# Re: Self Capacitance formulas

• To: tesla-at-grendel.objinc-dot-com
• Subject: Re: Self Capacitance formulas
• From: MSR7-at-PO.CWRU.EDU (Mark S. Rzeszotarski, Ph.D.)
• Date: Thu, 11 Jan 1996 09:43:09 -0500
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• Cc: msr7-at-slc5.INS.CWRU.Edu

```>>From Esondrmn-at-aol-dot-com 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 E-mail privately.
>
>
>I was looking at some formulas you published on 8-10-95 (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 10-30 picofarads to C to take into
account the capacitance of the large toroid.
Regards,
Mark S. Rzeszotarski, Ph.D.

```