RE: Trigger xfmr "grounding", and R-C protection networks

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Gary,

At 10:34 PM 1/12/2002 -0500, you wrote:
...
>This made me begin to question the necessity of the R's in the R-C network.  
>The question of just exactly what the R-C protection network protects the 
>NST against has not received a great deal of attention.  My personal theory 
>is that the offending thing is the brief but high voltage spikes generated 
>during the gap's zero-current crossings.  If true, bypass caps alone across 
>the NST secondary will shunt these spikes to ground.  When the gap is 
>conducting, its impedance is low enough that there are no hazardous voltages 
>across it to protect against.  And when the gap is not conducting, there is 
>no tank circuit and no HF oscillations, so again, nothing to protect against.
>
>So, am I overlooking something, or are the protection network resistors less 
>than essential?
>
>One further observation of a resistor-less protection network - the 2 NST 
>bypass caps would be in series and across the main spark gap.  Each time the 
>gap fires, the bypass caps would be discharged directly into the gap with no 
>current limiting, so perhaps some very low impedance resistor would be 
>advisable.  I don't know if this would have any other repercussions relating 
>to gap operation.
>
>Gary Lau
>MA, USA
>

The RC protection filter is just that.  Without the resistors, the filter
caps would be shorted directly by the main gap with very high currents
(especially if using pulse poly caps).  This would damage the caps, burn
the gap, and add to radiated noise.

Basically, "I" am trying to keep high-frequency high-voltage components
away from the NST.

http://hot-streamer-dot-com/TeslaCoils/Misc/Filter.jpg
http://hot-streamer-dot-com/TeslaCoils/MyPapers/rcfilter/rcfilter.html

The resistors control the cutoff frequency and do add some inductance.
Without them, a high energy pulse would hit the caps directly and probably
go through them since they are too small to stop much by themselves.

There is no doubt that protection filters do vasty reduce the number of
blown NSTs.  What is not known is if that is from the MOVs/safety gaps
insuring there is no over voltage, or if the high-frequencies really are
doing harm.  I would "guess" that 90% die from over voltage and 10% die
from high frequencies degrading the secondaries.  The filter was meant to
stop all known possibilities...

NSTs secondaries may have their capacitances and inductances distributed
enough that a high frequency spike is not concentrated in any one area.
Some of my testing suggests this.  If so, this was probably do to trial and
error.  It is noted that all NSTs have converged to almost exactly the same
design over the years...

So one may do fine with just a safety gap.  But the RC filter was meant to
protect against "everything" (one should use an input fuse on the NST too
to protect against shunt saturation).  It "may" be overkill, but "I" like
it :-))

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg

Cheers,

	Terry