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Re: Formula for true self capacity of a coil.
Hi Gavin,
On 29 May 00, at 12:27, Tesla List wrote:
> Original Poster: "Gavin Dingley" <gavin.dingley-at-astra.ukf-dot-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.
Thanks for doing that. I have to stress that those measurements
were taken under less than ideal conditions.
I just tried the formula on my ~10" coil and got a Ccox of 41.2pF
which seems believable (perhaps a bit on the low side so I will
recheck my computation). Obviously, trying to take meaningful C
measurements at home or in the workshop here is a total waste of
time. I will simply have to get the room.
Regards,
Malcolm