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Extended foil rolled cap construction
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From: Gary Lau 24-Aug-1998 1103 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Monday, August 24, 1998 10:14 AM
To: tesla-at-pupman-dot-com
Subject: FW: Extended foil rolled cap construction
>From: Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
>
>Comments added to original text below...
>Fr. Tom McGahee
>> 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.
>Gary, when the caps are designed to be connected in series, it is OK
>to use single sheets, and as you have found, at times it is desireable to
>do so. You get maximum breakdown voltage for your buck when the poly is
>thinner. Of course, if it is too thin then there are *other* handling
>problems. It is always a tradeoff.
>
>Is there some reason why you chose the 12" dimension instead of a larger
>width? With the extended foil construction you lose more of the
>width (as far as *active* plate area is concerned) than you do with
>the regular rolled cap construction. Since the amount of edge that
>gets wasted is fairly *constant* (as far as *width* is concerned...)
>then you get more capacitance for your poly buck if you make the
>width larger. Of course there are considerations such as the width of
>the aluminum foil, the size of the container, etc.
>> 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 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.
>Anytime you use tape to keep the alignment this crinkling will occur. The
>foil must be allowed to slip a bit, or else put up with the crinkling.
>The problem is that as the poly and aluminum foil are wound around the
>PVC pipe, the outer section 'grows' faster than the inner section. So
>there must be slippage to allow for this.
>
>If I were attempting to build a machine for making such extended foil
>rolled capacitors, it would have the following traits:
>
>1) The PVC pipe would be mounted so that a crank handle could be
>affixed. Turning the handle would cause the pipe to turn and the
>poly/foil would then wrap around the cylinder.
>
>2) The PVC pipe would be mounted such that it would not slide
>back and forth as it was rotated.
>
>3) There would be four feeder assemblies, two for poly, and two
>for foil. The feeder assemblies would take the form of troughs with
>a smooth, wide bottom and 'sides' that would guide the material
>through in straight lines, with no sideways deviation.
>
>4) the feeder assemblies would be stacked and aligned such that
>each material lined up properly.
>
>5) each feeder would have a means for applying a consistent
>friction to the material so that as the PVC was rotated, it would
>*pull* the material under tension. The sandwich of materials would
>then roll up without going all over the place.
>
>The capacitor would be 'wound' under a reasonable tension. When
>the end of the unit was approached the foil would be cut so that there
>would be proper end overlap.
>
>When the end was reached the capacitor assembly would have to be
>held together with bands or tape or something that will survive
>being immersed in oil.
>> 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
>The problem that you will always face when making such capacitors by hand
>is that you have a sandwich of four layers that is involved. It is
>difficult for one person to deal with all of this at once. Get several
>good friends to help you.
>
>Fr. Tom McGahee
I took my own advice and for each plate, used multiple 18" long segments
of foil, each held in place with two small bits of kapton tape on the
leading "short" edge, and overlapping the next 18" segment by about 1/4".
When I rolled this assembly up, the rolling was perfectly smooth - there
was no crinkling of the foil. I did have to start over several times as
it became apparent from the foil edge that the roll was becoming conical,
so in addition to a helper holding the far end of the sheets in
alignment, large amounts of patience are required. Short of good luck, I
found no tricks to getting the initial roll correct.
One thing I did note was the importance of setting the foil long edges on
a perfectly straight line. It is not sufficient that the margin be even
along the length of the poly. Since you're judging the "squareness" of
the roll by the foil edge (the poly edges are hidden) and since the poly
long edges may not be perfectly parallel, it's vital that the foil edge
be straight.
The reason I chose 12" wide poly was that I needed four poly dielectric
sheets to make two capacitors. I had exactly one 4"x8" sheet of poly, so
cut four ways is 12" wide. Also, this dimension worked nicely with the
standard 12" wide aluminum foil. And lastly, as I was targeting a pair
of .02 uF caps in series to yield .01 uF total, my chosed dimensions,
after rolling them up (but before oil immersion) yielded .0098 uF. Not
bad!
Next step is vacuum pumpdown and oil fill. This may take some time to
find the various fittings...
Gary Lau
Waltham, MA USA