Re: Yet another newcomer, and xfmr protection

From: 	Gary Lau  27-Aug-1997 1502[SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: 	Wednesday, August 27, 1997 2:22 PM
To: 	tesla-at-pupman-dot-com
Cc: 	lau-at-hdecad.ENET.dec-dot-com
Subject: 	Re: Yet another newcomer, and xfmr protection

>Your long message is very interesting. Perhaps you set a record for the
>most northerly tesla coiler. I'm near Huntsville, Alabama, USA. From my
>limited experience, I've found US neon transformers to be very easily
>burnt out by high voltage spikes; I recommend not only chokes, which you
>have, but bypass capcacitors (from each HV line to ground), and a safety
>spark gap set just high enough to prevent arcing at the normal output
>voltage of the neons. More experienced coilers tell me 5 millihenry
>inductance per choke and 500 picofarads each side of the bypass
>capacitor.  But I believe the safety spark gap, although the simplest
>precaution, is the most important....  <snip>

Agreed in full, but incomplete.  I was using a 15KV/30ma NST with 9 mH 
chokes, 600pF caps, and safety gaps on each side.  As such, I was seeing
terrible (2") arcing between my tank circuit and ground (like the bottom
of my secondary and gap cooling fan).  Malcolm Watts on this list pointed
out that my chokes were being shock-excited by the spark gap, just as my
tank circuit was, and were ringing at their self resonant frequency.
The solution was to add some high power resistors in series with the
chokes (I used 3K/50W).  This fixed the arcing.  I now don't believe a
choke/cap protection circuit should be used without series resistance.

Gary Lau
Waltham, MA