Fundamental spark gap confusion
From: Gary Lau 07-Aug-1997 1202[SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Thursday, August 07, 1997 11:09 AM
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?