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Re: Ctop Measurement vs. Calculation
Hi Bart,
For some time I have been an advocate of treating the
coil+terminal as a single entity with some combined area A as
defining the capacitance. This came about because my
experimental results showed that a small toroid's capacitance could
be totally swamped if its diameter was << than the diameter of the
coil on which it was sitting. Now the situation has changed somewhat
since Robert Jones pointed out that Medhurst's Cself (the one we've
all been using) is not the true Cself of the coil. While I still think the
structure must be treated as a single entity as regards its total
capacitance (a now somewhat different figure) only if the secondary
current is uniform (and in phase at the top wrt the bottom) at any
given instant can the real figure be used to calculate Vout.
Regards,
Malcolm
On 22 May 00, at 7:56, Tesla List wrote:
> Original Poster: "Barton B. Anderson" <tesla123-at-pacbell-dot-net>
>
> Dr. R, Ed S., all,
>
> I was experimenting a little this weekend with a sphere top capacitance.
> The sphere is 16" in diameter on top of my 12.5" x 44" secondary. D.C., I
> was referencing a post you sent to the list a long time ago. You measured
> the capacitance of a few spheres and toroids on an insulated column. Here's
> the data you posted:
>
> 7 inch dia sphere - - - - - - - - - 14pF
> 14 inch dia sphere - - - - - - - - 22pF
> 30 inch dia sphere - - - - - - - - 44pF
> 40 inch dia sphere - - - - - - - - 60pF
>
> 1.75 x 7 toroid - - - - - - - - - - 9pF
> 8 x 3 toroid - - - - - - - - - - - 11pF
> 12 x 3 toroid - - - - - - - - - - - 16pF
> 14 x 4 toroid - - - - - - - - - - - 25pF
> 24 x 6 toroid - - - - - - - - - - - 38pF
> 34 x 8.5 toroid - - - - - - - - - - 50pF
> 48 x 12 toroid - - - - - - - - - - 67pF
>
> The sphere I used is nowhere near the values above. My sphere (16") should
> be near 24pF (comparing to your 14" sphere above). However, using a large
> loop antenna and scope, I measured a near perfect 100kHz (the waveform was
> beautifully locked on). It should have been about 79kHz (Ls = 87.6mH , Cs =
> 21.6pF). It appears Ctop was only 9pF. This is far from what I expected. I
> expected some error from self distribution, but not this much. Sphere was
> set directly on top of the coil and run without breakout long enough to
> obtain measurements. I did this several times and got the same results
> using both 0.02uF and 0.04uF tank cap sizes (retuning).
>
> Also, I was looking at the measurements above and calcing Sphere Ctop using
> 1.414 x diam. At 14", the error is 30% but gets close with the 30" and 40"
> spheres. Is the equation incorrect? Is there a more accurate equation I'm
> not aware of?
>
> The toroid calculated values were worse. Errored 16% to 37%.
>
> The calculation used was:
> CpF = ((1+(.2781-d2/d1))x2.8)*(sqrt(2pi^2(d1-d2)*(d2/2)/4pi))
> d1 is the outer diameter of the toroid.
> d2 is the diameter of the cross section.
>
> If others have done similar tests, what were the results?
>
> Thanks in advance,
> Bart (happy to be coiling again!)
>