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Re: Unexplained arcing current

Hi Gary, All,
    An RCR configuration with a resistor between the transformer and
the bypass capacitor is what I had intended.  Sorry for the confusion.

    The secondary of a high voltage transformer sometimes possesses
considerable capacitance.  This stored energy gets discharged (limited
only by the resistance and inductance of the secondary) through the
protection network along with the bypass capacitors.  This also shock
excites the secondary into oscillations that can cause dissipation and
eventual thermal run away in the very poor tar dielectric. I did a
series of experiments and posted them here several years ago.  Look in
the archives under NEON.  These experiments demonstrated that adding a
capacitor to the secondary of a neon transformer (could be
extrapolated to any high voltage transformer) caused it's terminal
voltage to increase by resonant rise.  This extra voltage applied to
the capacitance of the secondary and bypass capacitor represents
additional energy that has to be dissipated somewhere.  The
transformer insulation usually heats up, carbonizes, and tracks,
shorting out the transformer. I showed that varisters will absorb some
of this energy, but as Dave Sharpe has pointed out, they have a
limited lifetime.  I also showed that resistors will absorb a lot of
this energy, thus protecting the transformer from itself, the bypass
capacitor, and the Tesla coil primary tank RF.




----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, May 27, 1999 7:02 PM
Subject: Re: Unexplained arcing current


Original Poster: Gary Lau  27-May-1999 0714 <lau-at-hdecad.ENET.dec-dot-com>

There WAS a resistor between the bypass capacitor and the gap.  The
two 450pF bypass caps were each across the NST terminals and ground,
and
the two 1.6K/113W resistors were from the NST terminals to the main
gap
terminals.

I believe this situation where the tank was disconnected resulted in
MUCH
hotter resistors than in normal operation.  I think the explanation
that
I gave where the bypass cap energy is dissipated in the R's adequately
explains the heating.  The same thing occurs in normal operation, but
since the BPS is so much lower, the additional power from this is not
that significant.  If I assume my main gap fires at 25KV 150 times per
second, my bypass caps would only contribute about 10.5 Watts to the
two
resistors.


Regards, Gary Lau
Waltham, MA USA

>Original Poster: "B**2" <bensonbd-at-erols-dot-com>
>
>Hi Gary, All,
>    What happens when you put a resistor between the bypass capacitor
>and the gap?


>>Original Poster: Gary Lau  25-May-1999 1022
<lau-at-hdecad.ENET.dec-dot-com>
>>
>>I believe I've figured out why my resistors get so hot.  Well,
almost.
>>
>>Recapping, with my tank circuit disconnected so that my 15KV/60mA
NST
>>just fires my 0.36" static gap through my RC protection network, the
>>series resistors (1.6K/113W non-inductive film) get hot very
quickly, but
>>if I disconnect the 450pF bypass caps, they do not.
>>
>>I realized that with my tank cap no longer in place, the bypass caps
will
>>charge many, MANY times per mains half cycle.  I quick simulation
showed
>>that if the static gap were set to 15KV, it would charge and
discharge
>>128 times per half-cycle.  The RMS current through the resistors is
over
>>250mA!  That will do it!
>>
>>Only problem is, I also have this hot resistor problem to a similar
>>degree when my tank circuit is in place and the bypass caps may only
>>charge the same number of times as my main cap does, a couple of
times
>>per half-cycle at most.  Still looking...