Re: electronic PWM variac

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Brady Hauth <bhauth@xxxxxxxxx>

Alright, any thinking about light ballasts is way off. What you would
use for a pole pig coil is an IGBT, and those are quite reasonable per
watt. <http://www.mpja.com/productview.asp?product=15539+QT> is 800
watts/$ at 120v. There's your solution, really. The small inductance
of the pole pig should limit current at a high enough frequency.
Putting in a small capacitor across the pole pig will prevent heat
buildup in the IGBT.

In terms of what you can then do - With IGBTs and a pole pig, you have
a flyback! Also, if you have a continuous arc, you can use that as an
audio transducer. (the limiting PWM would be an ultrasonic tone above
the audio input)

> The "buck a watt" figure is way high.
> They're very cheap and used in all of the screw-in fluorescents intended
> to replace incandescents.  Those retail for about 5 bucks.

I believe those cheap ones use magnetic ballast, and the 5 buck ones
are about 15 watts. It does depend on how mass produced they are - you
were talking about 100 pounds of MOT, which is bigger than
<http://www.adorama.com/AR560806.html>. Nevertheless, my figure was
high.

> This is a
> case where I suspect cost is almost independent of power output.

That would make little sense. The semiconductor cost is proportional
to about the max voltage * the max current, just like, in this case,
the max power is. If you have a circuit where this isn't the case, I
know some coilgun guys that would like to see it.