Re: Testing caps -> NST protection (fwd)

---------- Forwarded message ----------
Date: Wed, 29 Jul 1998 17:40:54 -0600
From: terryf-at-verinet-dot-com
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Testing caps -> NST protection (fwd)

At 09:43 AM 7/28/98 -0600, you wrote:
>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>To: Tesla List <tesla-at-pupman-dot-com>
>Subject: Re: Testing caps -> NST protection (fwd)
>Hi Gary,
>         Since I said it (and have lived by it.....
>> 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.

Hi Malcolm,
        I suspect that there may be high voltage transients that are very
high in frequency.  They may be so high in frequency that the gap cannot
fire and block them.  A separate protection gap my offer more protection do
to the fact that the gap may be cooler and smaller than the main gap and
thus capable of higher speed firing.  What is really needed is a good filter
to absolutely block the transients.  
	This subject is confused somewhat because some neons survive the transients
while others fail.  Thus, there are people that say theirs works fine with
no filters.  I don't think running without filters is a good idea.  I think
the people that don't have problems are just lucky.  Measurements and models
show that neons need good filters if their specifications are to be even
loosely adhered to.

	Terry Fritz