Re: HV Xfmr Protection networks

From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Thursday, June 26, 1997 2:31 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: HV Xfmr Protection networks

Hi Gary,

> From:   gary Lau[SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent:   Wednesday, June 25, 1997 10:51 AM
> To:     tesla-at-pupman-dot-com
> Cc:     lau-at-hdecad.ENET.dec-dot-com
> Subject:    HV Xfmr Protection networks
> All of the archived materials I've seen on this subject
> recommends using 3-4 barium titanate doorknob caps in series
> for the bypass caps.  I understand that these are considered
> "lossy" at high frequencies, desirable in this application.
> But is this necessary?  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.