereas the isolation transformer ensures you do not have a direct connection to the mains. (you can still get a shock though, if you hold both the live wires at same time.) The variac has no relevance to these parts. It simply provides a variable 0 to 240v AC supply, With a full bridge rectifier stuck on the end (which I often do) It doubles up as a DC variable supply as well. But if your only ever working on low current DC circuits then you do not need a variac or an isolation transformer. A variable DC SMPS instead is much cheaper and might be better for you. Hope that helps. Happy to reply to all questions.
Excellent video. Very informative-you've earned a subscriber! Is there a preferred order for a setup that uses an isolation transformer, a variac, and a dim bulb current limiter? It seems like the isolation transformer and in the variac combo would work in either configuration, no?
How is it not at earth potential if you connect earth from varisc to the leg of the isolation transformer? I'm building one with an almost identical wiring schematic only I wasn't going to connect the chassis of each. I have a 1kva isolation transformer as well I got in a trade but I'm not using that.
The casing is. But the secondary coil has no connection to earth. The earth wire on leg is purely for safety if ever any primary side live leads came lose and touched the casing. But you could leave it off. Will work fine without.
@Tech Tins cool. I'm going to go super lazy and have a closed loop stepper turn the variac. I have the motors and all that just sitting in boxes. It's never going to be moved after haha. I wish I could sell the 1000kva one I got recently. Traded a 3d printer for a tektronix tds2002c and that boat anchor of a transformer. I had just bought a new scope like a month prior for alot of quid and wish I could've still been within the return window. Haha. You guys have access to way better gear than us in north America. I'm trying to find a brymen bm869s and it's a bitch. Paying that vat seems bonkers. It's like some weird game of follow the cup as far as who it's good for haha.
Nice project done in a practical manor. This is similar to some commercially made unites of yesteryear I've seen once in a while. I'm wondering if you are going to use this in conjunction with a current limiter, like a dim bulb tester? You have a new subscriber.
Think you drawing is wrong for an autotransformer. As drawn, your output voltage could never exceed input voltage, only increase to equal it. I think it should be drawn so you input is between ground and what you indicate as TAP. Then, output would be across what you indicate to be AC input. Otherwise, excellent vid. Thanks!
Yes, my drawing's version as I have it, would not allow over mains voltage. But it is still an auto transformer, just not the same kind as most commercial ones you buy Ha ha!. Well spotted! and thanks for comment.
The drawing is correct. It depicts a fixed tap autotransformer, not a "variac". The input voltage is applied across all 20 turns. The tap is connected at exactly the center, 10 turns from the top and ten turns from the bottom. If you apply 100 VAC across the 20 turns, the voltage at the tap would be 50 VAC. On variacs configured to raise output voltage above the input voltage, there is typically a tap at 20% of the # of turns from the top, The input voltage is connected to this tap. The top of the winding (commutator in variac language) is left open. Thusly when the wiper is positioned below the tap, output voltage is reduced. When positioned at the tap, output voltage equals input voltage. When the wiper is moved above the tap, output voltage is increased to a maximum value of 20% above input voltage, corresponding to the input being applied at 20% of the turns ratio from the top terminal. There is also typically another tap positioned symmetrically at 20% of turns from the bottom of the commutator to allow the same function with CCW rotation. Always place the overcurrent protection device on the OUTPUT lead of a variac.
Good video! Whats the name of the terminals with the red lid your meter is connected to? I may be interested in one for a 400 vdc variable supply I am building.
They are as rare as hens teeth! It is an old style knife switch, don't know what its proper name is as it was given to me by a friend. But I to would love to know what you call them. I spent ages trying to find the same model online. But, I failed to find another one. I would like to buy a few more! So, If you find out please post back. And thanks for your kind comment.
Kevin Gittemeier : are you talking about the "Cliff quick test"? It's available many places in the UK, as that's where it's made. In the U.S., Amazon carries it. They even have one that is color coded for the U.S. and Canada (black/white/green). They're around $60.
Not claiming to be an expert here, but wouldn't there be no harm in feeding earth ground all the way into your device under test which would mainly be for the discharge of static electricity if it were to accumulate? At the device under test, the two hot wires are not referenced to ground I don't think, so it still wouldn't shock you by touching one of them and earth ground I don't think.
neat... one question, i'm not a newby but having made this apparatus i think you can answer me quickly: what happens on the second transformer if you undervolt it?... i mean, transformers are not "precision" devices but their sizing is all held togheter, by turns amount etc, i perfectly understand that the voltage will follow the variac but i'm not sure about the current capability, if they work like audio output transformers it should be a matter of turns ratio, changing just the voltage and not the max current capability, but maybe i'm missing something... or maybe it will be just a loss by the parasitic resistance of the windings of the isolation transformer (little i hope)
LOL @ "blueprint". Nobody uses blueprints. And if you don't understand how to wire a Voltmeter and Ammeter, then you DEFINITELY should not be fucking about with this kind of equipment.
