Re: Who needs a quenching gap ?

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

Original poster: "Bert Hickman" <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:
> 
> Original poster: "Metlicka Marc" <mystuffs-at-orwell-dot-net>
> 
> with this talk of first and second notch quench and coupling
> coefficients, i have a question pertaining to my proposed 13khz coil.
> in this design i will be placing my secondary inside of my tank former,
> these are 1" of fiberglass with a 1/8" PE  liner. in placing my primary
> on the outside, only 1 1/8" away from the secondary in a helical
> pattern, is there such a thing as over coupling? i feel that there will
> be no chance of flash over from primary to secondary, but will the tight
> coupling cause racing arcs to run down the secondary windings?
> should i try to design it differently?
> any input will, as always, be appreciated.
> marc

Marc,

Coupling governs how fast energy can transfer between the tuned primary
and secondary circuits. The degree of coupling is a function of the
shapes and relative positions of the current-carrying portions of the
primary and secondary windings. The more that the primary and secondary
"look" alike (in your case both helical), the closer they are oriented
so that one is symmetrically inside the other, and the closer their
respective diameters, the greater the coupling. The coupling coefficient
"k" is actually the fraction of the total magnetic field created by one
coil which interacts (or "links") to the other coil. Most 2-coil Tesla
Coils operate best with relatively low coupling, where typically only 10
- 22% of the fields from each coil interact with each other. This is
initially a bit surprising to new coilers, since we'd intuitively think
that close coupling should result in better system performance. The
reasons have to do mostly with the limitations of spark gap quenching,
and perhaps the physics of streamer propagation (as discussed in earlier
posts). 

It's very easy to get too much coupling! And, overcoupling can cause
major problems. First, quenching of the main gap becomes much more
difficult, and more of the available energy ends up being burned in the
main gap, reducing coil performance. Overcoupling can also cause
localized overvolting between sections of the secondary, resulting in
racing sparks that can eventually damage the secondary. Finally, voltage
breakdown may also occur between the primary and secondary if they are
physically too close together. This can become a problem with helical
primaries when the top of the primary is up too high on the secondary,
and voltage stress breaks down the intervening insulation. And, high
voltage RF is VERY difficult to insulate, especially along insulating
surfaces - an RF voltage that may jump only 1" in air, can easily flash
over 3-4" along the surface of a dielectric. 

As long as you can raise the secondary relative to the primary to bring
coupling within the proper range your planned approach should work.
Remember to start out with the secondary elevated (low coupling), and
tune for best spark before going to full power. Then you can slowly
increase coupling to get best full-power operation without racing
sparks.

Safe coilin' to you!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com