# Re: rolled caps (fwd)

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---------- Forwarded message ----------
Date: Mon, 15 Dec 97 17:24:17 -0500
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: rolled caps (fwd)

>I can't seem to find my information on how to calculate capacitance for
>a rolled capacitor.  Could somebody help?

Here is the standard MKS formula for a simple two plate flat capacitor:

er*e0*A
C [F] = ---------
d

Where:    C is the capacitor value in farads.
er  is the dielectric constant of your insulating material
e0 is the permittivity of free space =  8.85418781762 * 10^-12
A is the overlaping plate area in square meters
d is the plate separation in meters (dielectric thickness)

For a two plate rolled cap, you need to multiply the final value by 2,
since both sides of each plate are used, and you have twice as much
dielectric material as a two-plate flat cap.

If you are using more than two plates for your flat or rolled cap, you
need to add an (N-1) term to the numerator, where N is the total number
of conductor plates (not the number of dielectric plates). The above
equation assumes N = 2 (two plate cap).

If you are using *all inches* as measurements, the flat plate cap formula
is:

2.248965 * 10^-7 * K * A
C (microfarads) = -------------------------- * (N-1)      K=dielectric
const.
d

!!!Again, you need to multiply this result by 2 for a rolled up cap.
This is the formula I generally use when making homemade caps.

Here is a solved example for a rolled cap:

Dielectric plates : 8" by 72" 60 mil total LDPE, two used, K=2.0 (approx)
Conductor plates:   6" wide Al foil strips, 68" long overlapping region
Rolled up.

2.248965 * 10^-7 * 2.0 * (6 * 68)
C = ----------------------------------- * (2-1) * 2 =  0.0061 uF
0.060

BTW, This formula is EXTREMELY accurate for real world homemade caps,
once you assume a degree of sloppiness (e.g. K=2.00 for polyethylene,
instead of 2.26).  My flat cap that calculates to 0.02 uF and measures
0.01998 uF.  Pretty Good!

Enjoy!