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Re: electronic PWM variac

Original poster: "Sean Taylor" <sstaylor@xxxxxxxx>


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.
It's better to use four FETs and make a synchronous
buck converter with bidirectional switches.


Or you could stick with 2 FETs and have the bridge rectifier topology both
from source to load and across the load, alternating when each FET is on.
Perhaps a simple solution would be to use 2 IGBTs (antiparallel, no diode)
across the load, leaving the "correct" one on during each half cycle.  Or,
go with the buck converter topology :-)

I started down this road. But I soon realised that the
electronic ballast will never behave the same as an
iron one because it has nowhere to store energy.

Steve Conner 

I have also thought about (and designed) a full buck converter to serve as
a variac and ballast, but never got around to building it!  I really
should just sit down some time and build the thing, though I haven't
actually built a coil in so long (even more for a spark gap coil!) that I
don't entirely feel the need.  As you said Steve, you also lose the
ability to store energy, though you can fool with the current limiting :-)

Sean Taylor
Urbana, IL