Variac or Autotransformer Review and Teardown


Blog posting:

In this video I give a review of what is a variac (a.k.a autotransformer), how one works in general terms, some usages, a teardown and I finish with a demonstration.

You can order one of these nice 20 amp variacs from Circuit Specialists (link below).

They are a great local electronics supplier that I use and they ship worldwide.

Here is their selection of variacs:

And here is the variac I use in the video:

Channel:  Todd Harrison

Your initial explanation of an AUTO TRANSFORMER IS TERRIBLE.

Take some advice, Soend some time planning what you will say BEFORE STARTING YOUR PRESENTATION

Josh Cadwell 

Todd I bought a variac and am having some issues. I’m outside my return window so I’m going to have to deal with it. Can I email you some pics of what I’m working with? Thanks in advance.

محركات Elec Action 

Think s

Robin Banks 

Are those copper winding live would you get shocked when its on and touch it by accident without the case?

A M 

Output will vary for most everyone since power off the pole varies +/- up to 10v normally. More than that someone should contact their power company to adjust/repair the supply transformer. I was just at a location with 133vRMS toasting everything but the toaster. Peak2peak was slightly over 180v.


Nice review. My thoughts:-

* If the wires were spade connected, some clown would probably re-install them arse-about.

* Can you could lock the knob properly onto the shaft flat, and then fix the zero offset using the set screws on the wiper collar.

* I would install proper ground wires to all 3 chassis parts, also putting some extra length on the wiring inside the power cover.

* Maybe some hot snot would stop the output sockets flapping about.

Lester Barrett 

I think the dial is a percentage, since line voltage would be different in different places. So if your line voltage was 120V, the meter should read 120V or thereabout when the indicator is at 100 on the dial.

white vamp 

same lines as you mentioned of powering up a commodore.

powering up older test equipment to make sure you don't blow it up.


It is a Chinese variac lol...

Bud White 

I bought the exact unit, a 20 amp dual plug outlet model, and it came in all bent up on one side, ...seems to work okay though.


19:05 wtf was that jajaja.


I have an old adjust a volt standard electric variac. It will do 0-140V AC up to 7.5 amp. When I got it it had an really old 2 prong un-polarized plug. I redid that with a 16AWG 3C cord from an old microwave, then just took a 3 prong adaptor and bonded the ground to the metal frame. I then got a 7.5 A breaker to replace the fuse holder.

I fin interesting at one time I thought variacs were just some wacky part with not much use. I use it like all the time now though, I really like I can use it to test transformers. If I put 2V in and get 10V out then I know the ratio. One time I needed 480V for a project I connected an old microwave transformer to the variac then turned it up to like 20V and that gave me 480 V from the MOT.

The other day I used it I had had to test a transformer with a 208V primary. I had another one that had a 120 primary and 208 output. I took the 120 input connected to the variac for adjustment reasons, then wired the output 208V to the input 208V of the transfor

Enrique Valderrama 

did you measure the inductance of the primary? I guess is high, maybe Henry's but I have no idea how big.

Jp Phelps 

I plugged a receptacle ground tester into mine and under 60v it's says the ground is fine but anything higher it says it has a bad ground. I even scratched of the powder coat off around the ground lug & it still does it, any idea why?

F Dalia 

what is a chasy? or how u spell it...

Javier Rabba 

I think that for every given position of the dial the output voltage will vary depending on what load you connect. The bigger the load (lower impedance) the lower the output voltage will be. Just a reminder. Great video BTW, keep up the good content.

EDIT: I just realized after this vid that autotransformers are just variable inductive voltage dividers.


Great video. Mine never worked right. I used your video to clean the coils, remove the armature, adjust the tension rings, reset the armature and reassemble. It works much better now



h barton 

Hi Todd,

Thanks for the review of the variac. I was on the market for one and found your review and helped me pick the same one you have. On bit of good news - the company must have reviewed your comments and added a earth ground wire to the cage! I took mine apart to check the quality and when I took off the front panel, they had already added the extra Earth connection over to the cage (I did add a star washer to make sure there was a good electric path through the paint).

Thanks again for the review and take care,



How do you find that CSI3005X power supply in the back ground. have you done a teardown and review of it?

SwapPart TV Repair 

Great video! I have the SC-10T which is rated for 10A and is isolated, but the front panel is identical. I'm planning on modifying it to add a digital display with an ammeter and volt meter. Did you notice that if you pull the fuse, the light on the switch stays lit. They have the fuse after the switch (between the switch and transformer). Doesn't make much sense to me.

Ray M 

Another awesome video sir, I recently acquired 2 Superior Electric Co 3PN116B Powerstat Variable Autotransformers off ebay for 5 bucks a pice! Score ob my part, just need to replace old dry rotted cords. I did have a question for you about the scale on these units. The two i have go from 0 to 100 marked, but output 0 to 140 volts, as well as all of the pictures i have seen show a scale of 0-140 volts on them. Is there something odd about the ones i have, or else i have no clue! I took the covers off to check the windings, and the screws appeared to be the original ones. Any guess as to why the output would be labeled this way as to the others i have seen?..


20 amps? I've got a little 5 amp variac. It's good for a starter, but it's getting old.


Hi, just to add regarding the voltage not equal to the dial. In most cases you will find it will only be equal when the input voltage is correct to the system! ( eg. System equals 120v and calibrated @ this voltage from factory) So if the input is not 120v the dial will be out to the output!

damon btc 

if u unscrew and flip over the scale it may have percentage rather than voltage markings

Nuker 9000 

I got one for 5$



I acquired my variac pretty much in the same period as you did you clip, however this is the first time I see your video about the variac on Youtube.

