At 07:38 PM 2/2/00 -0700, measures wrote:
> >Did you see the precaution against using so-called equalizer resistors in
> >the post 1994 Handbooks? What value of R ans watt rating?.
> >thanks
Rich, I have seen precautions against using resistors and against NOT using
resistors. I have also sen precautions both ways for shunt caps. I have
seen no real proof of any of it. I have hard proof in the form of hundreds
of dead diodes that it is equally easy to blow diodes with any form (or
lack thereof) shunt equalization you could think of. In the 40 or so years
I've been digging into such things, I have noticed that almost without
exception commercial power supplies made from discreet diode stacks have
some form of equalization. Those supplies that utilize potted rectifier
assemblies don't seem to have any shunt R inside the units. I assume that
to mean that if you can't grade out the diodes, it might be a good idea to
at least attempt to equalize the PIV. I don't care a bit about individual
diode reverse current as long as it's well within spec.
The resistors I happened to use this time were 390k 3 watt metal film. I'm
tempted to take you advice, and rewire the assemblies without the
resistors. That way I can get twice as many diodes in! If it is/was a PIV
related problem that should help, but them I might never know why this
failure occurred.
> >You did not state the manufacturer.
That's correct, I did not. I would have if I had any idea who it was.
There is step-start in place and functional. There is glitch protection.
One more time, the failure DID NOT occur on start-up. It DID NOT occur
during a big-bang or other disaster. The whole thing was loafing along at
about 1/3 design limits when the damn diodes just rolled over and died.
Never before have I experienced diode failure at anything but turn-on or
big-bang. I have almost convinced myself that it was just a bad batch of
cheap diodes, but yet at the level it was running at, anything should have
survived.
So the question remains, if 3 amp diodes blew with only 700 ma. load, what
does it take to keep it running at 2 amps?
73,
Larry - W7IUV
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