Re: Recent s.s.t.c. work

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Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

I see that my last posting on this topic was in December.  In the 
interim I've been slothful to a fault (altho happily busy for 3 of 
the weeks entertaining our Most Perfect Granddaughter, 2 1/2, 
visiting from Berlin).  But I have now gotten to the stage where I'm 
ready to put the H.V. to it once again.  I've checked the gate 
waveforms & all 4 appear OK.  Next, it's... turn up the variac & 
watch for the smoke.  It's only trepidation, accumulated from years & 
years of such practice, that keeps me from doing it today; So perhaps 
I'll first just sit & think about it for a while...

Someone asked, after I reported my last failure (death of an IGBT 
brick), what might have caused it, and at the time I didn't 
know.  But while rebuilding the drivers, I discovered that I had 
positioned a wire-wrap pin, in one of the gate circuits, so that, 
when I fastened the board down above the mains capacitors, the pin 
pressed against one of the capacitor terminals--hidden from view, of 
course.  I didn't locate the source of the resultant smoke until I 
started rebuilding.

As I've already reported, I utilize NPN/PNP emitter-follower 
driver-pairs for each of the 4 H-bridge IGBTs, transformer-driven, 
with the 4 transformer signals always applied (from my "pilot 
oscillator")--and now rebuilt with opto-isolators acting to gate-on 
drive to the NPNs during the spark-event times.  That way, all 4 gate 
voltages are kept at -28 or so between sparks by the continuous drive 
from the PNPs.  As before, the continuous transformer signal also 
serves to keep the + and - drive-supply electrolytics charged up.

So stay tuned, so to speak--& don't be startled by smoke seen coming 
from the vicinity of California; it'll only be me once again.

Ken Herrick