Re: My first TC

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

Original poster: "William Swanson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <swansontec-at-yahoo-dot-com>

Hi All,
In order to answer your questions, I have experimented
with a number of different coil geometries on my
spreadsheet, and have come up with some interesting
insights. It turns out that the amount of wire per
square inch is the exact same over the entire area of
the coil. This isn't a big surprise. After all, the
spacing between turns is the exact same over the
entire coil. Therefore, if the amount of wire per
square inch were the only factor in determining the
inductance of the coil, the outer turns would have no
more inductance per square area than the inner turns.

However, the number turns per inch doesn’t seem to be
the only factor. I have found that the outer turns do
in fact have more inductance per square inch than the
inner turns of wire. It must be some sort of coupling
effect, since I can’t think of any other logical
reason. Perhaps curved wires don’t have as much
inductance as straight wires? All I know is what my
equations tell me, so if I’m wrong, it means that I
did something wrong when I derived the formulas for
the physical dimensions of a coil.

For anyone who wants to build a flat spiral secondary,
here are the equations that I worked out for dealing
with the physical dimensions of a flat spiral:
 
2(pi)rw/s=l
2(pi)rn=l
ns=w

r = average radius
w = width of coil on one side 
n = number of turns
l = length of wire
s = spacing between turns

The Wheeler formula is, of course:

L = (rn)^2 / (8r + 11w)

L = inductance in microhenries (uH)

The rest variables are the same as above. Units are in
inches.

The reason why I use a single wire for my primary has
to do with inductance. As you know, the resonant
frequency of a circuit is related to the product of
its inductance and capacitance. Since my secondary is
extremely high frequency, I didn’t have much “room”
for both a large inductance and a large capacitance in
the primary of my coil. I decided that since the
capacitor is what determines the amount of energy in
each burst, it would be better to use a small
inductance and a large capacitance. Thus, I used a
single turn of wire to “make room” for my 3.6 nF
capacitor. If you use a lower frequency or a smaller
capacitor, you could easily use several turns wire in
your primary.

On a sadder note, my capacitor was destroyed during
last night’s tuning session, so I will have to make a
new one. In the mean time, I won’t be able to run my
coil unless I go solid state. I have the parts for a
solid state driver sitting around, and I’ll put them
together this weekend. I’ll build a decent cap once I
figure out what went wrong with my last one...

-William

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