Re: HV Xfmr Protection networks
From: Gary Lau[SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Friday, June 27, 1997 8:40 AM
Subject: Re: HV Xfmr Protection networks
>> I was planning on making caps from
>> .09" thick fiberglass circuit board stock, easy to work with and
>> easy to get any reasonable value. With a dielectric rating of
>> 11.8 pf/sq inch (measured) and 700V/mil (from MATH.TXT), this
>> should be good to 63KV DC, maybe 30KV AC? As I'm using OBIT's
>> which have a center ground, I'd use a pair of these caps,
>> shooting for ~700 pf each to RF ground.
> I think your ratings for PCB are way too optimistic (from
> experience). It _might_ withstand the voltages you suggest for _some
> types and thicknesses_, but it completely ignores the high stress you
> get at the copper edges. I have blown the odd one at 18kV pk from a
> 12kV neon transformer running under oil.
Thanks for your reply. I was planing on etching a .5" margin
around the perimeter of the plates. Did your's have this or was
it just sheared? Was your failure arcing across the edge of the
board or was there an actual puncture?
Also, for a center-grounded xfmr like a neon or OBIT, these caps
should go from each HV lead to ground. For your 12kV neon
transformer, each cap should have seen (12KV x sqrt2)/2 or about
8.5KV peak. Or is there some mechanism that I'm not aware of that
applies much higher transients (18KV?)?