RE: Toroid Eddy Current?

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Several months ago I performed an experiment to determine things which
affect primary losses.  I charged my tank cap with a 9V battery and
replaced the spark gap with an ultra-low Rds on-resistance MOSFET (0.02
Ohms), driven by square wave generator to get a repetitive waveform.  I
then scoped the logarithmic primary tank ringdown with NO secondary
present.  When I placed large metal objects (like an NST or a sheet of
metal) within about a foot from the primary I could see the ringdown
time get shorter, indicating that eddy current losses were occurring.
But nothing so distant as the toroid at it's normal position had any
observable effect.  Bringing the toroid MUCH closer to the primary will
of course result in the observed losses.

That said, I didn't try any experiments with the secondary in place.  I
don't know if the presence of the secondary would raise the "active
field zone" to where the toroid could result in losses, and intuition
suggests that it may.

Gary Lau
MA, USA


>Original poster: "Ray Haynes by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <ray.haynes-at-home-dot-com>
>
>Doesn't the toroid form a single shorted turn that would dampen the
>secondary by lowering the Q? It seems that the toroid is close enough
to the
>secondary to have an effect. An interesting test could be performed if
>someone had a working coil with an identical spare toroid. That spare
toroid
>could be cut with a band saw radially toward the center stopping couple
of
>inches from the middle. Probably 3 or 4 (maybe more) equally spaced
cuts
>would be adequate and maybe dabs of epoxy to stabilize the cuts. Then
>install the modified toroid, retune the coil and compare the results
with
>the original toroid.
>
>Maybe I'm off base but I have never seen this discussed and if the Q of
the
>material for the secondary coil form matters, seems this just might.
>
>Ray