RF destruction of caps and trannies

From:  FutureT [SMTP:FutureT-at-aol-dot-com]
Sent:  Wednesday, January 28, 1998 9:14 AM
To:  tesla-at-pupman-dot-com
Subject:  RF destruction of caps and trannies


I've been giving some thought to the problems of capacitor and
transformer destruction in Tesla coils.  It is well known that high
break rates promote capacitor failure.  It also seems to me that poor
gap quenching may also lead to capacitor failure by *assaulting*
the cap for a longer period of time, and with more RF cycles, 
during each gap firing.  As an added benefit, fast quenching ability 
permits the use of closer coupling, which reduces the gap firing
time even further, protecting the capacitors and transformers even

High break rates and poor quenching may also help cause transformer 
destruction by the same mechanisms; the windings are beaten to
death by long and frequent RF pulse trains.  This may help to 
explain why neon trannies can gived long-lived service when used
with synchronous rotary gaps.  A sync rotory fires only once per
half cycle, and usually quenches quickly, thereby giving the tranny
only a small *dose* of RF.  Narrow static gap spacings may cause
the gaps to fire multiple times per AC half cycle.  I have seen neon
trannies destroyed
within a few minutes while running with narrow static gaps, yet
long-term operation using wide safety gaps and synchronous
rotary gaps have never caused a failure in my neon-powered Tesla
coils.  This too in my opinion favors the views presented here.

Another advantage of sync-gaps over non-sync gaps is that no
gap firings are ever missed or skipped. Mis-firings, which 
can occur in non-sync, or widely gapped static gap systems, can
permit the transformer's voltage to rise alarmingly to destructive
levels due to resonant charging.

Still another possible advantage of the sync-gap system was
suggested by Malcolm Watts some time ago;  Malcolm suggested
that because the gaps quench in a rotary system when the gap
spacing is very narrow, this insures that the voltage will be very
low at the quench, thus reducing destructive kickback voltages
into the transformer.  In contrast, static gaps quench with a wide
gap spacing, resulting in a higher voltage at quench, and higher
voltages kicking back into the neon transformer.

It is well known that magnifiers are rough on neon trannies.  It is
also known that magnifiers are difficult to quench because of the
close coupling.  It seems reasonable to me that this poor 
quenching, which permits the RF to enter the tranny for a long
period of time during each gap firing, may contribute to the tranny's
demise.  I suspect that the caps in a magnifier may also suffer
from the long RF exposures caused by the poor quenching.

It is quite possible that large, high powered magnifiers do *not*
suffer so much from these effects, because the large sparks and
lower frequency will assist the quenching, thus negating the 
destructive effects noted above. 

Another benefit of low frequency operation may be the reduction
of dielectric heating effects within the capacitors and the
transformer windings.  I usually run even small Tesla coils at
a maximum of 100 -- 200kHz.

John Freau