Re: Unexplained arcing current

To: tesla-at-pupman-dot-com
Subject: Re: Unexplained arcing current
From: Gary Lau 29-May-1999 0830 <lau-at-hdecad.ENET.dec-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Sat, 29 May 1999 20:19:39 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

>Original Poster: "B**2" <bensonbd-at-erols-dot-com> 
>
>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.

I'm not sure I see the difference, as far as what current pulses the NST
secondary sees,  between an R-C-R network, and an 2*R-C network.  With
the RCR network, the parasitic capacitance of the secondary is discharged
by a resistance of R+R through the spark gap.  With an RC network,
assuming the total resistance is the same, the discharge current is the
same as well.  Through simulation, with reasonable component and
parasitic values, I see no evidence of NST secondary oscillations at the
gap closures.  Can you suggest a circuit that may show this?

With reasonable break rates, the amount of energy dissipated by the NST
secondary capacitance (small compared to the bypass cap values) is
insignificant, not enough to carbonize the tar.

There is no doubt that a resonant voltage rise occurs when caps are added
in parallel to the NST secondary.  This is only evidant however if the
circuit is allowed to ring over multiple half-cycles, something that
cannot happen with a reasonable static gap, and has no bearing on
parasitic oscillations or power dissipated in the resistors.

Regards, Gary Lau
Waltham, MA USA