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Re: Overcoupling?
Hi Antonio,
Fascinating idea...
<snip>
> The other discussion about static charges in the surface of the
> secondary gave me another idea: These racing sparks can be charges
> deposited in the surface of the secondary, moving due to the
> coil varying electric field. A varnished secondary coil can store
> significant charge at the surface, capturing charges from the
> ionized cloud that surround an operating coil.
> The small distance to the primary would result in charge spraying
> over the secondary coil by corona, enhancing the effect.
> The combination of a grounded conductive layer (the coil wire) and
> a thin dielectric(the varnish) create a large grounded capacitance.
> These accumulated charges can create areas of relatively conductive
> material at the surface of the varnish, that serve as terminals and
> charge supply for the racing sparks.
> (A similar phenomenon is what makes MOS transistors conduct.
> Charges at the surface of a charged vertical capacitor move when
> excited by a lateral electric field.)
> If this is true, coils that show racing sparks would also
> retain significant charges after turned off. The idea of gluing
> plastic rings spaced along the coil, or blocking corona from the
> primary with well rounded surfaces and added insulation could then
> be effective against "racing sparks". Another, more strange,
> idea would be to make the surface of the varnish slightly
> conductive to impede the slow accumulation of static charges.
>
> Antonio Carlos M. de Queiroz
I have a secondary spacewound on some PVC drainpipe that stores a
significant charge in its dielectric in dry conditions. It's never
been pushed to the point where racing sparks occur. The pipe was
unvarnished when the coil was first wound and had just a couple of
coats applied afterwards. There are still gaps between the turns.