Re: Fundamental spark gap confusion

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Saturday, August 09, 1997 12:17 AM
To: 	Tesla List
Subject: 	Re: Fundamental spark gap confusion

Tesla List wrote:
> From:   Gary Lau  07-Aug-1997 1202[SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent:   Thursday, August 07, 1997 11:09 AM
> To:     tesla-at-pupman-dot-com
> Cc:     lau-at-hdecad.ENET.dec-dot-com
> Subject:        Fundamental spark gap confusion
> I have a rather fundamental question concerning when spark gaps fire.  Here is
> 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 voltage
> 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 half
> cycle, 120 times per second.
> On my 15KV neon xfmr, if I hold the two output leads near one another, a spark
> 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 .336-.360".
> Others on the list have advocated using as little as 0.150".  I'm having trouble
> resolving this apparent discrepancy.
> 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 cap
> does not try to continue charging in the direction of the current, remaining
> half-cycle, only to not reach Vgap and then have to DIScharge prior to charging
> in the opposite direction to Vgap in the next half-cycle.
> Right now, my coil has a 15KV/30ma neon and a 6 x .030" static gap.  The gap
> starts firing when the variac is at maybe 15% (rough est), so it's firing at
> a much smaller voltage than 15KV.
> 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 rating.
> I don't doubt that my logic is flawed, could somebody tell me where?
> Gary Lau
> Waltham, MA


Excellent question, Gary! When your neon secondary inductance (L)
resonates at 60 Hz with your tank cap (C), the voltage across the L and
C elements will attempt to reach Q times the source voltage. What's less
clear, is that the peak current will ALSO try to go to Q times the
normal shunt-limited value. Now although a neon transformer doesn't have
a very high Q, it can easily hit the 5 - 10 range. This means that under
these conditions, your transformer will deliver significantly greater
charging current to your tank cap. Since your tank cap recharges more
quickly between "bangs", you can achieve more breaks/second with a
static gap than if you were not running at 60 Hz resonance. 

Now, as you've indicated, as long as you don't open the gaps too widely,
the system will run great. But remember, you're still stressing the neon
secondary windings to higher current levels than designed-for during the
tank cap recharge times. BTW, I've found (the hard way... :^)) that 60
MA transformers survive longer than 30 MA transformers in this
environment probably because of the larger wire gauge used in their
secondaries. Using the resonant approach, I can run off a pair of
15-60's with 18 gaps (0.5" total) and still hit 300 - 400 BPS. Without
the addtional charging current, this would not be possible. However...
I've also reconciled myself that (per Richard Hull) eventually my neons
are doomed!

Safe coilin' to you!

-- Bert H --