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Extended foil rolled cap construction
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From: Gary Lau 21-Aug-1998 0912 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Friday, August 21, 1998 8:23 AM
To: tesla-at-pupman-dot-com
Subject: Extended foil rolled cap construction
About a month ago I was inquiring as to how to construct an extended
foil rolled poly capacitor. Fr. Thomas McGahee replied with a detailed
explanation which seemed quite doable. Briefly, the method uses household
aluminum foil rather than flashing, extending over the entire long edge
of the poly, and crimped to a PVC core pipe with a metal hose clamp.
I attempted a dry run last night. As I'm planning on two identical
units in series, my dielectric in each is a single layer of .04" poly.
I know, it's best to have multiple layers of thinner stock rather than
one thick layer, but this is what I have. A single sheet also makes
alignment during the rolling much more manageable. So each unit consists
of two 12" x 96" x .04" poly sheets, and two 12" x 92" foil sheets, the
foil hanging over it's poly sheet by 1.5". The should result in each
unit being about .02 uF.
The problem is that with the foil hanging out over both ends, you loose
your poly-edge reference and the roll end becomes conical.
Have others have found any tricks for avoiding this?
While one might expect that I could simply use the foil edge for
reference, it turns out that the foil doesn't roll as neatly as one might
hope. I placed small pieces of kapton tape every 2 feet along the foil
edge with the 1.5" margin to ensure that the margin doesn't shift.
During the rolling, the margin was maintained, but the foil had to
crinkle a bit to do so, and as a result, the extended foil edge was not
100% uniform.
My next attempt will use many short foil segments instead one contiguous
sheet, each segment's starting position set with tape, but it's "ending"
position overlapping and free to shift during rolling relative to the
next segment's start. Hopefully this will result in smoother foil
rolling and hopefully a more stable edge reference. Since with extended
foil construction, current only flows on the short axis of the plates, it
shouldn't matter that each plate is many individual segments, analogous
to individual plates in a stacked plate cap.
Gary Lau
Waltham, MA USA