I did modify the wiring so that the fuse is actually protecting the variac's output. Afterall, the variac's input is protected via the circuit breaker on which it is connected. If someone really wants to make it safier, a fuse on the input and one on the output is an alternative to consider too.

As you stated, and also reported by other buyers, the ground connection wasn't adequate. This is one of the first thing I checked when I acquired the variac and fixed it ASAP !

I would appreciate if someone could tell me if they experienced breaker trippings because of the inrush current when they turn on the variac ?

Even if the dial is at the lower setting, no load, the breaker trips when I turn on the device. This is fairly annoying.

Some hobbyists are using the light bulb trick to minimize the breaker tripping, but personaly I'd

su R 

Take care mate....looking forward for more:)

su R 

Thanks Todd,You are a great teacher.Keep it from South Australia


Thanks for the info, I recently found a powerstat 246 variable transformer while cleaning out a barn, unused still in the box. I had no idea what i was looking at, now I just need to figure how much it is worth.


I Like your presentation, Thank you for telling us what is a VARIAC . Great video.

 PSS, didn't know you are in Az..  Hello Neighbor, I'm here in Surprise Az.


Wish you would have shown the bottom of the unit so we can see how well the shaft is supported without a bearing like most other Variac's. The one I had there was problems with the knob having side to side play allowing the unit to lose connection between the brushes and the top of the transformer coil, even allowing some small sparks inside the unit.

Failok Temp 

chintzy is probably the word you were looking for


I just purchased two of these variacs from Circuit Specialists because of the video.


I have the same variac but in the 30 amp version. I'm pretty happy with it. It has wire nut terminals instead of the plugs, two handles on top, and is a bit larger. For the price ($120) plus like $12 shipping it's really quite nice, I've seen name brand variacs that are comparable go for more than double the price. Mine has 2 brushes as well, and the windings look just like yours. They have no problem carrying the current.

They had no provision for a ground so I ground a bit of paint off where a screw goes and ran my ground there.

Also on the accuracy of the dial, it does depend on the input voltage greatly. On the front mine says 110v. My mains is 120v so it's not accurate. My overwind also goes to about 145v which is pretty nice. I made up a little information panel with a secondary side fuse, larger analog voltmeter, and ammeter to make it easier to use. Also added was a bridge rectifier, which makes it a pretty nice power supply. All I'm missing are a few ballasts.

Shawn Hawkins 

The local radio station has a variac. They only use it for a window display along with an old school mixing board. I asked to buy it but they think it looks cool in their window. IDIOTS!!!


Hiya. Nice little review on this. A few things about it though which I'd be unsure about:

(1) That earthing/grounding.... I have my doubts that would pass UK safety standards. As a qualified Portable Appliance Tester (PAT), I'd be kinda inclined to fail that on the basis that there isn't a dedicated earth connection to the chassis. Relying on mechanical fixings to carry-on the earthing continuity like that is a bit naughty. With the way that socket was flapping around in there, I don't think it's going to be too long before that screw loosens-off, and the split second it does..... No earthing to that chassis. NAUGHTY! :-O I'd be straight in there to make a dedicated earthing connection from that earth wire, to the little removeable casing/terminal cover, and the base of the chassis onto which the outer casing is connected.

(2) What's all that hot glue around that power cord in there? Subjecting that cord to that kind of thing could've damaged the insulation inside the power cord. Let

John Seach 

Nice review and good to see its non bias even though its from a sponsor .  I bet this has made ppl save up for longer and get something to last them  I guess there is a good moral to this story DONT by cheap Chinese crap I would recommend to spend double that on a 50 year old one . most of those type are known to not even have an earth even thought there poor excuse for a socket would suggest otherwise .also you output could be high probably because  of poor build quality but you could be getting a 110 to 150V input ?

Mark Zacharias 

My 25-year old Tenma variac has exactly the same non-linearity with respect to scale markings. I adjusted it to be more accurate at the higher end.

Todd Harrison 

@Eric William, Thanks. True, 210mA seems low but it was a Harbor Freight item so maybe its broken. I ran my 5Hp air compressor at ~14A for 10 minutes on the variac and the variac didn't even get warm. Nice.

MKme Lab 

210 milliamps for a wood burning iron? I would have expected much higher. Gread vid and great piece of equipment. Cheers


I don't think there would be enough winding inductance for 240v.

Frank Sandqvist 

Yeah, non linear scale was pretty weird. Must have been a design goof! :)

Frank Sandqvist 

The inaccuracy is probably because of variations on the mains. Don't know about the US but +/-10% is allowed here, I think.

Always a joy to see your vids :D


I assume you meant "compared to no load" instead of no variac. Interesting that it doesn't vary too much with a load. Good job testing it to make sure! that's not a serious difference in voltage and I probably wouldn't mess with it either.


Prolly it'd be fine, ofc without the volt meter. You'd have to interpret the scale as a percentage scale.

Btw for the same power output in a 230v line you require half the amps you'd need in a 110v so you wouldn't need to bother about the fuse either.

Piku's Junk 

That 110V variac would work on 240v mains, wouldn't it? Only the scale on the dial would be wrong and the little volt meter would go "pop".

Or would the variac windings get hot and melt? If not, is the only difference between a 110V and 240V variac the label on the dial and the little voltmeter?


Excellent video. Shouldn't be too hard to modify a digital volt meter into the variac instead of the needle meter.

Edward Hartmann 

I think that I would glue a clock hand to the bottom of the knob. That would give you a more precise and much easier to see adjustment indicator.

Malc Tulloch 

Great video, very interesting, good job your lad was there to remind you that you were still recording LOL, many thanks.