Re: Tesla Coil toroid Size

To: tesla-at-pupman-dot-com
Subject: Re: Tesla Coil toroid Size
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Wed, 09 Sep 1998 19:53:23 -0600
Approved: terryf-at-verinet-dot-com

Hello John,

> Original Poster: "John H. Couture" <couturejh-at-worldnet.att-dot-net> 
> 
> 
>   Malcolm -
> 
>   Thank you for the toroid capacitance data from your measurements. I
> thought you would be interested in what I caculated. The following
> information is based on methods that have not been completely verified.
> 
>   The "In space" capacitances are from theoretical equations. I show a
> graph in one of my books based on these equations. The "On TC" capacitances
> are estimated for your toroid but based on what I have found from other
> coilers tests. Note that the toroid capacitance is reduced by the presence
> of the coil.
> 
>                        In space     On TC  
> 
>   Toroid #1 - 11 x 30   33 pf        26 pf
> 
>   Toroid #2 - 10 x 23   28 pf        22 pf
> 
>   Toroid #3 -  6 x 19   22 pf        17 pf

There is some clear difference between your theory and practice here 
because the isolated values you calculate for T#1 are less than what 
it actually adds to a number of coils. 

>   I would expect differences from your values because the tests differed.
> The above results (On TC) would be obtained by finding the actual operating
> frequency of the TC and using this info to find the effective capacitance
> of the toroid.

Exactly what I did. If tests don't agree then something is wrong with 
either one or both tests. There is a high degree of consistency in 
what I measured if you check for incremental differences between the 
capacitances as measured.

>    Cs = 1/(39.5 Ls F^2)  and  Ctor = Cs - Ccoil

Which is not a great deal of use for predicting prior to the fact as 
you said. For some time now I have viewed the situation as being the 
capacitance of the total structure. As you say, the presence of the 
coil reduces Ctop by some amount and I suspect that amount is a 
function of the effective area (sides and bottom) of Ctop as a 
proportion of total structure area. That includes shading by the coil 
of the bottom of Ctop. Interesting to note that a 6"x2" top place on 
the 16.7" dia coil doesn't add a jot of capacitance to the coil.

Csec is easily and accurately calculated using Medhurst's formula. 

Malcolm
<snip>