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RE: Coil success on first big run - but (help) safety gap fires a lot!

>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.
>
>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!
>
>Alex Crow

Hi Alex:

Your mention of a residual DC charge on MMC caps is interesting and cause
for concern.  I assume you are using an AC supply transformer, not
rectified.  How was this DC charge detected?  Did you just run the coil
and turn it off and quickly probe the individual caps?  It would be
reasonable to assume that at the moment you switched it off, the MMC had
been charged to something less than the gap's breakdown voltage.  Then
the two end terminals of the MMC would be effectively shorted by the xfmr
secondary and tank primary.  If all of the MMC elements were identical,
the MMC's charge would then be discharged, unless either there was some
rectifying mechanism, or all elements were not the same capacitance
value.

If there is a rectifying mechanism at play here, that would be bad news
for MMC's, and resistors in parallel with each series element would cure
that.

I don't see how the safety gap would fire any differently than the main
gap to indicate the presence of a DC charge.  Both see only the
end-to-end charge of the MMC.

Regards, Gary Lau
Waltham, MA USA