As long as it is inline then your good to go. Does not really matter that much, which comes first if you make sure they both bolted and wired up correctly within same box. Although I would place the smaller turns of coils unit as second so the first unit sees the inrush current, instead of smaller unit. Which is why I placed my Isolation second as its much smaller (turns of wire) than the variac.
Huh a diy that you have to buy. Wouldn't a better title be something like, "this is not a diy variac?" Also it doesn't require tons of copper.. if requires winding a large toroidal chunk of hard to aquire and probably home cast soft iron with tight and even spaced turns that I've battled with achieving. But its very much less expensive than buying one. Also the commercially available crap produced today allows for more hassle than benefit.
Agreed it is probably not the correct title, but on that argument, if your going to wind transformers (which is just a large component) why not make your own fuses and dial meters as well? In UK these variacs are not much more expensive than the copper used to make them. It is mass production which makes this so.
I love someone who isn't from India who can explain electrical components with competence
You explain the circuit very well but I have a suggestion to make. I would put the isolation transformer first and the variac second. Traffos are designed to accept a particular input voltage and there is a reason for this. If the input voltage is variable then the primary of the IT could struggle to produce a secondary voltage at low input voltages due to EMF. This will cause over heating in the isolation transformer primary and could in time cause " Brown out" . This is where over heating of the windings causes a failure . It's common in motors but that's a different matter. Variacs are designed to vary input voltages and prolonged use at low voltages will eventually cause a problem . Hope this comment is helpful.
Your right 100% agree
I did more of less the same kind of setup but i added 4 bulbs, 2 60w and 2 40w that I can switch in parallel to protect the tested device. I also added a small resettable breaker (1 amp). I can disable the isolation transformer if I want with a switch. I call it the ultimate dim bulb tester :)
thank you so much! for saving lives of young people that are experimating with such possible lethal devices! i hope you can understand my poor english ;)
i comme from Bavaria (Part of germany) so i have much difficult to follow your explnation...because german is more an "order" language than one to explain;)
best regards, a
That's cool how you incorporated all those cool bits into one box. All you need is 3 groovy light bulbs sticking out of the top. Cheers from Utah!
Best tutorial I've seen on variac/isolation transformer setup, and I've watched many in preparation for building my own.
As far as I've seen you've left nothing out!
I plan on using two identical, inexpensive 120/240 step up/step down transformers (1K - 3K VA) to isolate by connecting the low voltage coils together and using the high voltage coils as the input/output. Maybe $20 each. Seems like it will work. You are only limited by the size of the transformers, or the variac. My variac is 2KV, 220 V here in the Philippines - no ground!
Again, excellent post!
Many thanks Brooks. There are a few errors in that video. But I have listed them at bottom. Also big thanks for letting me know the name of the red throw switch. I been trying to find that out for ages.
If you have time, check out "bigclivedotcom". He's a big Scot and very knowledgeable and entertaining. You'd enjoy. I'm looking forward to seeing your other posts!
I know I subscribed to him a very long tome ago!
don't piggy back small transformers. They just make a lot of heat. If you can't find the real deal find an industrial control transformer that is like 120/240 to 120/240 or 120/240 to 240/480. You can ue 240 to 240 for 120/120 but you have to maintain the current limits which means you have to buy a bigger transformer. I see 1 kVA ICTs all the time for under a hundred bucks. Used for 500 watts it wil run cool and quiet.
Always, always include a fuse at variac output. If you fuse at input (say 15A) and you output at 1/3 voltage, you can output 45A from variac and fuse does not blow (it is a transformer after all, low output voltage=higher current at same inpu). However, variac does not handle 45A at the 1/3 of the winding that has output. I have seen variacs burned because of a fuse only at input, actually I think I burned one tiny variac that way myself. After that I never used variacs without a fuse at output.
