FW: Extended foil rolled cap construction
From: Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Saturday, August 22, 1998 10:39 AM
To: Tesla List
Subject: Re: Extended foil rolled cap construction
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