# RE: Method of measuring self C of secondary

```
To All -

The test to find the true self capacitance of the TC secondary coil is shown
in several publications including the Tesla Coil Construction Guide. The
real trick is to find this capacitance at the design stage before the TC is
built. You can now find this parameter to an acceptable accuracy by using
the JHCTES Ver 3.1 TC computer program that is now a free download from the
Internet.

The secondary terminal (toroid) is a program input. If this is made zero the
coil self capacity is shown in the output. If a toroid is used the
capacitance should be the free space (isolated) capacitance less 15 to 20
percent to allow for the effects of placing the toroid on the coil.

The test for the coil self capacitance and the operating frequency of your
coil (in use) can then be compared to the program. These are not easy
measurements as many things can go wrong and be misinterpreted. Using a
scope and Terry's special probe is a good start.

John H. Couture

----------------------------

-----Original Message-----
From: Tesla List [mailto:tesla-at-pupman-dot-com]
Sent: Monday, May 01, 2000 5:33 AM
To: tesla-at-pupman-dot-com
Subject: Re: Method of measuring self C of secondary

Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz>

Hi Robert,

> Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
>
> Hi Terry and all,   (combined reply)
>
> It hard to keep with Terry's prodigious output.  Thanks for the
measurement
> and mods to T5 or is it T6 with an option of voltage weighted  self (vwsC
or
> medC) or  self C (true or LC bridge C).  Does it  now output the self C
> (true)
> along the coil if so can I have a copy.  Should very useful.   How about a
> output for mathcad.  Does it now produce self C (true) of the topload. You
> should set the primary V to 0v because you dont know the phase relative to
> the secondary so zero will give the least error.
>
> The measured self C (true) confirms a fiddle factor in Medhurst but
Medhurst
> may be for an isolated coil.  In which case  the difference between
Medhurst
> and self C (true) will be dependent on the separation from the ground
plane.
> Does anybody know what Medhurst assumes and if he assumes a ground
> plain what was the separation.

He made no reference to it that I'm aware of. I've measured coils at
ground and elevated from it and found the differences on Fr to be
very small. The only requirement is that one end of the coil be
grounded.

> The self C (true) or better the equation is required  for simulation and
> maths models.
>
> The use of a voltage weighted C in a lumped resonance calculation or any
> calculation is totally new to me. As is the concept of a current weighted
L.
>
> The secondary can be shorted across the ends during C measurements to
> decrease L to 1/4 and hence reduce errors.

But is that how it appears electrically in use?

Regards,
malcolm

```