RE: Best cap size for a sync gap

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Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

To expand on my previous post:

 >>Also, did your safeties ever fire (especially at .02uf).  I would
expect the
 >>maximum voltage to be at 20nf (greater than the 21KVpk for a 15KVrms
source)

 >As the cap size was considerably LTR in all cases, the NST/cap tends
not to ring up >severely.  When I advanced the timing just beyond the
sweet spot, the circuit lapses >into a state where the gap presentations
occur at or near the zero crossings and the >gap does not fire at all
(scary!).  I measured what this voltage was, but my notes are >at home
and I'll post them tomorrow.  But they were not considerably in excess
of >the open-circuit NST voltage.

Oops, my bad!  This is the trouble with using an uncalibrated probe, not
knowing exactly what you're seeing.  Assuming that the measured
open-circuit NST secondary voltage -at-120V in was in fact 1.414 * 15kV, I
have applied this calibration to calculate the bang voltage (BV), peak
voltage when the gap doesn't fire (PDF), and 120*0.5*C*V*V Watts
delivered to the cap (W), all at 140VAC in:

Cp	BV	PDF	W
.02uF	24.4kV	29.2kV	714W
.03uF	21.9kV	31.8kV	863W
.04uF	19.6kV	32.6kV	922W
.05uF	17.1kV	????	877W

OK, the condition when the timing is advanced too far and the gap stops
firing IS bad.  But setting the safety properly fixes it neatly.

I was surprised to see that the "PDF" voltage *increases* with
increasing Cp.  I would have guessed that the more distant from the
mains-resonant value (.01uF), the less resonant rise there would be?

Today I quickly hooked up an analog watt meter with my 140V .04uF
configuration and noted that it was drawing 1.7kW at the "best" timing.
Power and current increased as timing advanced up to the sweet spot, and
when it reached the gap-doesn't-fire point, the power soared to 2.4kW.
It was gratifying to note that the power did not change as PFC caps were
added, but the current was distinctly reduced.  20A w no PFC, 16A w/
120uF, and 14A w/ 240uF.

I'll have more to report on 120V operation and power in a few days.

Regards, Gary Lau
MA, USA