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Re: Self Capacitance formulas

To: teslaatgrendel.objincdotcom

Subject: Re: Self Capacitance formulas

From: MSR7atPO.CWRU.EDU (Mark S. Rzeszotarski, Ph.D.)

Date: Thu, 11 Jan 1996 09:43:09 0500

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>>From Esondrmnataoldotcom Wed Jan 10 10:45 MST 1996
<snip>
>What is the equation for self capacitance? I assume the above formula is for
>close wound coils, is this corrrect? How do you do calculations for space
>wound coils?
>
>Thanks, Ed Sonderman
>
>
Hello Ed and others interested in self capacitance,
I use the formula attributed to Medhurst for self capacitance
estimation. It gives values which are fairly close to what I measure
experimentally. The formula does not take into account wire spacing, so it
may be in error somewhat for spacewound coils. I suspect space winding does
not substantially alter the distributed capacitance, however. I have seen
no formula for spacewound coils.
Medhurst's formula: C = K x D
where: C = capacitance in picofarads
K = constant which depends on the ratio of the coil height to diameter
x = means multiply K times D
D = solenoidal coil diameter in centimeters
H = coil height 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
Sorry, I do not have my references handy here to tell you where I got this.
If you want to know, send me Email privately.
>
>
>I was looking at some formulas you published on 81095 (Tesla Math
>Formulary). Re: Equation # 6, Frequency of Coil, does this formula take into
>account the self capacitance of the coil?
>
Regarding formula #6 for calculating resonant frequency, that formula
estimates resonant frequency based on a quarter wavelength of wire alone.
The resonant frequency of a secondary coil will be lower than that due to
distributed capacitance (above) and the toroid placed on top. It is more
accurate to estimate operating frequency using the f=1/(2 x pi x sqrt(L x
C)) formula, where L is the inductance of a solenoid in henries, and C
includes the distributed capacitance plus the toroid capacitance, in units
for Farads. To get a ballpark C value, compute the distributed capacitance
above and double it. If you use large toroids (good thing if you have
enough available power), add another 1030 picofarads to C to take into
account the capacitance of the large toroid.
Regards,
Mark S. Rzeszotarski, Ph.D.