[TCML] Re: Spark Gap Resistance
Barton B. Anderson
bartb at classictesla.com
Mon Dec 10 21:31:48 MST 2007
Hi Greg and Bert,
If the 1uH is close, the numbers across each gap:
(.65-.377)/4 = 0.06825 ohms (391V/gap).
It still appears high by 200%. But due to the original primary and the
fact that electrodes would glow red after a short duration, who knows,
it might be close at that point.
When you consider the 1uH around the gap, it's then very easy to realize
the ohmic losses in the primary (if the numbers are put to the tasks).
Of course, Lpri changes the transfer rate and one cannot rely on ohmic
losses alone. It must all be put into perspective.
Greg Leyh wrote:
> Hi Bert,
> I hadn't considered the possibility of the circuit path inductance
> through the gaps when making the measurement, although I don't recall
> the voltage and current waveforms being significantly out of phase.
> The total circuit path down through the front 14" rotor, across the
> electrode holders, and back up through the rear rotor is about 37",
> which could easily represent 1uH total. The reactance of 1uH at 60kHz
> is about 0.377ohm, so the inductance could in fact have had a
> significant effect. GL
>> Hi Bart and Greg,
>> I also found it very interesting that Greg saw a 2700 volt drop
>> across the primary gap. "Short" gaps used in Tesla Coils typically
>> drop only 150-250 volts/gap. Since Greg's primary rotary gap actually
>> consisted of four spark gaps in series, he was measuring about 675
>> volts/gap. However, this included the combination of electrode
>> resistances, gap wiring resistance, actual gap voltage drops, and the
>> total inductance of the rotary gap.
>> I suspect that the abnormally high voltage seen across the firing gap
>> was due, in part, to the effects of the abnormally large peak current
>> through the electrodes (tungsten?), and the associated gap and wiring
>> inductance. The latter might have had a disproportionate contribution
>> to observed voltage drop due to the relatively low inductance of the
>> remaining primary circuit and the high di/dt in the original primary
>> circuit. It would be interesting to see what the voltage across a
>> single spark gap might be in comparison when running at similar
>> current levels.
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