I made my setup quite differently. My setup has isolation transformer to mains and variac at the output side. No need to worry wou the isolation transformer handles overvoltage, I have variac set to output slight overvoltage like 275V on maxinum instead of normal 240V. My setup also includes separate isolating transformer for measurement devices. Connecting a grounded scope lead to device under test makes it mains referenced and if you touch open device in wrong place, you get shocked. Having scope isolated too avoids that problem. (At the time I made the setup, it was the required one for any repair shop, requirement included two separate isolation transformers).
Large transformers pull a huge spike from mains when powered on. I have also a slow start mechanism: resistor to limit inrush current and after a second or so contactor bypasses resistor. My lab setup is for 230V/16A which means very heavy transformers that take huge current spike at startup, especially with a transformer+variac combo. Without a slow start circuit, fuses are always blown with my setup when it powers up.
I have four fuses on it for each stage. Both input and output. Not in video or diagram though. I wrongly assumed everyone would understand to use fuses liberally. This also blows fuses regularly but that is okay. I've learned to turn up the voltage slowly. Been a great asset to me over last 3 years.
@Phil Allison I agree that training yourself to touch things is very bad because it is safe leads to bad habits. The fact you can do that doesn not imply you should train to do that.
Yet, when working with open unprotected circuits it is safer to make them safe, in case accidents happen like when adjusting something and your hand hits something unwanted by accident.
Isolation can be a safety addition like for example residual current braker.
Много добър уред и много хубав клип...
Dear. Congratulations on the great class. The knowledge you have and detachment in sharing demonstrates your personality.
Congratulations!!! Lots of light and harmony. Thank you very much.
Nice job, looks good, Ive had a variac hanging around for a while now that ive thought about doing this with. Isolation transformers seem a bit expensive though, so was looking at ways of rigging up a 1:1 arrangement using the coils of primary coils of 2 separate 240v transformers. One day hopefully I will get round to it.
try ebay they often come up on there. Use auction option only.
Best to use an incandescent light bulb or two in series with the load for current limiting rather than depending on an amp meter to indicate a short.
Its good for what it is but people have forgotten about magnetic amplifiers you can do all this with a normal pot at low voltages and control HV via a tiny transformer
Just came across this video. I'm quite impressed; you did a nice job.
great vid thanks. Only issue i had with the circuit is i would have put the power switch on the hot side instead of neutral side of the variac.
Yes your right. I do actually have power switch on the hot side. But the diagram shows live and neutral written down the wrong way around. Too late to correct it now. Well spotted! I have updated description. Thanks for letting me know.
I used a simple and cheap digital power meter. It measure current, voltage and power on an
LCD display. Con etiology is made with a magnetic loop and ferrite core. You will never use the 50 and 300 milliampere meters.
True! But they look good. ha ha!
perfect. Im in the process of making one of these but with the option to limit the current so I don't blow anything up!
I've just started playing with some old valve radios. thanks for making this video.
Hey that is very cool! Please post back here and let us know how you get on. Its a lot of fun is it not.
I'm supposing the variac effectively limits the current, especially being able to view the current on the meters.
Great informative video Tech Tins. I would put the fuse and switch on line A to variac (@11mins)
Hiya Tech tins! Thanks for your video, I’ve been planning to make an isolating to myself with the 2 primary windings of identical microwave ovens but I’ve not made much progress.😢. Nevertheless, quick query: on your diagrams around 9:20 you show an oscilloscope connected to the DUT. Usually the oscilloscope probe ground is connected to the oscilloscope earth point, hence connecting the earthed probe to the DUT effectively grounds that side of your isolation tx. Then if you touch any live part while in contact with any earthed device you have a circuit through the isolated side of the tx and you get a shock. It feels like I’m missing something or that when a device is connected that is earthed, then you give up the safety benefit. Appreciate your thoughts!
Awesome video! I have aquired all the ingredients, ie variac, dim bulb assy, except for iso trans. I plan to search eBay for 1:1 120v trans. Is that the way you would proceed? Thanks!
Howdy! Will I loose TORQUE when using using a variac? Keep up the good videos!
Good communication skills! If I can follow along and understand, anyone can!
Thank you very much!
Nice video--- thanks!
I just picked up a nice Superior Electric Company model 116U variac I got for free off Craigslist. Now I'm looking around for an isolation transformer and meters and I'll be building something similar to your equipment. Probably fewer meters though--- an ammeter and voltmeter after the isolation transformer I'm thinking.
