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Re: Determining self-capacitance of inductor
Subject:
Re: Determining self-capacitance of inductor
Date:
Wed, 19 Mar 1997 21:22:14 -0700
From:
"DR.RESONANCE" <DR.RESONANCE-at-next-wave-dot-net>
To:
"Tesla List" <tesla-at-pupman-dot-com>
To: Steve Falco
There are methods of calculating the self-capacity but several factors
need
to be known such
as the dielectric constant of the wire coating, etc.
The experimental method is usually easy and sometimes faster. Using
classic formula for a long solenoid (listed in detail in the ARRL Radio
Amateur's Handbook) calculate the inductance of your coil. Measure it
also
if possible. Drive the base of the coil with 2-3 turns of scrap hookup
wire connected to a signal generator. Keep the 2-3 turns below the sec.
coil winding about 1 inch. Using a scope connected in series with a one
meg resistor to the top HV electrode of your coil, determine the
resonant
freq of the inductor. Knowing the resonant freq. and the calculated
inductance you can easily use the classic equation for freq. to
calculate
the distributed capacitance of the coil.
While the coil is in operation the electric field above the HV terminal
will usually add a small amount of capacitance to the system. If you
have
a friend who works at a University or have some other means of using a
storage scope for a few evenings you can capture the resonant freq.
while
the coil is running. Take about 20 samples and average them for your
operating resonant freq.
You now have two resonant frequencies --- one in static mode and one in
dynamic mode. You can now calculate the additional capacitance of the
electric field and add this value to your static resonant freq. to
obtain
the total secondary capacitance while the unit is in operation.
This procedure is simple and usually fairly accurate for general TC
work.
Good luck with your research.
DR.RESONANCE-at-next-wave-dot-net
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: Self capacitance and Medhurst
> Date: Wednesday,March 19,1997 6:46 PM
>
> Subject:
> Self capacitance and Medhurst
> Date:
> Wed, 19 Mar 1997 06:13:34 -0500
> From:
> Steve Falco <sfalco-at-worldnet.att-dot-net>
> To:
> tesla-at-pupman-dot-com
>
>
> I have seen two methods of finding the self capacitance of a coil. One
> is closed-form:
>
> V = L/R (in inches)
> CD = 5.08*R*(.0563*V+.08+.38*SQR(1/V)) 'uufd.
>
> And the other is table-lookup, attributed to Medhurst:
>
> C = K x D (D in centimeters)
>
> H/D K
> 5.0 0.81
> 4.5 0.77
> 4.0 0.72
> 3.5 0.67
> 3.0 0.61
> 2.5 0.56
> 2.0 0.50
> 1.5 0.47
> 1.0 0.46
>
> These two methods agree at the low end of aspect ratio, but begin to
> diverge at the high end, with the closed-form yielding the smaller
> values.
>
> Does anyone on the list have a feel for which approach is more
> accurate? Does anyone know where the closed-form originated?
>
> Note: I have played around with gnuplot, and can easily come up with a
> closed-form which matches the table points much more closely. But I'd
> like to know if the closed-form above is better than the table, or is
> merely an approximation of the table.
>
> Steve Falco
> sfalco-at-worldnet.att-dot-net