RE:True Toroid Capacitance

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi John,
          I'd like to add a comment to your piece based on my own
experimentation. We have seen, from various sources, a percentage
reduction figure in Ctor when placed above a coil to account for the
disparity. But the problem is not that simple. Some time ago I
checked some extremes to determine the mechanisms involved. The most
radical of these was to pop a 8" (major) diameter toroid on top of a
17" diameter coil. The capacitance added was less than a pF
(according to the change in Fr) yet the same terminal caused a very
significant difference when placed above a 2" diameter coil (don't
ask me to quantify it - it was so long ago I forget the figures).
However, the 2" case was in the 10%-type range, in other words, more
than notable. Point is, a fixed reduction percentage figure will
apply only for a limited set of size - size ratios and is therefore
invalid in general. However, I see no reason why a figure for a
particular coil and torus cannot be calculated with a modicum of
precision based on a consideration of e.s. fields and possibly even
just a mechanical size vs size rule (including of course pacement
height of the toroid above the windings).

Malcolm

On 5 Nov 2003, at 11:49, Tesla list wrote:

 > 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
 >
 > ------------------------------------
 >
 >
 >