Re: rolled caps (fwd)

---------- Forwarded message ----------
Date: Mon, 15 Dec 97 17:24:17 -0500
From: Adam <absmith-at-tiac-dot-net>
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:

      C [F] = ---------

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 

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

!!!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

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!



Adam Smith
Epoch, Inc. Digital Music Project

www.tiac-dot-net/users/absmith/     Now with MP3!     Musik. Macht. Macintosh.