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RE: Formula for true self capacity of a coil.
Gavin -
You may be interested in how your calcs compare with the JHCTES Ver 3.1
program.
Coil Parameter Gavin JHCTES
# 1 L 17.87 17.87
Cmed 8.8 8.9
fr kHz 401.43 403.6
# 2 L 29.3 28.82
Cmed 9.9 9.79
fr kHz 298.00 299.58
# 3 L 42.25 42.67
Cmed 11.19 11.09
fr kHz 230.41 231.38
Very good agreement considering different equations were used.
John H. Couture
----------------------
-----Original Message-----
From: Tesla List [mailto:tesla@pupman.com]
Sent: Monday, May 29, 2000 11:27 AM
To: tesla@pupman.com
Subject: Formula for true self capacity of a coil.
Original Poster: "Gavin Dingley" <gavin.dingley@astra.ukf.net>
Hi Bob, Terry, Malcolm,
I think I may have found a formula for true self capacity of a coil,
well this is it:-
C = ( (11.26 * H) + ( (16 * R) + ( 76.4 * R^(3/4) ) ) /
(sqr H)
C is the coil capacitance in pF, R is the coil radius in meters and H
the coil height in meters.
I hope that makes sense?!
It came from a TCBA article written by D. C. Cox. Pres., Resonance
Research.
Measured Values for Capacitance of Spheres and Toroids
by: D. C. Cox. Pres., Resonance Research
Volume 17, #3 TCBA News 1998
A photocopy of the article was sent to me by Don Butler, a fellow coiler
in the U.K.
I had sent the article to Bob Jones, but there appeared to be
complications in reading it, so I tried the formula my self to see what
would happen, the result looked good.
I wrote a program that calculated coil inductance, Medhurst C and "Cox"
C, along with the lumped LC resonant frequency using Medhurst and
resonant frequency using the Jones formula with Cox's self capacity of a
coil.
I then plugged Malcolm's coil data from an earlier post into the program
and this is what came out:
Coil 1)
Hs = 21.75", Ds= 4.75", N approx 870t, L = 17.87mH
1/4 wave = 394.5kHz
Cmed = 8.8pF
Ccox = 22.18pF
Lumped LC (using Cmed) fr = 401.43kHz
Jones (using Ccox) fr = 397.07kHz
-------------------------------------------------------
Coil 2)
Hs = 22.75", Ds= 5.95", N approx 910t, L = 29.3mH
1/4 wave = 269kHz
Cmed = 9.9pF
Ccox = 24.64pF
Lumped LC (using Cmed) fr = 298kHz
Jones (using Ccox) fr = 296.678kHz
-------------------------------------------------------
Coil 3)
Hs = 16.2", Ds= 9.05", N approx 650t, L = 42.25mH
1/4 wave = 212.7kHz
Cmed = 11.19pF
Ccox = 33.6pF
Lumped LC (using Cmed) fr = 230.41kHz
Jones (using Ccox) fr = 208.78kHz
-------------------------------------------------------
The Jones formula is;
fr = 1/ (4 * sqr(L*Ccox) )
Well it appears to work, and using Medhurst C in the Lumped formula also
seems to work two. Of course, the Jones formula is more versatile when
calculating wavelengths other than 1/4-wave.
Regards,
Gavin, U.K.