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Re: Data from Ed Phillips
Tesla List wrote:
> Date: Sun, 3 Nov 1996 10:56:37 -0800 (PST)
> From: Edward V. Phillips <ed-at-alumni.caltech.edu>
> To: chip-at-poodle.pupman-dot-com
> Cc: ed-at-alumni.caltech.edu
> Subject: Help!
>
> Here is the first bunch of stuff which I thougt might be
> useful:
<SNIP>
> NOTE:
>
> The effective capacitance (the value to add to the
> distributed capacitance of the coil to determine the resultant
> resonant frequency) of a terminal on top of a coil will be less
> than the capacitance given here. These capacitances are the
> values which would result if all of the electric field lines
> from the terminal terminated in ground infinitely far removed
> from the terminal. In the case of the coil, some of the field
> lines will terminate along the coil, so that some of the capacitance
> will be across only part of the coil, reducing its effect on the
> resonant frequency. This is kind of a lame explanation, but isn't
> too far from being correct.
>
<SNIP>
> "The effective capacitance of a toroidal upper terminalis
> approximately equal to the O.D. of the toroid, less about half of
> the diameter of the coil."
>
> This is for measurement at low voltage where there is no discharge
> with resultant ion cloud with its added capacitance,
>
> Does anyone have any measurements to confirm or modify
> this?
I've found that, depending upon the toroid height vs the top of the
coil, the effective toroid capacitance seems to fall within 70-85% of
the isolated value. I've used a formula for toroid capacitance (I think
Bert Pool originally generated it) which seems to give values reaonably
close to those in your table. The predicted capacitance for my 8" x 32"
toroid is a little over 35 pF, but it behaves as though it had only
about 26 pF or about 75% of its isolated capacitance (at low power).
However, my toroid may be a positioned a little lower versus the top of
the coil, since I'm seeing about a 9 pF drop on my 10" coil. Maybe
there's a way to "tweak" for height as well as coil diameter...?
<SNIP>
> MEDHURST'S FORMULA FOR DISTRIBUTED CAPACITANCE OF A SOLENOID:
>
> Cd = 5.08R(.0563L/R+.08+.38SQR(R/L)) uufd. R,L IN INCHES
> Cd = 2R(.0563L/R+.08+.38SQR(R/L)) uufd. R,L IN CENTIMETERS
>
> Where:
> R = Radius of solenoid, L = length of solenoid,
> and SQR(XX) means square root of xx.
>
> I have found that the calculated resonant frequency using
> these equations matches measured values within a few percent,
> which is the experimental error.
>
> That seems like a lot of text, but I think it would help
> some of the guys who seem to be in the dark about some of these
> matters and who haven't taken the trouble to get hold of some of
> the computer programs which are supposed to "do it all for them"
> and substitute for thinking and understanding basic principles
> of electricity.
>
> Best wishes,
> Ed Phillips
Great Post Ed!!
Most of this information is not readily available to folks without a
fair amount of digging. Thanks especially for including Medhurst's
formula - didn't have this one yet! I agree with your comments above -
the original formulas are essential for "sanity checks" and work great
in spreadheet modeling.
Safe coilin' to ya!
-- Bert --