Re: Topload and Arcing

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Danny,

Tesla list wrote:

> Original poster: "Danny Stone by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<dansto-at-pacbell-dot-net>
>
> I am in the process of designing a coil, using the wonderful software
> available online.
> The coil is 6 inches in diameter, and the estimated spark length is about
> 40 inches. Using an aspect ratio of 1:4, the coil length is 24 inches.
> Naturally, I'm adding some space at the top and bottom of the coil form,
> for a total form length of about 30 inches.
> You probably can guess my question: With the topload (a toroid) sitting on
> top of this form, how can I avoid arcs to the primary, since the distance
> is less than the estimated spark length?

You can't always avoid sparks to the primary. Sparks should eminate from
the outer center of the
toroid as this is where the greatest charge on the toroid is seen. Much of
the time, sparks
eminate outward shooting up and yes, down. They will seek the primary now
and then. Some coilers
use a strike ring (usually a single copper tubing mounted a couple inches
about the outer primary
winding leaving a 2 inch gap between ends and tying the ring to rf ground).
Some use a plexiglass
sheet over the top of the secondary which kind of disperses the strike over
the surface of the
plexiglass. And some just let the strikes hit the primary. I kind of like
the plexiglass idea (but
is the only one of the three above I haven't tried yet, so my liking it is
premature).

> The obvious solutions are
> insulation (of the primary) or providing a grounded point to attract arcs.
> Neither are highly desirable for my application.
> I've wondered why some of the coils I've seen on the web have two toroids,
> and I recall reading somewhere on the list that this configuration can
> prevent (or alleviate) arcing to the primary.

Hmm, maybe some tried it to increase the distance from strike to primary,
but I think many used it
to increase the top capacitance.

> Is this correct? If so, how
> does this work? Is this primarily why the stacked toroids are used?

Answered above.

> Finally, how do I calculate the total capacitance of the stacked toroids?
> Do I treat them as two separate toroids, calculating their free-space
> capacitance (from the equation) and summing them as two parallel capacitors?

The testing I've done has shown that stacked toroid capacitance will be .5
*(2*(C1+C2)). Stacked
toroids typically indicated 50% of their summed individual free-space
values. I've checked this
with 2 different toroid sizes and close and far proximity to eachother. The
value changed very
little.

Take care,
Bart