RE:True Toroid Capacitance

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

Original poster: "John H. Couture" <couturejh-at-mgte-dot-com> 


To All -

Now that we can find the capacitance of the toroid to many decimal places in
the theoretical world lets see if we can find the true toroid capacitance
when the toroid is used as a topload on the Tesla coil secondary. I have
tried to do this by combining three well known TC computer programs.
    1.  JAVATC10
    2.  JHCTES Ver 3.42
    3.  E-Tesla6

The JAVATC10 default Tesla coil was used as a good example of the typical
problem of toroid capacitance reduction in a TC environment. The crucial key
to the solution of the problem is the measurement of the operating
(resonant) frequency after the TC is built. This test appears to be the only
way to accurately find the true toroid capacitance when it is on the TC. The
operating frequency test automatically takes care of the effects on the
toroid due to the surrounding conditions.  Bart said he had made this test
and found the operating frequency to be 238.25 KHz.

To find the reduced toroid capacitance a TC program similar to the JHCTES
program is necessary. The JHCTES inputs for the JAVATC10 default Tesla coil
are the following parameters:

   Primary  .025  4.86  0.0  3.74

   Secondary  2.27  900  44.6  0.0  15.60

   Outputs  238.23 KHz
              7.78 Pri turns
             18.78 Sec Ind
             Etc, Etc,

It can be seen that all of the JHCTES outputs are about the same as the
JAVATC10 except for the mutual inductance and coupling which need to be
changed in the JHCTES.

The key to solving the toroid capacitance reduction problem is the 15.60 pf
shown above for the JHCTES Secondary Terminal (Toroid) input. Changing this
input changes the operating frequency in the outputs. This input was
adjusted to 15.6 and the 238.23 showed for the Operating Frequency in the
outputs. The 15.6 pf is the new reduced toroid capacitance that is required
to make the TC secondary circuit resonate at the operating frequency that
was found in the test. This capacitance with the 8.16 pf coil self cap gave
a total Sec circuit capacitance of 23.76 pf.

The JAVATC10 default toroid is 4.5 x 19.25 with a theoretical capacitance of
21.2 pf. The new toroid capacitance of 15.6 pf  would then give a toroid cap
reduction of 26.4%.

     1-(15.6/21.2) = .264 = 26.4% reduction

A check with E-Tesla6 and using the same inputs gave 25.59 pf capacitance
and 229.58 KHz resonant frequency. The 8.16 is the secondary coil self
capacitance. The toroid cap reduction would be

     Sec term = 25.59 - 8.16 = 17.43 pf

     1- (17.43/21.2) = .1778 = 17.78% reduction

However, this was at 229.58 KHz and not at the measured frequency of 238.25
KHz. I do not know why E-Tesla6 shows a different resonant frequency with
the same toroid and Sec inductance of 18.78 mh because the surrounding
conditions are the same.

This new TC research needs to be verified with additional testing and all
comments are welcomed.

John Couture

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