The renowned wrote:

>     NEGATIVE!!!!   
> First off Variac is a trade mark of ( I forget but think it was General
> Radio)  AND MOST IMPORTANT!  are not isolated but are a single winding
> on the core with a brush riding on a track for the output. So one terminal
> is common to input and output, this should always be the "neutral" line.

There's a few manufacturers, Staco and Superior (I think the latter is the
original.. could be wrong tho)

The schematic looks like this (often, though some don't have the extended
winding from A to B): 

B   (
     )<----------o Out L
C   (
o----------------o Out N

C goes to the incoming neutral. If you connect incoming Line to B you get
something like 0~140 VAC out (with 120V in), if you connect it to A, you
get 0-120VAC out with 120V in. 

As others have said, there is no isolation, the winding is shared between
primary and secondary (an autotransformer), and as has already been said,
the wiper is a weak point. The Out L line should be fused because it's
quite possible to draw excessive current through the wiper and burn the
winding at that point. 

Best regards, 

Spehro Pefhany --"it's the network..."            "The Journey is the reward"             Info for manufacturers: ""
Embedded software/hardware/analog  Info for designers:  ""
Contributions invited->The AVR-gcc FAQ is at: ""

Date: Thu, 13 Apr 2000 18:15:18 GMT

Original Subject: Re: What is a Variac?

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