Re: design presentation, and 1 question (Cap size)

Hi Terry:

I think that your advice about using the 2X rated cap may only be valid
when using a sync RSG, where only one bang per half-cycle is possible.
For static gaps, as in the case of the original poster, if the gaps are
spaced to fire at or near the xfmr peak voltage and the cap is sized for
resonant charging, then the gap will fire multiple times per half cycle,
resulting in the same (or better?) average power throughput.  The danger
of course is that when resonant charging, setting the gap too wide could
lead to cap-popping, NST-toasting 60 Hz resonant rise.

Gary Lau
Waltham, MA USA

>Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
>	I use a 60mA transformer with a 17.05nF cap.  It works great and charges
>the cap just fine.  In fact, I really should go to around 19.5nF to get the
>best charging but my cap value will not adjust that high (thus my new cap
>project).  19.5nF will charge the cap to around 20 kV from my 15kV/60mA
>neon with the system starting from zero volts after the last gap firing.  
>	If I use a 10nF cap, my output power drops dramatically.  Probably by
>about 1/2 since the stored energy in the cap is reduced by that ratio too.
>With a 10nF cap I could use resonant charging to get to a higher voltage
>and higher energy but I would need 30kV peek voltage to get to the same
>energy level.  This would substantially exceed the ratings of the
>transformer and challenge the primary caps.  Also, the safety gaps would
>have to be set to say 35 to 40kV in that case.  So if the gap failed to
>fire, the transformer could see very high over voltage.  I don't think the
>added current is of great concern to the transformer in this situation but
>it does add to heating...
> I have been recommending using as high of cap value as possible that still
>allows the capacitor voltage to reach the transformer's rated peak voltage.
> I get great energy and the system operates within rated voltages.  Since
>the load is great enough to prevent over-charging, the system operates very
>safely.  In limited testing and calculation, this has worked out to about
>twice what the classic load matching equation suggests.  Unfortunately,
>computer models that have the complete primary charging system (especially
>the transformer) modeled are needed to calculate this accurately or you
>need a high voltage (the regular high resistance kind) to check the voltage
>at the caps.  I have not done enough testing to be sure that the "twice
>what the classic equation rule" works for all neon transformer types.
>I use a 120 BPS rotary sync gap in this system.  The gap fires about 5.5
>mS after the transformer input AC peak due to the delay caused by the cap
>charging.  The rotor is set to delay the firing ~1/6 of a rotation behind
>the AC line (1800 RPM).  Perhaps a smaller cap could fire faster and
>deliver the same power with some good safety margins? 
>	Terry