I emailed Kevin (creator of Teslamap) the exact question a few years ago.
Here is his reply...
Hi Dan,
All the top load capacitance formulas are here:
http://www.teslacoildesign.com/design.html#design_topload
As you can see, there's several different formulas to calculate the
capacitance of relatively large and small top loads. Empirical testing by
others has shown that the formulas are all fairly accurate. However, I
don't believe any of the formulas have proven to be consistently more
accurate, so I simply use them all and average the results.
The "optimum top load capacitance" is the capacitance that will tune the
Tesla coil to 1/4 wavelength frequency - however, it is not necessary to
tune to 1/4 wavelength. Tuning to 1/4 wavelength is very common in antenna
design, and it was once thought that Tesla coils should be tuned to 1/4
wavelength. However, it's been shown to be unnecessary. But many successful
Tesla coil designs have a top load capacitance that is very close to the
capacitance required for 1/4 wavelength tuning. I'm not sure if it's a
coincidence or maybe just some Tesla magic? Anyway, I have included the
"optimum top load capacitance" output in the program for those who wish to
use it, and also because it generally seems to be a good value to shoot for
in Tesla coil designs.
Optimum Top Load Capacitance = Required Secondary And Top Load Capacitance
- Secondary Coil Capacitance
where...
Required Secondary And Top Load Capacitance = 1 / (4 * 3.14^2 * Quarter
Wavelength Frequency^2 * Secondary Coil Inductance)
Secondary Coil Capacitance = (0.29 * Secondary Coil Winding Height) +
(0.41 * (Secondary Coil Form Radius)) + 1.94 * Sqrt(Secondary Coil Form
Radius^3 / Secondary Coil Winding Height)
I hope that helped. Please let me know if you have any more questions.
Thanks,
Kevin :-)
On Thu, Mar 4, 2021 at 11:32 AM Futuret via Tesla <tesla@xxxxxxxxxx> wrote:
> AJ,
> You can often use the topload size to control how many simultaneous sparks
> are produced. With a smaller than optimal size, you'll get more sparks at
> one time, but they will be shorter. With a large enough topload you can
> get only one spark streamer at a time, but it will be longer. A certain
> size might give two simultaneous sparks that sort of alternate from side to
> size in and nice way. One will grow while the other shrinks, etc. This
> effect might be best seen at low break rates. I like this effect but it's
> also nice to see a great many sparks. It all depends on the effect you
> want. In any case, a certain amount of input power will be needed to
> drive a certain size topload. I had written some rules of thumb for this
> sort of thing in the past. I don't have the information handy at the
> moment. In general, lower break rates tend to produce slower lazier
> streamers, but fast breakrates tend to create more frantic streamers, but
> this is not always the case. There's interactions between the various
> parameters.
> John
>
>
> -----Original Message-----
> From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Adam James
> Sent: March 4, 2021 10:57 AM
> To: tesla@xxxxxxxxxxxxxxxxxx
> Subject: [TCML] Topload Question
>
> Teslamap gives [Optimum} topload. It is often difficult to achieve the
> optimum.
> Can someone explain results of going a little under vs. a little over for
> coil performance?
> AJ
>
>
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>
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