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Re: Formula for true self capacity of a coil.
Hi Malcolm,
your coil data was the only reliable measurements I could test this equation
by. My own measurements of coils are dubious due to poor equipment and
practical experience. As an aside, when I first got into coiling I was
subscribing to the ITS discussion list. I needed a method to calculate the
self capacity of a coil (this was about three years ago), you sent me a qbasic
program that worked it out. You must have been the only one on that list who
knew anything about TCs.
Anyway, the formula looks o.k at the moment, but Bob will verify it for sure.
Regards,
Gavin
Tesla List wrote:
> Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz>
>
>
> >
> > 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