Re: Cap Question

To: tesla-at-pupman-dot-com
Subject: Re: Cap Question
From: Hollmike-at-aol-dot-com (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Sat, 03 Oct 1998 14:46:47 -0600
Approved: terryf-at-verinet-dot-com

In a message dated 10/3/98 5:23:58 AM Mountain Daylight Time, tesla-at-pupman-dot-com
writes:

> Reinhard,
>  My calculation came up with 9.8nF, near what you measured. There got to be
>  something wrong with your calculation:
>  
>      .224 * k * A / d * N
>  
>   = .224 * 2.0 * 929 / 2 * 2 / 1000000 = 9.8nF
><snip>

>  Bart
>  
Bart,
    Where did you get that equation?  I have always seen  C =.224*k*A*
(N-1)/d.  Is that equation you posted a variation on the plate cap equation
for a rolled cap ? It seems like you divide the plate area by 2.  Why is that?
Also, if you use a dielectric constant of 2.1, the result is 10.4nF which is
quite close to the measured capacitance.  I am not doubting you, but I have
never seen that equation before.
I have not made a rolled cap, but I have heard that it supposedly causes a
doubling of the capacitance by creating a virtual third plate (then N =
3)since the backsides of the plates "see" each other once rolled up.  I
wondered about this, but many seem to subscribe to this notion.  This may be
the reason the calcs are double what Reinhard actually measured.
   I made an earlier post thinking that perhaps the total thickness used in
the calcs may have been in error from imperfect thickness in the poly or air
space between the numerous layers, but now think that it is just a calculation
problem.
Mike H.