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Multi-Rolled Cap



Subject: Multi-Rolled Cap
  Date:  Tue, 13 May 1997 15:41:01 -0400
  From:  "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:  <tesla-at-pupman-dot-com>


Fellow Coilers,

Fr. Tom here with what I hope will be a useful addition to our set of
techniques for making capacitors.

I note that a number of coilers are attempting to use thinner than
usual polyethelene to make rolled capacitors. It is difficult to roll
too many layers at once! But you don't really HAVE to. Here is a
method for making multi-section ROLLED capacitors that reduces the
number of poly sheets that must be handled.

*Note: wherever the text refers to a sheet of poly, you MAY in fact
be using two or more sheets. For example, your "sheet" may be four
layers of 6 mil poly.

First the facts: multi-section caps are actually more robust than a
single section cap. The high voltage rating is attained by having
many lower voltage sections connected in *SERIES*.

You can make a "rolled" multi-section capacitor that contains two or
MORE sections in a SINGLE roll. When completed it will have only TWO
connections. I will attempt to explain the construction of such a
capacitor, as I have never heard anyone mention the construction
details of such a capacitor on the Tesla list.

Let's assume you wanted to make a FOUR section capacitor using one
foot wide sections of 30 mil sheet polyethelene. Let's further assume
that you have two long sections of 1 foot wide polyethelene already
cut and loosely rolled up, and two rolls of aluminum foil that you
have already cut down to a width of 10 inches.

Let's wind the capacitor on a piece of 1" diameter copper pipe that
is 1 foot 2 inches long. First TWIST the end of a bare copper wire
(at least size #10) that is about ten inches long around one end of
the copper pipe, about 1/4" from the edge. SOLDER the wire down well,
making the solder flow as evenly as you can ALL AROUND the circle of
wire so that you have a connection with good surface area. The free
end of the wire will eventually be trimmed to a shorter length and
used to interconnect the finished capacitor to the tank circuit. The
copper pipe is going to provide us with a first class electrical
connection to one side of the capacitor.

To make it easier to manipulate the capacitor roll as we assemble it,
shove a broomstick or wooden or metal pole or pipe up inside the
copper pipe and then use wedges, shims, or whatever it takes to get
the pole temporarily firmly attached to the copper pipe. This will
give us a "handle" that we can use so that the turning of the roll
can be done more easily.

Use scotch brite or steel wool or sandpaper to clean the outside of
the copper tube so that it can make a GOOD electrical connection all
along its outer surface.

Take one of the aluminum foil ends and center it on the copper pipe.
Wrap several turns of foil TIGHTLY around the pipe. This gives us the
good electrical connection between this first foil sheet and the
copper pipe. Use a strip or two of 14" long duct tape to tape the
entire width down. We will call this piece of foil FOIL 1. Tape the
end of one of the polyethelene rolls ON TOP of Foil 1 using a piece
of 14" duct tape. This will be poly roll 1. Tape the end of poly roll
2 even with the end of poly roll 1. ROLL the assembly slowly around
THREE full turns. 

Now insert the end of foil 2 *in between* poly roll 1 and poly roll
2. Use a 10" strip of tape to hold it in place. You NOW have a
sandwich that consists of foil 1, poly 1, foil 2, and poly 2. Slowly
roll the assembly, making sure that the foil sheets stay centered,
and the poly sheets are rolling up smoothly and evenly. Apply a
slight tension to the assembly as you roll it, and smooth out any
wrinkles if they begin to develop. 

Ocassionally check the capacitance between the copper pipe and foil
2.

When the FIRST section reaches a value that is FOUR TIMES the desired
final value, pause. (4/1=4)

*Note that to get a valid value for the capacitance rolled so far you
must have the remaining foil in two rolls that are separated by about
a foot.

Since section #1 began with foil 1, carefully cut foil 1 free. Foil 2
remains intact at this time. Roll around at LEAST 1 complete turn of
poly 1, foil 2, poly 2. Now re-insert foil 1 (use tape if needed),
and continue to roll the capacitor.

Ocassionally check the capacitance between the copper pipe and foil
1. 

When the combined FIRST and SECOND sections reach a value that is TWO
TIMES the desired final value, pause. (4/2=2)


Since section #2 began with foil 2, carefully cut foil 2 free. Foil 1
remains intact at this time. Roll around at LEAST 1 complete turn of
poly 1, poly 2, foil 2. Now re-apply foil 2 (use tape if needed), and
continue to roll the capacitor.


Ocassionally check the capacitance between the copper pipe and foil
2.

When the combined FIRST, SECOND and THIRD sections reach a value that
is FOUR-THIRDS TIMES the desired final value, pause. (4/3=four
thirds)



Since section #3 began with foil 1, carefully cut foil 1 free. Foil 2
remains intact at this time. Roll around at LEAST 1 complete turn of
poly 1, foil 2, poly 2. Now re-insert foil 1 (use tape if needed),
and continue to roll the capacitor.

Ocassionally check the capacitance between the copper pipe and foil
1. 

When the comined sections reaches a value that is the SAME as the
desired final value, STOP. 



Since section #4 began with foil 2, carefully cut foil 2 free. Foil 1
remains intact at this time. Roll around at LEAST *TWO* complete
turns of poly 1, poly 2, foil 2. The extra is insurance. If you are
using particularly thin poly, you may want to increase the number of
outer layers.

You now need to attach a connection to the remaining intact foil. (In
this case Foil 1). This will form the second capacitor lead. One good
method of getting a low impedance connection is to use a strip of
flashing that is about 1" wide and as long as needed. Extend the
remaining foil a couple of extra inches and then *tightly* roll the
flashing inside the extra aluminum foil. I like to have a piece of
the flashing extending below that I bend over after wrapping. This
prevents the flashing from pulling free accidentally. Use duct tape
to hold everything together. Wrap AROUND the entire assembly. Several
layers here makes a lot of sense. Cut away any remaining poly. You
might want to wrap a protective layer using a piece of thicker poly,
if available. Use more duct tape to hold the outer layer together
firmly.

You now have a single rolled capacitor that is actually FOUR
series-connected sections.

You can extend the basic idea for more or less sections. The
dimensions given are just for the sake of clarity.

If you are rolling along and you come to the end of a poly roll, you
can carefully overlap by AT LEAST 1/2 turn of poly, and then continue
as usual.

It is higly recommended that the capacitor be immersed in oil, as is
usual, and vacuumed down to help remove bubbles. But feel free to
modify the rolled cap as needed. What this capacitor assembly method
seeks to do is make the job of making multisection capacitors a bit
easier by reducing the number of layers of poly that are being
rolled. The finished cap will require the usual break-in procedure
before you attempt to run it full power.

Please note that if you do not have a capacitance meter, you can use
the actual LENGTH of each section's FOIL to determine the approximate
capacitance. Not as accurate, but close enough.

Hope this concept helps somebody.

Fr. Tom McGahee