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Re: [TCML] IGBT variac dimmer?



OK guys,
You'll have to forgive me, I am but a humble spark gap coiler, and know little of IGBTs, so I will tell you more precisely what I have in mind (schematic to follow when I have time.) OK, 240V into a rectifier, filter cap bank, nice smooth lethal DC and 300+volts (should drop as the coil runs, and including bleed resisters through some kind of led for safety.) That will be fed to two IGBTs, one to the positive leg of the cap, the other to the negative, the "grounds" of the IGBTs will of course be hooked to the "primary" of the pig. This should allow for voltage throttling in both directions of the AC cycle, assuming the IGBTs have built in freewheel diodes, which I believe they can. As for the driving part, what voltages slash currents are required to drive an IGBT of say 75-100 amps? I was planning on generating a signal using an ATMega driven ARduino, I can program it to sweep a duty cycle from zero to 100 and back to zero in any length of time I wish, PWM frequency by default I believe is 500 Hz, I can the program another output to do the same thing, but after the first one turned off (feeding the other IGBT) so this should mimic a sine wave, as long as the "on" and off period for any given side is 1/60th or 1/50th of a second (experimenting with other frequencies can come later, since this is going to be asynch gap, there isn't much point in changing the frequency.) This only gives me 4 pulses per half ac cycle, which concerns me, I can increase the PWM frequency as high as I want but it requires much more difficult programming. Anyway I suppose we could use this for current control as well, but I would rather use an inductive ballast to limit to 40-80 or so amps, so my IGBTs and a shorted pig are never the only thing between my fully charged filter cap (pop goes the IGBT.) I think this should work, I see no reason it would not, other than I doubt an ARDuino can drive an IGBT that size, but I could build a tiny amp/filter for each output, that should not be too difficult. The way I figure, this should be substantially lighter, about the same cost, and a much much greater learning experience than a standard variac, the only down side is no overdrive... OK guys rip it apart and tell me if this is a waste of time, or a potentially good idea...

Scott Bogard.

On 9/17/2011 4:49 PM, Jim Lux wrote:
On 9/17/11 7:33 AM, dave pierson wrote:

As an aside I believe that the humble dimmer switch contains a
thyristor (SCR) so the wave form which it produces is a chopped
version of the mains AC, not a sinusoid of lower magnitude.
     I suspect, more commonly, a triac
     (think: bidirectional SCR.).  The comment on
     chopped/distorted sine is well made.
\

OTOH, a good low pass filter would turn that phase controlled sine into a nice sine..

Considering that you might already have a honkin big inductor (technical term, that) for current limiting, such a LPF might not be a big deal.

You could also do what utilities do for cycloconverters and DC links...

Build tuned filters to suppress the specific harmonics..
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