16th August
written by Todd Harrison

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. They are a great local electronics supplier that I use and they ship worldwide. Here is a link to see their selection of variacs and this is the variac I use in the video.


Some user feedback on YouTube comments compelled me to run the below tests.

Testing variac under loads:

First to recap, the best I could adjust the knob and cage with original voltage scale was still 13v off at 120v when tested against a Fluke 117 and Kill-a-watt. I then marked in my own scale 0-145v in 10 volt majors and 5 volt minors.

For loads I I tested a vacuum cleaner at 30,60,80,100,120v ranges from 0-3amps. My custom voltage scale was never off by more than ~0.3v under load.

I repeated the same test and ranges with a 10 inch miter saw with a max of 4.5amps and my scale was within ~0.8v under load compared to no load readings.

The final test was using my 5 horse power Craftsman compressor as seen in an older repair video. Tests where done with the tank ~90% full to maximizes the loading. It pulled 13amps at 120v which is the only voltage I dare test with my compressor. My scale was off by 1.5v under this load. I even ran the compressor for 5 minutes on the variac and it was stable the whole time with a temp increase of 5 degrees F for the variac windings.

I repeated the testing of the same loads without the variac. Each device was connected directly to mains. My mains reading with no load was 119.8v. With the vacuum cleaner there was no change in line voltage. With the 10 inch saw the lines dropped 0.3v. The air compressor pulled the lines down the hardest at a 1.0v drop to 118.8v.


The variac did have some performance impact but not as much as I was thinking it would for higher loads. I would say this 20amp rated variac could have handled a lot more with no worries. I would also add that my custom scale is very reliable as long as I use it at my bench which has a a very short dedicated 20amp drop to the service box literally on the outside wall behind my bench wall. I could have tested more loads at once to get closer to 20amps but my lights take a few amps on this circuit and I can just extrapolate that at ~18amps load my custom variac scale would be reading ~2.3v high. So for higher amps a volt meter or kill-a-watt would be recommend if you need the exact voltage readings. The original scale, even under max loading, would still be set 10v too high if used as the 120v reference point. And that is after tweaking the cage and knob to bring it as close to a true reading as possible.



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