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Re: Cap Question

Reinhard,
Oops! I screwed up my calculation below. My appologies. I didn't get the
dielectric of 42mils correct. After redoing the math with the correct
numbers, I
did get the equivalent of 19.8nF, near what you originally calculated. I
also put
in the wrong numbers on my other post on the Java calculator because I was
using
input values from my previous post below. The Java calculator when given the
correct input values also came up with 19.8nF. So, ignore my previous post,
it's
all messed up, however, the formula .224 * k * A / d * N is correct for your
rolled cap. You should also come up with 19.8nF on a calculator using this
formula.

Well, now I understand a little of why your scratching your head on your
measured
value. If your meter is measuring correctly, I would have to say that the
problem
has got to be the dielectric spacing between plates. You did use 24 sheets at
1.75mils ea. I have to believe that the actual combined dielectric
thickness of
42mils is way off. If you run the math backwards to find the dielectric
thickness
from your measured value of 10.5nF, the dielectric thickness ends up at about
80mils, almost double the thickness of what your think is in there. You didn't
happen to measure 24 plys of poly before you rolled it up did you?

My mumble appologies for the previous numbers,
Bart

Tesla List wrote:

> Original Poster: "Barton B. Anderson" <mopar-at-uswest-dot-net>
>
> 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
>
> k = dielectric constant (2.0)
> A = plate area in square inches (78.74 * 11.8)
> d = distance between plates (.00175 * 24)
> N = Number of plates (2)
> / 1000000 = nF
> Bart
>
> Tesla List wrote:
>
> > Original Poster: RWB355-at-aol-dot-com
> >
> > Hi All,
> >
> > Iīve got a really wierd problem. H E L P !!!!
> > I decided to "oil in" one of my test caps. So I bought myself a piece
of PP
> > pipe, two end caps and proceeded on.
> >
> > The cap specs:
> >
> > Al foil: (2x) 78.74" long and 11.8" wide
> > PE sheeting: (2x) 24 sheets of 1.75mil PE (42 mil per set)
> >
> > I started with 24 sheets of PE. Next is one AL foil. Then again 24
sheets of
> > PE and stopped with the last AL foil. I used a wooden rod to roll the
> thing up
> > nice and tight. I didnīt have very much slippage (PE sheets) and it
ended up
> > rather nicely.
> >
> > Hereīs a bad ASCII Drawing:
> >
> > _________________ AL foil
> > _________________ PE (24 sheets)
> > _________________ AL foil
> > _________________ PE (24 sheets)
> >
> > I put some tape and wire ties around it and removed the wooden handle. The
> > electrical connection where placed in the middle of each AL foil sheet
> (across
> > the full width). I figured this would give me better discharge and
> inductance
> > values.
> >
> > Here the mystery starts.
> >
> > 1.) I had calcīd this cap to be aprox 21 nF. It turned out to having only
> 10.5
> > nF. Thatīs only one half. I scratched my head, unwound the whole thing and
> > tried to rewind it tighter. It didnīt help at all. Why? Is there a
> possibility
> > that the two AL sheets are acting capacitivly on each other, because I
> didnīt
> > cover the top AL foil with more PE sheets effectivly reducing the
> expected 21
> > nF to the measured 10.5nF ?!?. I donīt think so.
> >
> > 2.) I plunged it into my PP container and filled her to the rim with
mineral
> > oil. The capacitance didnīt change at all. I ran a line from the vacuum
port
> > to a glass full of oil. From the glass I went to my trusty vacuum pump
> (an old
> > fridge pump...works pretty good for the $10 it cost me). Next mystery. I
> > pumped it down and measured the capacitance while pumping down. I
figured it
> > would go up as trapped air was being removed. Nope, it didnīt!! It
actually
> > went down aprox 2nF in the beginning. After 30 min of running I removed
the
> > vacuum pump and let the cap suck back the oil in the glass jar. There was
> > still oil left after the vacuum was back to zero, so I figured it was
> (almost)
> > full. The cap now measured 10.8nF. !!#~-at--at--at-##+!! I reconnected the vacuum
> pump
> > at let her have it another 30 min. As the pump started making vacuum
again,
> > the cap value started to drop again, stopping at aprox. 10.2nF. I redid
this
> > twice more. I let the cap stand for two full days and measured it
again. Now
> > it has 10.95nF.
> >
> > 1.) Why doesnīt the cap have 21nF or at least somewhere near this?
> > 2.) Why didnīt the cap value increase, as it was being filled with oil. I
> > figured the PE and the oil should have a K of around 2 at least ?
> > 3.) Why did the cap value drop as I removed the air ?
> > 4.) Why didnīt the cap change itīs value (not much at least) after
> standing a
> > few days?
> >
> > Please help a poor coiler who is scratching his head in vain, chewing
on his
> > finger nails trying to figure out what he did wrong.
> >
> > BTW:
> > 1.) I calcīd this value on my homebrew tc prog, but other progs gave me
the
> > same values, so it shouldnīt be a program bug.
> > 2.) Yes, the vacuum pump really did pump out air. This was clearly
> visible in
> > the glass jar where the oil foamed and bubbled as air was being removed
from
> > the PP container.
> > 3.) No, the PP container does not have an air leak. I greased the o-rings
> > thoroughly before assembly. During the vacuum run I put a thin film of oil
> > along the port and both end caps. No oil was sucked in and I didnīt
hear any
> > air being pulled in. So I figure it was vacuum tight.
> >
> > coiler greets from germany,
> > Reinhard