Original poster: "Peter Terren" <pterren@xxxxxxxxxxxx>
How often have you had a circuit work by sheer luck Steve?
As Samuel Goldwyn said, "The harder I work, the luckier I get". I
had constant monitoring of the spikes on the CRO and had snubber and
MOV's in place. It was driven by a PCM motor speed controller from
a TL494 and had output transistors in the driver.
I guess you lose a lot of power in the snubber ?up to 50%, but I
didn't blow an IGBT.
I wasn't seriously expecting this to work well. If it was this easy
clever people would have already designed a simple circuit on a
small board that can be made for a few cents in China and everyone
would be selling their variacs on eBAY.
>Re "electronic variac" using a bridge rectifier and a
> single PWM FET.
If this circuit works with inductive loads, then it's
only by sheer luck. There is no path for the current
to free-wheel when the FET is off, so you wind up
either destroying it, or having to use an oversized
snubber that wastes a lot of power to keep it alive...
Steve Conner