In pre-testing a junkbox Variac before using it for filament control, I
watched it arc and spark and burn out the winding near the high voltage end.
Does anyone do Variac repair and rewinding? Or is that another "lost art?"
As a side question, I was running the variac into a step-up transformer,
which was unloaded. Does the primary of an unloaded transformer present a
significant load? Some Specifics: 5A Variac driving a 120:240VAC transformer
of unknown power capability, weighing probably about 10 pounds. I know if I
put a 2.5 A load on the 240V side, I'm drawing 5A through the primary, so
I'm staying well below that level. The question is, do winding resistance,
core losses, etc, add a large enough load to fry the Variac in front of the
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