Re: Testing caps -> NST protection (fwd)

---------- Forwarded message ----------
Date: Tue, 28 Jul 98 13:35:28 EDT
From: Gary Lau  28-Jul-1998 1326 <lau-at-hdecad.ENET.dec-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: Testing caps -> NST protection (fwd)

>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>Subject: Re: Testing caps -> NST protection (fwd)
>Hi Gary,
>         Since I said it (and have lived by it.....
>> 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
>> >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
>> it.
>If there is that kind of voltage across the main gap, why does it not 
>re-ignite? Why should a gap several metres downstream do so? I'm keen 
>to know.

But I said it does re-ignite.  Let's re-examine the gap dynamics.

At each gap current zero crossing, the primary c-self is charged to the
identical voltage as Ctank.  When the gap current is zero, it briefly stops
conducting and Ctank is no longer in parallel with the primary, but
instead in series.  The voltages across c-self and Ctank are equal but
opposite, so the effective series voltage (Vgap) is zero, so it can't
re-ignite.  Primary c-self is now free to ring with the primary.  180
degrees into this ringing, the polarity of Cself reverses and being still
in series with Ctank, the series voltage (Vgap) is now 2Vpri.  In a
private exchange with Terry Fritz, he provided some scope traces of Vgap
that indeed showed Vgap bursts of 2Vpri at each zero current crossing.
Unclear is exactly when the gap re- ignites, at 180 degrees into the
c-self/Lpri oscillation, or shortly thereafter.  But the important thing
is that the 2Vpri bursts are real, supported by measurement and theory.

I never suggested that a gap several metres downstream (safety gap?) might
fire while the main gap did not.  What I am suggesting is that NST damage is
being caused not by some effect due to inductance in power supply wiring,
but rather from the 2Vpri bursts described above, and that damage from this
phenomenon is easily avoided with a simple RC low pass filter, and not
just by using short power supply leads.

Gary Lau
Waltham, MA USA