Re: HV Xfmr Protection networks
From: John H. Couture[SMTP:couturejh-at-worldnet.att-dot-net]
Sent: Thursday, June 26, 1997 5:58 PM
To: Tesla List
Subject: Re: HV Xfmr Protection networks
At 04:42 AM 6/26/97 +0000, you wrote:
>From: gary Lau[SMTP:lau-at-hdecad.ENET.dec-dot-com]
>Sent: Wednesday, June 25, 1997 10:51 AM
>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.
>For the safety Gaps, do these need to be anything more robust or
>complex than a short piece of heavy copper wire at the proper (?)
>spacing across each bypass cap?
I have found some of those doorknob capa work very well with small coils.
In fact I have found some of them have less loss then more expensive
capacitors. The only way to tell how good they are is to test them at high
voltage. I show how to do this in the TC Design Manual.
The test is simple and easy to implement. Obtain a DC power supply of
about 10 KV to charge the capacitor. Connect a voltmeter and time the
discharge to a certain drop in voltage. The voltmeter must provide
negligible load, otherwise the voltmeter load will discharge the cap instead
of the cap losses. The voltmeter can be an electrostatic meter or an
electroscope. The TCD Manual shows how to build an inexpensive box
electrosope that can be used in this test.
This test will show very cleary which caps are the best to use. The test
is DC and not RF but I have found it very reliable. A cap that is good with
a DC test usually tests good at RF tests. The only way to do the RF test
properly is to use the cap in one or more Tesla coils which introduces so
many more variables that the results usually cannot be relied upon.