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Re: again!!
Tesla List wrote:
>
> Original Poster: mark <moyson-at-tig-dot-com.au>
>
> hello all!
> ..........does anyone know the formula for the inductance of a cone
> shaped primary?...
This is a non-trivial computation. I would suggest calculating assuming
it is squashed flat in two cases. The first would be with the same
outside diameter (so the turns are closer together than they are in the
cone). The second would be with the turns the same spacing, but the OD
expanded by the appropriate amount. Or you could approximate it as a
cylindrical coil. The actual inductance will be of the same general
magnitude. You'll have to tune anyway, so just leave yourself a bunch of
adjusting room and then go to it.
The quickest way to do this whole thing is to make a primary out of bare
#12 or #14 house wire and cardboard in the right general shape. Then
tune that to find out how many turns you need. Then, make the fancier
primary with the same dimensions.
My impression, by the way, from reading the list over the last year is
that a saucer primary doesn't buy you much over a flat spiral primary,
and is much, much harder to make. Make a flat spiral with the OD the
same as the height of your secondary. (i.e. if you've got a 2' high
secondary, make the primary 2' in diameter.) My own experience with
primaries (which is quite limited) is that it made no difference in
output whatsoever (other than changing the tuning). I started with flat,
added capacitance, so I wound a new primary in a cone with more turns,
tuned it and it worked better. Then, when moving, I inadvertently
squashed part of the primary flat. Retuned, it worked the same. Then,
ripped the original flat primary off, put a new flat primary on with
more turns, tuned, and it works just fine (but is much less fragile).
Of coure, the cone or saucer primary can look a lot cooler...and, the
aesthetics of building a coil are not an insignificant aspect of the
process.
>
> also, what is the mutual inductance between the two coils have to
> be?.......or can it be anything?....it doesnt matter does it?
> alex
It does matter, but, if you make a secondary that is about 4-6 times as
long as it its diameter, stick that in the middle of a flat primary, the
coupling will be about right. And, you can always play around later to
move it farther away to reduce the coupling.
--
Jim Lux Jet Propulsion Laboratory
ofc: 818/354-2075 114-B16 Mail Stop 161-213
lab: 818/354-2954 161-110 4800 Oak Grove Drive
fax: 818/393-6875 Pasadena CA 91109