Re: Plate thickness
Subject: Re: Plate thickness
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Date: Thu, 1 Feb 1996 08:38:52 +1200
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Mark Graalman writes...
> I have a thought that I'd like to run by you all, the calculated skin
> effect depth of penetration at 100 KHZ is about .011" in copper, if a
> capacitor is built using .002"thick foil as plate material would it be
> possible that since the foil is thinner than the depth of conduction at this
> frequency there would be a loss of current discharge capability to due an
> increase in the "RF" resistance?
That is a really good question. I've studied the skin effect in some
depth (npi) and have a few ideas. Firstly, It is going to be virtually
non-existent in plates of this thickness. That means that the DC
resistance of the plates will be nearly identical to the AC
resistance. So the discharge current should not be affected by
the skin effect as such. But the resistance of the plates is not
going to be optimally low either. My personal opinion is that
it shouldn't matter esp. for extended foil and plates.
> I would think that in order to obtain maximum current capability
> at these low frequencies that plate material should be thick.
If you do make the plate thickness comparable with the skin depth,
you'll be in much the same position, so I'd say that you could
lower the resistance further. But much thicker than this, and
the effect starts happening. The consequence is you get into an
area of diminishing returns. For example, if your plates are
5 depths, you are not going to lower the resistance by a factor of 5
because the current is then crowding in the surface.
> This is just something I`ve been thinking about, it is
> obvious that foil capacitors work, I'm just thinking that a thin foil would
> have the effect of increasing the effective ESR of the capacitor.
I think it may not be optimally low, but beside dielectric losses
and lead losses for the types of caps mentioned, I think it shouldn't
make much difference.
My 2 centum.