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Re: electronic PWM variac



Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 04:18 PM 8/18/2005, you wrote:
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


Ah yes... but a TC with a pole pig isn't particularly inductive. At least, it shouldn't be.. the pole pig has fairly low leakage inductance. There's not much energy stored in the capacitor charging part of the primary, so freewheeling might not be much of an issue.

Driving an unloaded NST is another matter.