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RE: New Spiderweb Primary/OK to use Flat spiral Formulae?



Original poster: "Ken Stevens by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bluewaterdiver-at-earthlink-dot-net>

Forgive my naiveté, but what is a Spiderweb Primary?

Are there any pics of such?

Ken Stevens 

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com] 
Sent: Sunday, October 13, 2002 10:13 AM
To: tesla-at-pupman-dot-com
Subject: New Spiderweb Primary/OK to use Flat spiral Formulae?

Original poster: "Jolyon Vater Cox by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>


Are the rules concerning spiderweb coils really different to those for
flat
spirals?

Having just wound a 9-slot spiderweb primary for a mini-coil, I have
found
calculating the length of wire needed to wind the coil using the formula
wl = a
* 2 * pi * n (where wl is wire length, a is the average radius of the
coil and
n the number of turns) gives remarkably accurate results.

The spiderweb is wound with 20 SWG tinned copper wire on 80 mil
polystyrene
sheet or "former"- because the 20 swg TCW is not insulated, the slots
are each
divided into 22 holes 1/8" apart to keep the wire from short-circuiting
as it
passes up and down through the former. There are 22 turns of wire in the
entire
coil.

I suppose the thickness of the sheet (80mil) times the number of holes
(22)
times the number of slots (9) should rightfully be considered in the
calculation of the inductance -but is this really necessary when the
board
thickness is but a small proportion of the average radius?

The official formula for the inductance of a spiderweb is
L=a^2*n^2/(135a+433c)
presupposing that c -the width of the inductor- is less than 0.2 times
a- the
average radius;

this is apparently true if the coil stack is high -the only way I can
see for
this to be true with a spiderweb would be if the coil were wound on a
thick
former.

This is does not seem to be the case with my coil so I presume the
official
formual does not apply well in this instance.

Therefore wouldn t the ordinary flat spiral inductance formula L=
a^2*n^2/(8a+11C) be better used to describe the L of a spiderweb where c
is
greater than 0.2 of a?