Re: Best cap size for a sync gap

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Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

HI Gary,

Questions and comments are interleaved.

 > Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com>
 >
 > I use an unmodified 15/60 NST, cranked up to 144VAC.  I had available two
 > .02uF caps, and one .01uF cap.  With these I measured the bang voltages
 > using .02, .03, .04, and .05uF.  The peak-to-peak bang voltages measured
 > were 688, 618, 552, and 482 mV respectively, as directly indicated on the
 > scope.

mV???? as in millivolts?
Shouldn't the bang voltage be in the order of 10's of KV or is this the
uncalibrated optic probe voltage?

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)


FWIW, if I scope just the unloaded NST secondary -at-120VAC input, I
 > get 598mV p-p.

Ahh!  This is your calibration.

 >
 > If I calculate the relative bang size with a simple scale-less formula of
 > C*V*V (mV*uF*uF), I get

I think you mean (uF*mV*mV) ??

 > .02   9,467
 > .03  11,478
 > .04  12,188
 > .05  11,616
 >
 >  >From this I conclude that using a .04uF cap with my 15/60 NST will
result
 > in the highest power throughput.
 >
 > Has anyone else performed such an experiment?  Just trying to understand
 > why my result is so at odds with the widely suggested value of .028uF for
 > the same power supply.  Hmmm, wonder what I'd have gotten if I had tested
 > at 120VAC?
 >
 > Regards, Gary Lau
 > MA, USA

I've done your experiment but only on a computer.  I'm wondering if the
halogen load is getting the Cp completely discharged in an equivalent amount
of time as the energy transfer time in a real TC.  I would expect the
voltage to drop off more rapidly as Cp was increased from .02 to .05 uf

Seems like it might be possible to repeat the experiment with a real TC
since you are optically coupled.

Gerry R