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Re: Cap Question
Mike,
Hopefully you read my last post on this. I had errored my input numbers
(amazing
what not paying attention to what you are typing can do). Reinhard hard
mailed me
off the list also questioning. I had found the problem and posted to the
list the
error about 20 minutes before I recieved his mail. I told him I would
probably get
hit with a lot of (wrong answer) posts, as I should have, it was wrong.
Yes, the equation is a variation and I got it off this list of course. It
is ONLY
for rolled caps. The equation C =.224*k*A*(N-1)/d is for plate caps . As Bert
Hickman responded, for a rolled cap you should replace (N-1) for (3-1) or (2).
This is why the equation I posted is C = .224 * k * A / d * 2, the same as C =
.224 * k * A * (3-1) / d. Their both identical. I just screwed up my use of
it in
that post.
Also, a "rolled" cap has 3 dielectrics where the charge is stored between
the 4
sides of the 2 plates. Actually, as I think about this N is probably better
represented as the quanity of individual dielectric charges upon final
configuration. If we used N in this manner, we could use the same equation for
plate or rolled caps.
I believe you were right in your first post questioning the dielectric total
thickness. I think actual vs. calculated was not correct (or as Bert posted,
trapped air has reduced k). Also, when the oil was added, capacitance
decreased.
But then it should as the poly soaked in the oil and further seperated the
plates.
Sorry if I confused you or anyone else,
Bart
Tesla List wrote:
> Original Poster: Hollmike-at-aol-dot-com
>
> In a message dated 10/3/98 5:23:58 AM Mountain Daylight Time,
tesla-at-pupman-dot-com
> writes:
>
> > 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.