electronic PWM variac

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

A few months back, someone mentioned that they had built an 
"electronic variac" using a bridge rectifier and a single PWM FET. 
(I've done this for non-tesla coiling loads, like motors.)

I was curious about the ratings of that FET?

If I wanted to build something along these lines rated at, say, 50A, 
what sorts of inexpensive switching devices are there out there?  It 
would be driven by something like an 555, and the chopping rate can 
be pretty low (few kHz to maybe 20 kHz or so), so it's not quite like 
running an SSTC at 300 kHz.  I'd use a isolated power supply to power 
the 555 and FET drive circuitry.  The 60A, 600V bridge rectifier's 
not a problem.


How simple can this be made?  A 555 can swing the voltage to turn 
most FETs on, but does it have the drive capability, and the 0-100% 
duty cycle you'd want? I don't know that you'd care if the switching 
frequency changed (over a reasonable range), so maybe you could use 
dual timer (556).. shoot out fixed 100 microsecond long pulses at a 
variable rate.  Do you need a more FET friendly drive waveform (spike 
and back porch?)

Has anyone tried to turn one of these into an "electronic ballast" 
that synthesizes the effect of series inductance into a SG coil?

Jim...