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

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

Original poster: "rheidlebaugh by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>

The resistors used are wire wound resistors that form a low Q series RL
circuit with "no" shunt capacitance to stop the reactance from isolating the
NST from the pulse current of the tank circuit.
  Robert  H

> From: "Tesla list" <tesla-at-pupman-dot-com>
> Date: Sun, 13 Jan 2002 13:08:09 -0700
> To: tesla-at-pupman-dot-com
> Subject: RE: Trigger xfmr "grounding", and R-C protection networks
> Resent-From: tesla-at-pupman-dot-com
> Resent-Date: Sun, 13 Jan 2002 16:23:35 -0700
> 
> 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
> 
> 
> 
>