Re: Very strange problem, please help

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 8/12/01 1:45:56 AM Eastern Daylight Time, tesla-at-pupman-dot-com 
writes:

>  start out with 10 inch arcs, and they grow, over about 15 seconds, to 
about 
> 36
>  inches long, so long they started striking the fluorescent light above, 
also
>  the capacitor safety gap starts firing about once a second, so I shut down.
>   
>  Transformer is a 15/60 neon, cap is a 0.01uf 20 KVAC Plastic Capacitors,

David (?),

I've seen this "growing of sparks" in other coils too, and I'm 
not 100% sure what causes it.  It could be caused by the caps
if their value changes as they heat up.  This would depend on
the cap's dielectric.  In some coils, I suspected the break-rate
may be increasing as the gaps heat up and fire at a lower
voltage, but this seems less likely in your case, because the
cap value is not small.  Here's a question?  Does the frequency
or pitch of the sound change as the spark gets longer?  This
would suggest that the breakrate is changing.  If the pitch
doesn't change, than it may be a tuning change that occurs
after it runs awhile.  Does the cap get warm in operation?
The cap may be somewhat underrated  voltage-wise, but 
whether it survives depends a lot on your gap spacing.  
A narrow gap spacing will keep the voltage low and protect
the cap.  I think you said the gap spacing was quite narrow
overall, at less than 1/4".  I'm surprised the coil even gives
36" sparks with such a narrow gap.  In thinking about this
more, I must have mis-read your gap spacing, because 
since your cap gap begins to fire at 1/2", this means your
main gap must be wider than I thought.  Do I understand
that your cap gap is a safety gap across the cap only?
It's a good idea to put a suitable resistance or inductance
in series with the cap gap to prevent abuse to the cap
when that gap fires.  Alternatively, you can put the
safety gap across the main gap, so the inductance of
the primary reduces the cap current when the safety
gap fires.  Some say this may subject the cap to a higher
voltage in certain cases. A 1/2" gap is probably too wide for
that capacitor.  Is that cap a part of a bulk buy that took
place a number of years ago?  Some of those blew up
for various reasons..... a combo of poor cap design, and
over-volting perhaps.

John Freau