RE: Secondary capacitance

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

Original poster: "John H. Couture by way of Terry Fritz <twftesla-at-uswest-dot-net>" <couturejh-at-worldnet.att-dot-net>


There is another method to measure the coil self capacitance that may be
more accurate. This method is shown as Fig 39 in my Tesla Coil Design
Manual. The test uses the same frequency equation but finds the frequency in
a different way. Another interesting test shown on Charles web site (#5) for
the coupling is shown in my Tesla Coil Construction Guide Fig 26. However,
this test is also different than the #5 test and is easier to perform and
may be more accurate. I was happy to discover that Charles schematics were
similar to the ones I had chosen out of the dozens of tests methods for
finding these parameters. Some day I hope to get these tests on my site.

John Couture

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-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, January 17, 2001 10:17 AM
To: tesla-at-pupman-dot-com
Subject: Re: Secondary capacitance


Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<paul-at-abelian.demon.co.uk>

Charles Hobson wrote:

> I did a crude set of measurements on my TC to determine self
> capacitance. Perhaps you can comment on them. It is in my web
> site
>
> www.charles.a.hobson.btinternet.co.uk/MEASUREMENTS.htm

Charles,

Funny that, I was admiring this page just the other day.  Looks like
a good intro to the subject of measurements. The C that you calculate
is based on the low frequency coil inductance so it may be out by
some -30% to +15%. I'm working on a way to estimate the effective
inductance from the coil dimensions, eg something like

 Les =  Ldc *  ( 1.5599 + 4.6321 * h) ^ 0.0263 *
               ( 6.75 + 4.191 * h/d) ^ -0.8836 *
               ( -0.0021 + 1.3341 * b/h) ^ 0.2486 *
               ( 1201.79 + 19.4111 * awg) ^ 0.1498 *
               ( 48.77 + 6.8 * sr) ^ 0.0142

 where h = coil length, d = diameter,
 b = height of coil base above ground,
 sr = spacing ratio = turns * wire_diameter/h,
 awg = wire gauge,
 Ldc = inductance at DC, ie Nagaoka value.
 and ^ means 'to the power of'. Coil length is in metres.

valid for coils without topload, might give a reasonable estimate of
the effective series inductance Les close to Fres - it gives a peak
error of 2.2% on a large number of simulated coils, but is untested
on real coils.

Another way to get the effective capacitance and inductance is to
measure the ratio of top voltage to base current. The former is not
an easy measurement, but the ratio does tell you quite a lot about the
resonator. Try measuring C (= Ibase/(2 * pi * f * Vtop)) this way and
see if it agrees with the C necessary to resonate with the Les above.

I particularly like the primary/secondary ringing stuff. An excellent
illustration of the energy exchange. By doing a bit of cycle counting
you could determine quite a lot of info about the coupling conditions.
Could make a good setup tool for experienced coilers as well as a neat
demo for the beginner. Maybe you could replace the outboard audio
oscillator with a 555 timer so that you can play with the quenching
point.

If anyone follows the instructions on your web page, I don't think
they'll go too far wrong. They might after a while notice that things
don't quite add up, but by then they'll be hooked on measurements
(which as everyone knows is much more interesting than just making
sparks) and be led to investigate the subtleties of resonating coils
a bit more closely.

On a note of style, you might do better with a less obtrusive
background and text color. Regarding the images on the web page, you
ought to use gif rather than jpeg [*], which will eliminate the
ringing which is making the formulas hard to read. There won't be a
size penalty, and you can make the gif backgrounds transparent so that
the diagrams and formulas will blend into the page better.

[*] jpeg is ideal for photos. gif is better for diagrams. The run
length compression within the gif format will make short work of the
large areas of uniform color.

Cheers,
--
Paul Nicholson,
Manchester, UK.
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