On 2008-07-09, flipper wrote:
On 9 Jul 2008 09:41:29 GMT, Jasen Betts wrote:

On 2008-07-06, flipper wrote:

Having the fuse on the input to the variac provides progressively less and
less protection as the voltage setting is reduced - finally providing none
at all.

Well, even an output fuse isn't 'perfect' because max current at mid
scale is less than at 1-1 so if you 'protect' mid scale you limit the

assuming the device is wound all the way with the same gauge of wire
max current output near mid scale is more than anywhere else (except at
exactly 100%, or 0% where the only limit is the current handling capacity of
the wiper!)

You need to tell these folks about it, then.

http://www.ietlabs.com/Variac/Variac_How_To_Select.html

"An autotransformer cannot supply as much current at midrange settings
as it can at full-voltage setting without overheating. "

my statement is in full agreement with theirs

At full (100%) setting the autotransformer is simply acting as a shunt
choke, it's not participating in the load current at all.

But, I now see that even though the lowest currents are flowing
in the windings with the wiper in the middle, total I-squared-R peaks
there, so although that gives the most even distribution of heat it
also gives the most heat.

So even though the individual turns are putting out nearly 4 times as much
heat when the knob is at 95% than they do at 50% that localised
heating isn't an issue as it's only 5% of the turns?

I had assumed that with a variac, given that it has a single layer of
turns most of the cooling would be from the indiidual turns straight
to the air. and that localised heating would be more significant than
total heat. Was I wrong?

Bye.
Jasen