What I have there is three meters for current on a simple three way switch and one meter for voltage, so if your measuring low current you can switch between them to get a more accurate reading (ie 50mA, 300mA and 3A scales). But you could install just one active digital meter and that would give you all resolutions you need plus voltage. Here is an example for ya www.amazon.com/DROK-80-300V-Voltmeter-Multimeter-Transformer/dp/B01MRZAFAF
I prefer analogue so no option but multiple meters for current. It is fun to build one of these. Enjoy your build and thanks for your post
I don't have any meters lying around, so I'm considering using a cheap multimeter that can be switched from one function to another.
Thanks for your reply!
That will work. My first version did not have any meters. Have fun!
Since you are honooring me by replying to my posts, I'll ask another question:
Based on the video, the most important ways to protect yourself and your equipment from the hazards of powering up unknown or experimental equipment are:
1: The hazards of being shocked by being connected to high voltages
2 The hazards of sort circuits or unplanned high current loads through unknown defects in experimental or unknown equipment.
#1 is best corrected with an isloation transformer
#2 Is commonly remedied by two suggested methods ---a variac auto transformer and gradually running up the input voltage to identify possible problems AND installing a current limiting device such as an incandescent light bulb in series with the 120 VAC voltage input.
However, it seems to me that recommend both a variac and the current limiting light bulb are two solutions to the same problem.
Wouldn't it be satisfactory to have the isolation transformer and the current limiting light together, but not bothering with a variac?
The variac might indeed be nice to have, and I am lucky to have found one just this week. But it seems to duplicate most of the protection that the current limiting light bulb provides.
Depends what projects your working on. Current limiting bulb for AC will alert you of a direct short and brighten up taking all the power instead of the shorted current catching the wires on fire (a direct short on 240Volts AC will either cause the power cable to catch fire or burn out your valves/components) So its very good to have one if your working on old AC valve equipment. But it does not give you anything else. (Think of it like slow safety fuse that does not blow out but uses all the available power up safely)
I need the variac for setting different amplitudes of the 50hz AC voltage on different projects (not just valve radios). Very useful bit of kit. I use it all the time. But the isolation transformer (like the light bulb) is only for safety.
Think of it like this, the light bulb stops wires from catching fire, whereas the isolation transformer ensures you do not have a direct connection to the mains. (you can still get a shock though, if you hold both the live wires at same time.) The variac has no relevance to these parts. It simply provides a variable 0 to 240v AC supply, With a full bridge rectifier stuck on the end (which I often do) It doubles up as a DC variable supply as well.
But if your only ever working on low current DC circuits then you do not need a variac or an isolation transformer. A variable DC SMPS instead is much cheaper and might be better for you. Hope that helps. Happy to reply to all questions.
You have a 1A isolation transformer, so at 12V it’s12 VA. Switch both transformers and you will have a 3A/200VA output.
Excellent video. Very informative-you've earned a subscriber! Is there a preferred order for a setup that uses an isolation transformer, a variac, and a dim bulb current limiter? It seems like the isolation transformer and in the variac combo would work in either configuration, no?
You only need the lowest current ampmeter + switch to select 0 & 2 parallel resister shunts (x1, X10 and X100)
@@set3777 yes that is true. But I like having lots of them.
Great vid.Will build mine just like this.Thank you.
How is it not at earth potential if you connect earth from varisc to the leg of the isolation transformer?
I'm building one with an almost identical wiring schematic only I wasn't going to connect the chassis of each.
I have a 1kva isolation transformer as well I got in a trade but I'm not using that.
The casing is. But the secondary coil has no connection to earth. The earth wire on leg is purely for safety if ever any primary side live leads came lose and touched the casing. But you could leave it off. Will work fine without.
@Tech Tins cool. I'm going to go super lazy and have a closed loop stepper turn the variac. I have the motors and all that just sitting in boxes. It's never going to be moved after haha. I wish I could sell the 1000kva one I got recently. Traded a 3d printer for a tektronix tds2002c and that boat anchor of a transformer. I had just bought a new scope like a month prior for alot of quid and wish I could've still been within the return window. Haha. You guys have access to way better gear than us in north America. I'm trying to find a brymen bm869s and it's a bitch. Paying that vat seems bonkers. It's like some weird game of follow the cup as far as who it's good for haha.
Nice project done in a practical manor. This is similar to some commercially made unites of yesteryear I've seen once in a while. I'm wondering if you are going to use this in conjunction with a current limiter, like a dim bulb tester? You have a new subscriber.
Well thanks for your nice comment. Yes you can do that as well.
Think you drawing is wrong for an autotransformer. As drawn, your output voltage could never exceed input voltage, only increase to equal it. I think it should be drawn so you input is between ground and what you indicate as TAP. Then, output would be across what you indicate to be AC input.
