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Re: A s.s. head-scratcher

Original poster: Ken or Doris Herrick <kchdlh@xxxxxxxxx>
Well, as the Bard wrote, it's a tempest in a teapot. Mea culpa, as I no doubt had confessed numerous times to my Latin teacher all those many years ago. At least, it would appear to be so: I have mis-wired. For convenience, I've set up 2 mains-supply pairs of big capacitors, one for each side of the push-pull circuit. Do you start to get the drift? I've connected the primaries & IGBTs cross-wise betwixt the two supplies. I should have seen a bit of current thru the common small-gauge leads from the mains-rectifiers but my ammeter was in an inconvenient position, my eyeballs are weak, and perhaps I didn't notice it. At any rate, before I get my knickers further in a twist, I'll hook up the thing correctly & see what happens.
As to the zeners, Steve, I do have them in series; should have made that more plain. 

KCH

Tesla list wrote:

Original poster: "S&JY" <mailto:youngs@xxxxxxxxx><youngs@xxxxxxxxx>

Ken,

This is probably not your problem, but if your "back to back zener
protection" diodes are in parallel, that will be a problem, because one of
them will conduct at about a normal diode drop of 0.7 volts or so.  The
Zener diodes need to be in series which will break down at their rated
voltage for one of them plus about 0.7 volt for the other if you need
symmetrical protection for signals that can go positive or negative.
Otherwise, just use one Zener to limit the positive and negative excursions,
respectively.
--Steve Y.

-----Original Message-----
From: Tesla list [<mailto:tesla@xxxxxxxxxx>mailto:tesla@xxxxxxxxxx]
Sent: Wednesday, March 14, 2007 7:19 PM
To: <mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx
Subject: A s.s. head-scratcher
Original poster: Ken or Doris Herrick <mailto:kchdlh@xxxxxxxxx><kchdlh@xxxxxxxxx> 

Steve Ward et al-

This one stumps me but perhaps not someone else:  I'm implementing my
push-pull s.s. primary circuit, using two Powerex CM300DY-24H
IGBTs.  Previously it appeared to function but not satisfactorily so
I've redesigned and rebuilt the drivers.  Waveforms now, right at the
IGBT gates, appear perfect.  However...  The IGBTs will conduct no
current; nada, zip, neither of them.  Nothing within them is shorted,
per my ohmmeter, and the gate:emitter nodes are "open" per the ohmmeter.

In order to accommodate my revised driver circuit, which has
back:back zener protection, I had carefully removed the 10K resistors
that were protecting the gates.  Was I not careful enough, do you
suppose?  Could the gates have actually opened up within the
packages?  >Both< of them??  Not hardly, I should think: they'd be
shorted, if anything.  Does anyone have a clue, before I have to
un-mount them & jury-rig a test-setup?

Ken Herrick