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. "


Think about it: When the output voltage matches the input voltage,
the transformer isn't even used. It is simply an unloaded transformer
primary connected to the AC line voltage. At any other voltage, the
current is determined by wire size. If the Variac is configured to put
out more than the line voltage, you have the same limit at the highest
voltage as you do at the midpoint.


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