Re: My first TC

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

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

Hi William,

Tesla list wrote:

> Original poster: "William Swanson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <swansontec-at-yahoo-dot-com>
>
> Hi All,
> Here are all the juicy details you wanted about my
> Tesla coil with the flat secondary...
>
> The secondary is 45 turns of 24 AWG magnet wire spaced
> 1/3" apart, for a total wire length of about 200 feet.
> The coil is wound on a form consisting of sixteen 1/4"
> dowels 24" long arranged like the spokes of a wheel.
> The "hub" of the wheel is a piece of pipe with an OD
> of 1.67 inches.<snip>

Using the Archimedes Flat Spiral equation: LuH = NR^2 / 8R + 11W, where:
N = number of turns
R = average radius
W = width of coil on one side

I calc 1.295Mhz and 603.4uH.

This is something I didn't realize at first sight. This is an extreme high
frequency coil! This is an
unexpected surprise.

> The secondary has an inductance of 610 uH (Wheeler
> formula), and a self-capacitance of 25 pF (back
> calculated from an observed frequency of roughly 1.2
> MHz). The primary has a 3.6 nF capacitor and 6.97 uH
> of inductance. (My first statement that my capacitor
> was 6 nF was incorrect. Sorry.) I calculated the
> inductance of the primary using the formula for the
> inductance of a single straight wire, as opposed to
> the Wheeler formula. My thinking was that the Wheeler
> formula would be inaccurate for a single turn of wire,
> since it was designed for a coil.

I agree.

> I calculate that I
> need 17 pF of topload capacitance for proper tuning,

Yes, that's an even better way of tuning on this puppy I think. Do it with
the top load. I would imagine
a top load would create a large change to the Fr where you could possibly
easily tune it with top load
proximity.

> As for the connections of the coil, you are quite
> right that I'm taking my arcs off of the center of the
> coil. The outer turn of wire is grounded, just as
> Tesla showed on his drawings.

I would imagine Tesla tried both and found this the best for the purpose he
was using the coil for.

> I think that since most
> of the inductance of the coil is in the outer turns,
> it is appropriate for the primary to be right on the
> outside of the coil where it can get some decent
> coupling.

I think there would be plenty of coupling on the inside as well (maybe too
much, but it would be just a
matter of easy height adjustment).

> My primary gap is static, and is connected
> across the transformer as opposed to the more popular
> wiring method with the capacitor across the
> transformer. My thinking was that the gap would act as
> a safety gap as well, and would protect the
> transformer from high frequency by short-circuiting it
> right out of the resonating system.

Actually, the gap in parallel with the xfmr is the popular method used.
Your reasoning about the
transformer is why it's recommended by many - but it isn't a safety gap.
It's still a good idea even with
your system to provide one at the xmfr itself.

> The capacitor is a
> homemade mica job. I hope that this is enough
> information. :-)
>
> -William

Excellent William! Thanks for sharing. I am kind of curious about skin
effect R considering the Fr. But
then, there's only a couple hundred turns. Keep us informed of any
improvements, changes, or strange
effects you encounter. I bet this won't be the last coil of this type built.

Take care,
Bart A.