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Static gap/cap size simulations
I have done some PSpice simulations of my own relating to the optimum
size for one's capacitor, and have documented the waveforms and results
on my web page: http://people.ne.mediaone-dot-net/lau/tesla/tesla.htm
While I don't believe I've broken any new ground, there are some
observations I've made that I think ought to be underscored, ESPECIALLY
point 4 below:
1. For the given 15KV/60mA NST, there appear to be two optimum
capacitance values which deliver maximum power through the capacitor:
between 4-5nF and 8-9nF, neither one being the 10.8nF value required to
achieve mains resonance.
2. Without exception, the higher the spark gap firing voltage (i.e. the
wider one spaces the spark gap), the more power will be delivered to the
capacitor, with comparably better performance. The obvious downside is
that this places much higher stress and voltages on the transformer and
capacitors. It is all too easy to get carried away with wider and wider
gaps, giving one longer and longer arcs, until something blows.
3. Static gap firings are chaotic and will not ever settle down to a
regular timing pattern. The goal of setting a static gap so that it fires
once each half-cycle at the peak of the AC waveforms is an unattainable
fantasy.
4. Estimating the required voltage value of one's capacitor based upon
the transformer's voltage ratings is pointless, IMO. Using
static gaps, the peak voltage has an upper limit imposed by the
transformer and capacitor combination, and can be far in excess of 1.414
x NST RMS-Vrating. In the simulation examples above, using any capacitor
less than 20nF with a 15KV NST allowed a 30KV gap to fire regularly.
Spacing the gap even wider would have permitted even higher peak
voltages, particularly with smaller-sized caps. Estimating the peak
capacitor voltage at 1.414 times the NST faceplate voltage is wrong. The
absolute upper limit of the peak voltage that the capacitor must
withstand is mostly determined by the gap spacing. Now if only there were
a definitive table relating gap spacing (at sea level) to breakdown
voltage...
5. While it may appear that certain-valued smaller caps can process just
as much power as larger ones (and thus be more economical), beware that
the higher BPS firing rate that will necessarily occur with smaller caps
will make them far more likely to fail.
Comments and opposing points of view welcome!
Regards, Gary Lau
Waltham, MA USA