Re: Testing caps -> NST protection (fwd)

---------- Forwarded message ----------
Date: Mon, 27 Jul 98 09:07:53 EDT
From: Gary Lau  27-Jul-1998 0854 <lau-at-hdecad.ENET.dec-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: Testing caps -> NST protection

>From: Terry Perdue <terryp-at-halcyon-dot-com>
>Subject: Testing caps
>I posted a message a few weeks ago about some oil-filled 10nF 80kvdc "xray
>machine" caps I have, asking if they might be suitable for TC use. Several
>people thought it was worth a try, but suggested a safety enclosure, in
>case they overheated and exploded. My question is this: 
>Is there a way I can test them for suitability before building the whole
>system? I have a NST, and could fashion a temporary spark gap. I'd like to
>run a minimal system long enough to give me confidence that these caps
>will survive before going any further, as I'm not interested in rolling my
>own cap. 
>Two other questions: 
>I had hoped to use 10" diam acrylic tubing for the secondary, but at
>$40/ft, may have to use PVC. But I'm considering getting some sheet
>acrylic and melting it around a form, possibly in half-cylinder sections,
>then cementing them together. I've been pretty successful forming
>plexiglas in this way in the past, and assume that it is the material of
>choice. Has anyone tried this? (I'm assuming that the sheet stock is much
>less expensive.) 
>Finally, I think I read that if the spark gap is close enough to the
>transformer, secondary RF suppression isn't necessary. Is there agreement
>on this? 
>Thanks for your comments.
>Terry Perdue

Re. RF suppression, I've heard this suggestion too and I believe this is
misguided advice.

Terry Fritz has made some interesting measurements and discoveries about
the nature of primary gap/tank circuits, in that with each zero-current
crossing, the gap extinguishes, each time exciting high frequency
oscillations due to parasitic L-C components.  Measurements with his
fiber optic voltage probe of Vgap show brief high voltage bursts of 2X
Vpri at each zero-current crossing.  This is due to C-self of the primary
inductor, fully charged at a zero-current crossing, resonating with the
primary inductor.  180 degrees into this VHF oscillation, the voltage
across C-self reverses and since it is in series with the tank capacitor,
the two caps in series now present a voltage of  2X Vpri to the gap 
AND TO THE NST POWER SUPPLY, causing the gap to re-ignite.  This phenominum
is not influenced by lead length or inductance between the gap and power
supply (although other oscillations do arise due to this).  A train of 2X
Vpri (~40KV!!!) voltage bursts applied to an NST is not too healthy for

The up-side to this however is that the very high frequencies involved
are very easily attenuated with a simple R-C low pass filter, since it's
tens of megaHertz, not kiloHertz, that we're targeting.

Gary Lau
Waltham, MA USA