Rheostat or ???

[abstruse1]

I'm using an inductive heater driver to drive the FB that runs my CW multiplier, powered by 5, 12v SLA batteries in series. I can tap the battery bank wherever I want to for varing voltages at 12 v. intervals.

The driver is rated for 48 vdc input, but I use up to 60 vdc for short periods while monitoring the temperatures of the MOSFETS and resistors.  I've been using a large 15 ohm rheostat to vary the voltage (mostly used in the <6 ohn range).  Amps run around 15 (~900 watts).  I paralled an 8 ohm fixed resistor with the rheo to spread the load across more windings.

My rheostat gets really hot and, in one case, flashed the windings at one end.

Large, low resistance rheostats are hard to find and expensive.  I suppose that's because they're becoming obsolete in favor of semiconductor devices.

My question is, are there semiconductor devices that would work for me?  Thanx!

[klugesmith]

You could replace rheostat with power transistors operating in linear region.   But they would dissipate just as much power as the rheostat, and fail at junction temperatures that would not bother the rheostat.  Might be easier to cool your rheostat with fans, air blowers, or oil immersion.

Or you could make a duty-cycle-controlled dimmer, using power transistors driven to very low resistance when ON.   That would be a respectable engineering project by itself.

How about an array of big power resistors that are switched in or out of the circuit?

I don't understand the circuit and purpose of your rheostat to "vary the voltage".   Is it in series with the battery, which you are switching in steps of 12 volts?  Is there no power control knob on your inductive heater driver?

[abstruse1]

Nope, no pwr control on driver.

I've thought about an array of power resistors, and that would work, but not cheap.

Yes, I pick the voltage from the battery box (usually 60) and run that in series with the rheo.  That way, I can tune the voltage to  the driver to whatever I want.

I also thought about a hefty bridge rectifier with capacitor to change the output of my Variac (an honest-to-god old one) to D.C.  Would that work?

[MRMILSTAR]

You could use a variac with rectifier and smoothing capacitor if the current rating of the variac is high enough. I don't know how much current your multiplier needs but I would shoot for at least a 10 amp variac. You would have to be very careful adjusting the variac though. One small mis-adjustment of the knob and you could easily fry the MOSFETs in you ZVS driver with over-voltage.

Unless you have a real need for fine adjustment for your voltage multiplier I would just stick with your batteries. They already give you 12 volt tap points. I would select 24, 36, or 48 volts and be done with it.

Have you actually measured the AC output of your flyback transformers? You don't want to exceed 1/2 the voltage rating of the capacitors. Maybe 24, 36, or 48 volts already will yield that maximum AC voltage. My 14-stage VM only needed 30 volts to achieve the maximum safe voltage for my capacitors.