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Re: [TCML] Coupling

It also needs to be said that highest coupling is not the goal - more is
not necessarily better.  Performance will generally increase with
increasing coupling, until an ill-defined threshold is reached.  Increasing
coupling beyond that point will result in what's called "racing sparks" -
sparks across portions of the surface of the secondary coil that can
quickly destroy the secondary.  You need to design the coil in such a way
that the distance between primary and secondary can be experimentally
adjusted to vary the coupling, typically by raising the secondary.  For
most coils, using a flat spiral primary provides adequate coupling when the
plane of the primary is near the bottom end of the secondary.

Regards, Gary Lau

On Wed, Oct 14, 2020 at 4:26 PM jimlux <jimlux@xxxxxxxxxxxxx> wrote:

> On 10/14/20 11:10 AM, Douglas Johnson wrote:
> > What could one consider a good coupling starting point for a 3.5"
> > secondary? Even with the primary, maybe a little above or below?
> > Also, using tesla map, I see quite a difference in tuning by changing the
> > center hole size. Can someone explain this to me?
> >
> The coupling is determined by how much of the magnetic flux from one
> coil (the primary) is in common with the flux from the other coil (the
> secondary).
> A primary with a giant hole has lower flux in the center than a primary
> with a small hole - because the flux is distributed over a larger area.
> Consider a vertical solenoid primary - that will have the highest flux
> in the center, and is the highest coupling.
> I think someone (Greg Leyh?) built a coil with a solenoid primary
> *inside* the secondary.
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