Re: Latest coil(and racing sparks)

Tesla List wrote:
> Original Poster: CHURCHMON-at-aol-dot-com


> With two 15/60's the coil had major "Racing sparks"  knowing
> that the system was LTR  there were choices to work with,  either lay the
> primary
> down flat(which I made adjustable for this reason)or raise the secondary.
> So I tried this  tweaking  tecnique and it helped dramatcally by laying it
> down,next I borrowed a .01 cap from Dave McNamee and put it in place of
> the.028 .
> Instantly the "racing sparks" were gone and I was getting 4'arcs.
> It appears that too much LTR cap size along with overcoupling for a given
> coil
> size may be the problem and not static electricity. Just a possibility not a
> fact!
> Hope this helps,Mike Church (CHURCHMON)

Tesla List, Mike, all:

I've done some testing charging a capacitor up to full voltage.
Upon reaching a certain voltage level (15 KV), it was then
discharged.  The capacitor along with the primary coil were
purposely NOT resonant with the secondary.  The capacitor was
about 7 times that needed to be resonant.  When the circuit
would discharge (only a single discharge - not continuous as
in the TC), there would be very large "racing" sparks along
the secondary coil.  What I think is causing the problem of
the racing sparks, is the impedance of the secondary
being very high at the time of the discharge - zero current
flow.  All of a sudden - there is a huge L*(DI/DT) present on the
secondary and a huge impedance - very large immediate voltage
differentials.  The reason that I think that the sparks go
along the secondary is the same principal as found when a
lightning impulse penetrates the winding of a transformer.  You
get an impulse voltage per turn which is far greater than the
normal operating voltage of the transformer.  It takes a little
while for the impulse to penetrate into the winding. The same goes
in the tesla coil - all of a sudden there is a huge voltage (L DI/DT)
differential traveling up from the bottom of the secondary coil.
This wave will be only along a portion of the entire winding
(to start with) as is found in transformers.  Thus, this is why
the sparks are only along a small portion of the winding.  It will
then "penetrate" up the winding.  The larger the capacitance for a
given primary and secondary inductance, the larger this effect will be.

I think that by somehow testing the voltage gradients present in
the secondary while doing similar types of tests may lead to some
insight of this phenomenon.  Especially if the voltage could be
measured via several channels on a storage scope with the each
channel representing a given portion - say 1/8 of the winding
starting from the bottom up.

There is a great book on the subject of transformer
windings being subject to transients and impulses - it is
Electrical Transients in Power Systems by Allan Greenwood, 1991
John Wiley & Sons - EXCELLENT book.

I also think that this book explains the same phenomenon on why the
HV transformers fail.  If the spark gap experiences a reignition upon
quenching of the arc, there will be a transient build up to
approx. 3 (PI) times the input voltage (for a single phase system
like that typically used in a tesla coil) - additionally, the effect
being greater when the cap is resonant with the transformer.  This,
I think, is why the caps have to be rated for typically about 3 times
the operating voltage.  This may be reduced (haven't tried it yet) by
inserting a pre-ignition resistor across the spark gap itself, for our
case it would have to be pretty high in resistance so it woudn't load
the spark gap down too much.  Although high in resistance, this may
help to dampen the transients (and any reignition) coming from the 
gap and a voltage buildup back to the high voltage transformer.

I may be way off base but, that's my 2 cents on the subject.

David L. McKinnon
D&M's High Voltage

E-mail:  DavMcKin-at-ix-dot-netcom-dot-com