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Re: Coil success on first big run - but (help) safety gap fires alot!



Tesla List wrote:
> 
> Original Poster: Alex Crow <user-at-alexcrow.clara-dot-net>
> 
> All,
> 
> Just to say I've got my coil up and running and am working on tuning. I
> have got 29 inch sparks for 1000w in, so it's only a start. It was
> really beautiful once it got dark though! I was wondering how critical
> the tuning was - would say 1/2 a 1/4 turn adjustment on the primary make
> a lot of difference? I have it at 6.5 turns at the mo with a small
> topload (a garden soil riddler!). I may try going for the big 24 inch
> toroid tomorrow, but I will need to set up a stike rail before this.

You may need to consider the lead wire to the tap as a turn of wire. 
Putting the tap on fractional turns may just be changing the length of
wire in the tank not the actual number of turns.
> 
> Also, after about 5 seconds the safety gap would begin to fire for a
> second or two at a time, when the main gap would stop firing. After this
> I noticed that some of my MMC caps had quite a residual DC charge on
> them, so I was wondering if this was building up during this first five
> seconds and then the Safety gap was cutting in, draining it off and
> starting the cycle again. There must be some rectification here. Any
> ideas? Would equailising resistors help here by draining this charge
> away as it builds up? This is the only thing that is worrying me right
> now!

A residual charge on series connected caps is normal (always).  The last
bang never discharges the cap completely, and with a series string, all
caps cannot discharge into the transformer.

You are probably seeing the effects of dielectric heating in the caps. 
As the cap plays for awhile, the dielectric heats, and the constant and
plate spacing change slightly.  I use some ceramic RF doorknobs that
exhibit this phenomenon.  I get around it by leaving in a little less
inductance (one turn, out of nine).  When the dielectric heats, it seems
to gain capacity, and efficiency (with the ceramics I use).  The sparks
get longer after it runs awhile.  

Try tuning one turn higher or lower and see if you don't notice the
tuning improve with heating. With only six or seven turns in the
primary, each turn is a large proportional change.  It may be a good
idea to decrease the cap and increase turns, for troubleshooting
purposes.

Good luck
bob