Variac Wiring Question


2016-03-13 1:52 am

I recently acquired an old Standard "Adjust-A-Volt" variac which I've rigged up in a case with volt and ammeters output plugs and posts, to make it useful for amp work.

My problem is that I can't seem to figure how to wire it correctly to get the voltages out that I'd expected.

This one is a little different than some I've seen in that instead of a single panel of numbered terminals, it has 5 terminal posts arranged around one half of the perimeter of the transformer body. But they're numbered 1-5 so I thought my job would be easy.

Based on reading a lot of threads on variacs, I figured that if I wired it like this:


I'd get out a linear voltage rise from zero to about 140V.

Well, that t'aint what happened. :)

What I get out is 20V at the 0% setting, which drops to 0V as I bring it up to about the 15% mark. Then it starts to rise again and tops out at the 100% setting giving me my full house voltage of about 122 V.

Ok, so that's not the end of the world. I at least can get a smooth 0-122V range, but it bugs me that it's not done correctly and I get this bit of opposite phase voltage at the bottom of the range.

So, based on my incomplete understanding of how these things are wired, I figured I could just eliminate the "boosting" effect and wire it like this:


That was not a good idea. :( I got weird readings, very briefly, from the volt meter and then my 2A fuse blew. :dunno:

So now I'm pretty convinced that my variac's numbering of the terminals is not really the same as all the others I've read about.

But I can't quite figure out how they must be based on the results I'm getting.

The one thing I think I'm sure of, is that there is a big copper bar running from Terminal 3 to the center shaft, which makes me somewhat confident that 3 is indeed the wiper, as every other Variac I've read about.

Can anyone help me get over my mental block and see what's going on? How should I wire this thing?



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2011-04-29 8:37 pm
Last edited:
If the variac is a '' open type '' ( no housing )

Turn the knob full counterclockwise ( in this position output voltage should be zero ) locate the wiper .. find the terminal number closer to the point where the wiper is ( the wiper is in contact with the winding .. close this spot on the winding there must be a wire connecting to the winding .. this wire run to a terminal .. ) this terminal should be connected to mains neutral and also to load neutral terminal .

Turn the knob clockwise to the point where you expect to have output voltage at 100% ..... locate the wiper .. find the therminal numer ..( the wiper is in contact with the winding ..there must be a wire connecting to the winding close this spot .. ) this terminal should be the '' live '' from mains

load of course connected to wiper ...


2016-03-13 1:52 am
Thanks guys!

Here are a couple of pictures:


The terminals:


And the terminals from the rear, centered on "3" which I believe is the wiper. (You can see the copper strip that runs to the center spindle.)



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2016-03-13 1:52 am

Here are some more pictures.

A better view of the front of the device, showing the terminals and their leads down into the winding:

A general shot:

With the wiper turned all the way to 0%, it actually seems to line up with Terminal 2, giving me a glimpse at why I'm probably seeing these weird readings:

And here's where the wiper would stop if I turn it to 100%. Going PAST 5, around to 4. Maybe it's just that the numbers don't correspond at all to the numerical percentages of output and I was too silly to grasp that before... :eek:


As soon as I get it wired up the way you suggested, I'll get back with the voltages.


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2016-03-13 1:52 am
With my current limiter (300W bulb) in series in the mains hot line, connected to Terminal 4, and the house mains common at Terminal 2 I get...

1 -- 2: 18.1 V
1 -- 4: 98.3 V
1 -- 5: 79.0 V
2 -- 4: 117.5 V
2 -- 5: 97.4 V
4 -- 5: 19.0 V

And I get a nice linear sweep from 0V to 122 V as I move the dial from 0% to 100%! Great!

I imagine there's another way yet to wire this to get that boost up to 140V, but as I've not yet figured out why I'd want that, perhaps I don't care. :)

Thanks guys!

You have maximum voltage betwen #2 & #4 ...these must be the 2 end of the winding

A low voltage betwen 1 & 2 so #1 must be a tap close to #2

Alow voltage betwen 4 & 5 so #5 must be a tap close to #4

#3 is the wiper

So neutral of the source and neutral of the load should be at #4 ( 2 wire on this one )

Hot side of the LOAD at #3 with a fuse selected according to the variac nominal current

Then the mains ( HOT side ) connect to #2 with a second fuse selected according to the variac nominal current ... and output voltage will be ajustable from 0% to 100% ( no overvoltage possible )

To make overvoltage possible move the main from #2 to #1 ( dont forget the fuse )


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