Re: Fundamental spark gap confusion

From: 	FutureT-at-aol-dot-com[SMTP:FutureT-at-aol-dot-com]
Sent: 	Friday, August 08, 1997 6:07 AM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: Fundamental spark gap confusion

> I have a rather fundamental question concerning when spark gaps fire.  Here
> my present understanding on how a static gap operates:
> The cap is selected to be resonant with the HV xfmr secondary at the mains
> frequency, lets say 60 Hz.  The voltage across the cap is then 90 degrees
out of
> phase with the xfmr primary and peak cap voltage occurs as the primary
> crosses zero.  The spark gap is adjusted so that it fires at a voltage
> slightly less than the peak capacitor voltage.  Thus, the gap fires every
> cycle, 120 times per second.


This is correct.
> On my 15KV neon xfmr, if I hold the two output leads near one another, a
> will form when the seperation is _roughly_ .75 inch.  The doc's for the RQ
> static gap suggest using 12 gaps of .028-.030" for 15KV, a total of
> Others on the list have advocated using as little as 0.150".  I'm having
> resolving this apparent discrepancy.

On my 15kV, 60ma set-up, I used 4 or 5 gaps, each gap was
about 1/8" wide.  Safety gaps are useful for protecting the transformer.
IMO, longest sparks cannot be obtained unless one is willing to open
the gaps quite wide and risk damage to their transformer.  Just make
sure you have some extra transformers...and you won't worry about it
too much.  I now use a synchronous rotary gap which gives good 
performance and I have had no transformer problems while using this
> It would seem to me that setting the gap to less than the free-air sparking
>distance will result in dumping the cap's charge well before it attains it's
>maximum potential value, though it would be firing at a rate correspondingly
> faster.  But then, what's the point of sizing the cap to resonate at 60 Hz?
> I thought at least part of the reason was so that following a firing, the
> does not try to continue charging in the direction of the current,
> half-cycle, only to not reach Vgap and then have to DIScharge prior to
> in the opposite direction to Vgap in the next half-cycle.

I agree.
> Right now, my coil has a 15KV/30ma neon and a 6 x .030" static gap.  The
> starts firing when the variac is at maybe 15% (rough est), so it's firing
> a much smaller voltage than 15KV.

My gaps started firing at a much higher variac setting...about 80 percent
of max variac.
> I've seen numerous cautions about opening the gaps up too much, in doing so
> risking blowing one's xfmr and/or caps.  Is this simply to limit the peak
> charging voltage on the cap to something less than the peak xfmr voltage?
> If so, then it probably doesn't make any difference what voltage one's xfmr
> is in terms of whether the cap dielectric will survive, since the peak cap
> voltage is now a function of gaps (plus resonant rise), not secondary

Yes, and assuming the caps are properly rated, the cautions are for 
protecting the neon sign transformer.  I've heard that at high
voltage/power levels, enough RF can get back into the transformer 
to break down the tar and maybe even the wire insulation, to ruin the
transformer.  Chokes andresistors can be used also to help reduce
and reject these RF frequencies.  Safety gaps are "must have" item.

> I don't doubt that my logic is flawed, could somebody tell me where?

I found no flaws.   :)

John Freau
> Gary Lau
> Waltham, MA