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Re: Rolled PE Caps



Kevin,
As I have received several inquiries about my cap design, I thought I
would post it to the list. I fire my coil almost daily for several
months now, and have never had a puncture or problem. If you have any
further questions I'd be glad to help any way I can.

Sparkin'
Bob Volk 
> 
> The measurements I used for my rolled poly caps are as follows:
> 
> The foil was Reynolds heavy duty aluminum foil (12" width- not the
18")This is available at any well stocked grocery store beside the regular
thickness foils.
> 
> The polyethelene was a standard Home Depot or Ace Hardware package of
10'X 25' .006 mil plastic.
> 
> I used Mike Hammers suggestion for multi-layers, multi-cap design. I
needed .01 uF for my 15K/.060 NST. I figured three caps in series would
give me plenty of puncture resistance.
> 
> The foil was cut 68" long ( .224 X 2.2 X 816 sq in X 2/.024 = .033 uF
approx.)
> 
> The poly was opened up and layed out on the floor, using a long
straight-edge to cut with. I allowed 1 1/2" overlap beyond foil for
flashover prevention - so I measured and cut 8 strips per cap (24 strips in
all). If you cut it in half lengthwise, you'll get 2 - 6' X 24' long sheets
of poly. Cut your 24 strips from this , 15" wide. You'll have plenty left
over ( you could get up to 38 strips this way )
> 
> So when done, you should have 24 strips of 15" wide by 72" long poly.
> 
> I used 3/4" pvc pipe as rolling core for my caps. I laid out 4 sheets of
poly, aluminum foil, four more sheets poly, aluminum foil. I taped two
terminals to foil sheets at opposite ends, taped one end of all poly/foil
assembly to the core and rolled it up TIGHTLY. Side note: I was having
great difficulty getting it to roll properly without foil or poly skewing
to the side (one advantage of having that 1 1/2" overlap!). By having my
son grab and hold the BOTTOM sheet of poly ONLY, I was able to keep tension
and roll theses caps very tightly! After rolling, I secured with 6 plastic
ties pulled as tightly as possible, excess tie length then cut off. These
will fit perfectly inside a 3" PVC pipe. Another added advantage to using
plastic ties: if you rotate the six tie connectors about 30 degrees each,
the ties will provide perfect standoff from the inside edges of PVC,
allowing adequate oil circulation!
> 
> The terminals I made by using a 3' length of heavy duty aluminum foil. I
folded it in half so it was 18". Then I folded it lengthwise about 3/4"
wide, over and over , the whole width of foil. This gave me a thick 18" X
3/4" wide terminal strip (2 of these per cap). If you wrap tightly, then
use the ties pulled as tightly as possible, these terminal strips work
GREAT! No soldering, no screw heads. Strictly friction fit. And as the poly
swells from oil, it improves connection.
> 
> The last step was cutting 3- 18" sections from 3" PVC pipe. Glue 1/2"X  1
1/4" X 1/4" thick plexi (scrap) radially  on inside of bottom cap (to
support poly roll and allow oil circulation). Be careful not to glue
supports too close to outside edge of caps to allow PVC pipe to slide all
the way down when inserted. Liberally apply glue to cap and pipe, insert
and let dry overnight. Insert poly roll. Connect the two aluminum terminals
to 2 - 1/4" 20 brass screws 1" long, inserted through other cap and glued
with epoxy, held with a nut. You can either carefully drill a 1/4" hole in
foil terminal with a SHARP drill bit, or modify a punch. You do need to be
gentle when working with these, they are not like flashing and will break
if you're rough - but they carry the voltage/current fantastically for me.
> 
> One other thing I did: I cut a piece of scrap 1/4" plexi 3" long X 1"
high, and wedged it in between the two aluminum straps going up to the
terminal connections.I feel this is added protection against any flashover
between terminals.
> 
> Attach top cap, fill with oil and vacumn out air. Or you can use my
approach if you can't afford a pump.
> 
> Use a 4" palm sander (if you don't have one you can borrow it from a
friend or neighbor - otherwise they run about $40.00 at Home Depot). Turn
it upside down with pad facing up, no sandpaper. With oil filled to within
1" from top (do not glue top cap down yet), turn sander on, grasp capacitor
firmly with both hands and place on top of sander pad. You can vary
intensity of vibrations by how hard you apply pressure downward. You will
visibly see the oil level drop as the air bubbles rise to the top. Refill
as necessary, and continue until bubbles cease to rise when you apply
medium pressure. By doing this, I was able to use my capacitors the same
night I filled them, and have not had any punctures or problems. Just make
sure all bubbles have quit rising to the surface.
> 
> Hope this info helps!
> 
> 
> Bob Volk