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Re: Unexplained arcing current

Gary Lau wrote:
>> I just saw something I can't explain.  For as long as I've been coiling,
>> the resistors in my NST protection circuit have been getting hotter than
>> expected.  The power defined by I**2 R, I being the NST current, never
>> came close to explaining how hot they got, so I assumed that the
>> additional wattage was due to a reverse transformer action.  After the
>> gap quenches, the energy in the secondary could couple back to the
>> primary, although it won't be tuned any longer now that the gap is open...
>> Not a very good theory, I know.
>> 
>> Tonight I shorted my gap, essentially shorting the NST, but having the
>> current pass through my RC network.  My R's are each 1.6K, 113W, non-
>> inductive.  C's are 450pF.  Metering the current, I got 73mA (mine is a
>> 15KV/60mA NST), and the resistors barely got warm.
>> 
>> Next I removed the short across the gap but broke the connection between
>> the tank L and C, so that the gap arcs, but no cap charging/discharging.
>> Now the resistors get hot as a pistol in no time.  I tried measuring the
>> current to the gap, but my digital meter just wigged out, I'm probably
>> lucky it still works.
>> 
>> So, what's different that causes the R's to see so much more power going
>> to an arcing gap?  It's just down to 60 Hz stuff now.



Antonio Carlos M. de Queiroz <acmq-at-compuland-dot-com.br> wrote:

>A very possible reason is DC in the resistor due to repetitive firing
>at low voltage:
>
>Assuming that the series reactance of the transformer is much higher
>than the resistance or the resistors, with the gap shorted you have
>only the current limited by the reactance:
>Ioutrms=Vrms/(w*L)
>Where Vrms is the open circuit rms voltage of the transformer,
>w=2*pi*60, and L is the transformer output equivalent series 
>inductance. Vrms=Vpeak/sqrt(2).
>
>But, if the gap fires at low voltage, instead of the steady-state
>current, what appears is a sequence of transient currents.
>Assuming a sinusoidal input, and the gap firing at 0 volts
>at every cycle (simplification that results in the worst case),
>we have:
>Vin=Vpeak*sin(w*t)
>Iout=Vpeak/(w*L)*(1-cos(w*t)) (cosinusoid + DC)
>Ioutrms=Vrms/(w*L)*sqrt(3) (rms value of the above waveform)
>
>The dissipation R*Ioutrms^2 at the resistors is 3 times higher!
>
>Make a test: If the gap fires at high voltage, the current
>waveform shall be close to the steady state waveform, and the
>resistors would heat normally. With the gap firing at low 
>voltage they shall heat more, up to 3 times more.
>
>Antonio Carlos M. de Queiroz


I performed a test.  With the gap spaced about 1/16", the resistors ran
just warm, not much different than if the gap were shorted.  If I open the
gap up to 3/8", then the resistors get hot rapidly.

Gary Lau
Waltham, MA USA