Re: Rolled cap construction techniques

From: 	Chip Atkinson[SMTP:chip-at-pupman-dot-com]
Sent: 	Sunday, November 30, 1997 11:02 AM
To: 	tesla-at-pupman-dot-com
Subject: 	Rolled cap construction techniques

======= DANGER -- 80 character wide ASCII art follows!!! =========== :-)


I just finished a successful test of a new idea to ease the winding and
tightening of rolled capacitors.

As anyone who has rolled one of the TCBOR style capacitors knows, winding
up a sandwitch of two layers of Al flashing and two layers of 60 mil
(0.060", or 1.5mm) LDPE (forget HDPE or PP) is a wrestling match that does
not have a guaranteed winner.  Performing this operation by hand requires
at least 4 of them, and 6 hands are even better.  Furthermore, there is no
guarantee that the bundle will be tight enough to fit in the 6" pipe the
first time it's attempted.

Another problem that I have encountered in the past is getting the lead
wires to the plates to be opposite each other (across the centerline  of
the capacitor roll).  Due to problems with irreproducible levels of
tightness in the roll, the leads may end up on the same side of the roll
or in the same quadrant.  This is a problem because the connection lugs on
my capacitors are across the top from each other. (See illustration below)

(Graphics aside: If you have the pbm plus package on your machine, you can 
 use some drawing program to create a drawing, save it as a pbm file, 
 and then use a command similar to the one below to come up with an 
 80 character wide ascii art drawing like the ones below.
[chip-at-poodle chip]$ pnmscale -width 45 Grafix2.pbm | pgmtopbm | pbmtoascii | more
          oo""                """oo
        o""                       "oo
      M"                             Mo
    oM                                 Mo
   M                                    "o
  M                                      "M
 M"               ooo"oooo                 M
o"              oM        "Mo              M
M              o"           "o             o"
M   M""M       M             M     oM""o    M
M   MooM      "o Window      M     "ooM"    M
M              M            o"             o"
M  Lug 1        Mo        oo"     Lug 2    o"
 M                ""ooooo""                M
 "M                                       M
   M                                     M
   "M               oM"oo              oM
     "o             "o o"             M"
      ""Mo           ""            oM"
          "oo  Fill Plug        oo"
            """ooo o      ooooM"
                  " """"""

A further problem is the restriction that the materials be LDPE
only.  This restriction is due to the stiffness of the materials
being wound.  

For the test that I performed, I used 1/16" PolyPropylene, which is
quite stiff, around the stiffness of HDPE.  I cut the 4'x8' sheet
into three 16" wide strips 8' long and rolled them up into as tight
a bundle as I could and let them sit in my garage for about a year.
Now the plastic has set into a bundle that naturally curls up into a
bundle about 1' in diameter.  However, I believe that the technique
that I'm going to describe below will work fine on plastic that
doesn't have a curl to it.

Furthermore, I used the technique to tighten up an already made (but
not finished) capacitor so that I could more nearly align the lead

OK, now to describe how I did it.  First, get a piece of PVC pipe about
1.5" diameter and about 25" long.  I used schedule 80 electrical conduit
because I had it, but you could probably get away with schedule 40.
It is probably important to have a fairly thick wall because it recieves
a fair amount of torque, and there is a slot down the length of the pipe,
which significantly reduces the rigidity of the pipe.

This piece of pipe must have one side sawn through.  I used a table
saw and put an off center groove down the length of the pipe.  
Note the gap in the circle drawing below.  This slot is about 1/8"
wide (one sawblade width).  

                       ooM"         |       """oo
                    oM"             |           ""oo
                  o""               |              "Mo
                oM"                 |                 Mo
               M"                   |                  "o
              M                     |                   "M
             M                      |                    "M
            M"                      |                     "o
           oM                       |                      M
           M                        |                      "o
           M                        |CL                     M
           M                        |                       M
           M                        |                      o"
            M                       |                      M
            Mo                      |                     o"
             M                      |                    oM
              Mo                    |                   o"
               Mo                   |
                "oo                 |
                  "Mo               |
                     Moo            |            o
                       """oo        |       ooM""
End view of pipe. CL is the center line(s)

Lay out your capacitor sandwich and feed one end of it into the
slot.  With the picture above, you'd feed it in from the right.  Put
some hose clamps on the pipe on either side of the sandwich and
tighten them up.  This grips the sandwich tight enough to wind up
the cap.  

Roll up the capacitor loosely, using a modest amount of strength.
You should end up with a bundle about 1' to 1.5' across, which is
quite easy to achieve.

Now I have a lathe, so I chucked up one end of the pipe, and put the
other end in the tail stock, but I didn't even really use the power.
I had the lathe on the lowest speed (20 rpm setting) so it wouldn't
turn backwards very easily.  I had rubber gloves on for gripping,
and proceeded to tighten the bundle up by turning the outside of it.
As I tightened the thing up a little, I would tighten the zip ties
that hold the whole bundle together when the tension is off.  I just
kept tightening until the following criteria were met.  It must be
smaller than 6" so that it will fit in the pipe, and the leads
should be in the correct position relative to each other.

I believe that all one really needs is some method of holding the
pipe securely.  A vise on one end, and some sort of support on the
other should work beautifully (a helper would probably be adequate).

Hope that helps people make better and easier rolled caps.  I'm
really pleased with the results.

 Chip Atkinson 
 http://www.pupman-dot-com and http://bhs.broo.k12.wv.us/homepage/chip/info.htm
 --- If I can't fix it, I can fix it so it can't be fixed --