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(Fwd) Re: Toroid Question
Subject: (Fwd) Re: Toroid Question
Date: Wed, 30 Apr 1997 12:03:51 +0500
From: "Alfred A. Skrocki" <alfred.skrocki-at-cybernetworking-dot-com>
To: tesla-at-pupman-dot-com
On Wed, 23 Apr 1997 12:30:16 -0600 (MDT) Kristopher Larsen
<larsenk-at-rastro.Colorado.EDU> wrote;
> I've got a tesla coil that I'm fixing up for work and am having
> a little difficulty with the terminal. It originally had a Al sphere about
> 12" in diameter and I want to switch over to a toroid design. I'm
> wondering how I decide the correct size of the toroid given my components.
> Here are the specs on my current system.
>
> 15,000 V transformer
> 15 pf capacitor
> 12" diameter 14 turn primary
> 5" diameter, 36" length secondary
>
> I think that covers the specs. If anyone has any ideas on how
> big a toriod I can construct, please let me know. Thanks.
>
> Kris Larsen
Hi Kris, in the Volume 7, #2 (April, May, June) issue of the T.C.B.A.
is the second part of a two part monograph entitled "Torids and Their
Tesla Coil Applications" by D. C. Cox. He suggests that the size be
determined in relation to the diameter of the coil so that it
functions as an electrostatic shield to deter coronal leakage until
maximum power is established, to this end he suggested the following;
Coil Form Diameter | Toroid size
----------------------|----------------
2-4" | 2 X 6", 2 X 8"
5-6" | 3 X 12", 5 X 14"
7-12" | 5 X 14", 5 X 20"
13-16" | 5 X 20", 7 X 30"
17-24" | 7 X 30", 8 X 48", 12 X 48"
25-36" | 12 X 48", 20 X 36"
I have personally found these suggestions to be good. Smaller toroids
yield decreased spark length due to coronal leakage and larger toroids
also yield decreased discharges due to insufficient power. In the
case of the coil you are presently working on it would suggest a
3 X 12" toroid. If you stick a thumb tack on the side of the toroid
(point facing away from the toroid) you will usually get a few
percent longer discharge than the toroid alone.
BTW the coil you are working on is not exactly the best design, the
Length/Diameter ratio is too high! This coil has an L/D ratio of 6,
you shouldn't use anything larger than 4 and preferably it should be
around 3. If you look at all of Dr. Tesla's coils they are barrel
shaped this is in part due to the fact that the input to output ratio
of a Tesla Coil is determined by;
Voutput = Vprimary X SQRT(Lsecondary/Lprimary)
consequently we want as large an inductance in the secondary as
possible and it turns out that the optimum L/D ratio for maximum
inductance for any given length of wire is roughly 3. The primary you
stipulated of 12 turns is also a bit high, referring to the input to
output ratio given above it can be seen that the maximum output will
require as small an inductance as possible on the primary. I have
found it best to keep the number of primary turns to 6 or less, a
Tesla coil really dose'nt need much of a primary only enough to
transfer the primary circuits energy to the secondary and at
frequencies of 100 Khz and above you don't need much more than a turn
or so. I couldn't say if your primary capacitance is right or not
without knowing the current rating of your transformer to calculate
it's output impedance.
P.S. The input out put ratio equation can also be expressed as;
Voutput = Vprimary X SQRT(Csecondary/Cprimary)
This is because at resonance the capacitance and the inductance are
holding exactly the same amount of energy which for a capacitor is
given by E = .5C X V^2 and for an inductor it is given by
E = .5 L X I^2 where C is capacitance, V is voltage, L is inductance
and I is current. Realize of course that these equations are
theoretical maximums and you will never actually reach these values
due to the numerous losses in the circuits involved.
Sincerely
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Alfred A. Skrocki
alfred.skrocki-at-cybernetworking-dot-com
.ooo0 0ooo.
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