Otherwise, excellent vid. Thanks!
Yes, my drawing's version as I have it, would not allow over mains voltage. But it is still an auto transformer, just not the same kind as most commercial ones you buy Ha ha!. Well spotted! and thanks for comment.
The drawing is correct. It depicts a fixed tap autotransformer, not a "variac". The input voltage is applied across all 20 turns. The tap is connected at exactly the center, 10 turns from the top and ten turns from the bottom.
If you apply 100 VAC across the 20 turns, the voltage at the tap would be 50 VAC.
On variacs configured to raise output voltage above the input voltage, there is typically a tap at 20% of the # of turns from the top, The input voltage is connected to this tap. The top of the winding (commutator in variac language) is left open. Thusly when the wiper is positioned below the tap, output voltage is reduced. When positioned at the tap, output voltage equals input voltage. When the wiper is moved above the tap, output voltage is increased to a maximum value of 20% above input voltage, corresponding to the input being applied at 20% of the turns ratio from the top terminal.
There is also typically another tap positioned symmetrically at 20% of turns from the bottom of the commutator to allow the same function with CCW rotation.
Always place the overcurrent protection device on the OUTPUT lead of a variac.
Good video!
Whats the name of the terminals with the red lid your meter is connected to?
I may be interested in one for a 400 vdc variable supply I am building.
They are as rare as hens teeth! It is an old style knife switch, don't know what its proper name is as it was given to me by a friend. But I to would love to know what you call them. I spent ages trying to find the same model online. But, I failed to find another one. I would like to buy a few more! So, If you find out please post back. And thanks for your kind comment.
Kevin Gittemeier : are you talking about the "Cliff quick test"? It's available many places in the UK, as that's where it's made. In the U.S., Amazon carries it. They even have one that is color coded for the U.S. and Canada (black/white/green). They're around $60.
Many thanks for that Brooks!
Thanks!
Great job my friend, very good clear explanation.
Thanks Jace, I value your opinion very much.
hello you can put the link where you can buy the variac and the transformer? thank you
Google, you lazy bastard.
Not claiming to be an expert here, but wouldn't there be no harm in feeding earth ground all the way into your device under test which would mainly be for the discharge of static electricity if it were to accumulate? At the device under test, the two hot wires are not referenced to ground I don't think, so it still wouldn't shock you by touching one of them and earth ground I don't think.
Thanks a lot for posting this, I like your analog meters!
Isn't your variac drawing musing a primary winding?
Excellent job, that gets you a thumbs up and a subscribe with notification!
neat... one question, i'm not a newby but having made this apparatus i think you can answer me quickly: what happens on the second transformer if you undervolt it?... i mean, transformers are not "precision" devices but their sizing is all held togheter, by turns amount etc, i perfectly understand that the voltage will follow the variac but i'm not sure about the current capability, if they work like audio output transformers it should be a matter of turns ratio, changing just the voltage and not the max current capability, but maybe i'm missing something... or maybe it will be just a loss by the parasitic resistance of the windings of the isolation transformer (little i hope)
What do you mean by undervolt it? Don't understand your question sorry ;-)
Do you have a blueprint for fitting the ammeter and voltmeters?
they're very easy to attach- just google how
LOL @ "blueprint". Nobody uses blueprints. And if you don't understand how to wire a Voltmeter and Ammeter, then you DEFINITELY should not be fucking about with this kind of equipment.
I'm green but since safety for our equipment and mainly US!
Shouldn't the isolation transformer be 1st?
As long as it is inline then your good to go. Does not really matter that much, which comes first if you make sure they both bolted and wired up correctly within same box. Although I would place the smaller turns of coils unit as second so the first unit sees the inrush current, instead of smaller unit. Which is why I placed my Isolation second as its much smaller (turns of wire) than the variac.
plug it into an rcd for extra safety
"amperage" or "current" not "ampage"
Coil of “WAA”. 🤣
Huh a diy that you have to buy. Wouldn't a better title be something like, "this is not a diy variac?" Also it doesn't require tons of copper.. if requires winding a large toroidal chunk of hard to aquire and probably home cast soft iron with tight and even spaced turns that I've battled with achieving. But its very much less expensive than buying one. Also the commercially available crap produced today allows for more hassle than benefit.
Agreed it is probably not the correct title, but on that argument, if your going to wind transformers (which is just a large component) why not make your own fuses and dial meters as well? In UK these variacs are not much more expensive than the copper used to make them. It is mass production which makes this so.