Fr. Tom: Rolled Extended Foil Capacitor

From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Thursday, June 25, 1998 2:49 PM
To:  Tesla List
Subject:  Fr. Tom: Rolled Extended Foil Capacitor

Fellow Coilers,
Gary Lau asked some questions about extended foil capacitor
construction. I though others might also be interested, so I
wrote up this post covering the topic.

The reason for wanting the extended foil construction is that
the regular rolled cap has the plates arranged in a sort of
spiral. This adds inductance to the capacitor. This means that
the capacitor actually acts like a capacitor and an inductor,
and so it has a self-resonant frequency. In addition,
the current is only removed at the extreme ends. When the cap
is discharged, the current density along the foil is not
very uniform!

The extended foil construction greatly reduces the inductance
of the capacitor, and allows for much more uniform current flow.
The result is a capacitor that has excellent pulse

by Fr. Tom McGahee tom_mcgahee-at-sigmais-dot-com

The method for providing extended foil connections is actually
not all that hard to implement.

The usual rolled cap construction has the foil coming out the ends.
The extended foil construction has the foil coming out the entire
length of the sides. One foil plate hangs off one side, and the other
foil plate hangs off the other side. You *DO* have to increase the
insulation distance of each plate from the *other* side of the poly

It is best to illustrate the construction with a simple example.
Let's say we have two strips of poly 24" wide by 96" long. You would
cut 2 strips of aluminum foil the same width, but 8 inches shorter.
That means the aluminum foil strips would be 24" wide by 88" long.

         - -------------------------------------------
         |                                           |
  LEFT   |                                           | RIGHT
         |                                           |
         |                                           |
Use the above dumb ascii drawing to identify orientation info
included below.

(The sizes given are ONLY so that you can visualize the process
more easily. Where I say poly, you may very well be using multi
layers of poly instead of a single layer.) When calculating
capacitance, the area of the plates would refer ONLY to the
area of the plates that OVERLAPS. This is one of the drawback to 
this design. You use more aluminum foil, and the amount of poly
dedicated to side insulation is greater, but in return you get
a much better pulse capacitor.

Lay down the first poly sheet as shown. On top of this poly sheet 
lay down one of the aluminum sheets so that it overhangs the TOP
by two inches, and is 4 inches in from both the RIGHT and LEFT
sides. On top of this lay down the second poly sheet, and align it
exactly on top of the first poly sheet, with the first aluminum
foil sheet sandwiched in between. On top of this second poly sheet
lay down the second aluminum sheet. This second aluminum sheet
should overhang the BOTTOM by two inches, and should have its
left and right sides in 4 inches from both the LEFT and RIGHT sides.

Starting at the RIGHT hand side, carefully roll up the capacitor.
It is useful to roll it up on a piece of PVC pipe with an OD of
1 inch or more, as this allows you to roll the assembly more tightly.
make the PVC pipe long enough so that you will have handles on both
sides. This allows you to get a good grip as you roll.

Begin rolling towards the LEFT side, using a steady pressure to
keep the assembly tight. Keep an eye on the layers, and do not let
them slide around.

When you get near the end, you will discover that the ends don't
"match up". If necessary, trim the respective aluminum foil plate
LEFT hand sides so that neither of them is closer than 4 inches
from the LEFT side of the 'shortest' poly end. (you can trim using
scissors, or you can just fold the 'extra' foil back on itself, if
that is more convenient at the moment).

Tape around the poly or use tie wraps to hold the wrapped assembly
permanently together. (The PVC pipe stays where it is, but later
you can hacksaw off the 'handles' if desired.) 

You will notice that the aluminum foil extends from the TOP and
BOTTOM. What you now want to do is get all the foil on the TOP
adjusted so that it all connects. This will effectively convert
the spiral into a bunch of concentric cyclinders, and will greatly
reduce the effective AC resistance of the cap. I normally begin
by first taking a utility knife and carefully cutting at least 
4 slots in the foil. These slots are parallel to the inner PVC
pipe, and extend from the edge of the poly outward all the way
to the edge of the foil. The purpose of these 4 slots is to later
allow a path for air to get out and oil to get in, and they also
make it easier to get the foil bent! Once the slots
have been made, start from the outside of the assembly and with
your fingers, push the foil down. Try not to tear the foil,
but be vigorous in getting it squished down along the side
of the assembly and then outward, along the PVC pipe.

Do this for all sections. 

Repeat for the BOTTOM foil side.

To make electrical contact with the aluminum edges that you have 
just formed, you can use one of those automotive hose clamps
that has a screw adjust, and use it to clamp a heavy wire or
even a strip of flashing to the extended foil. Be careful not
to destroy the aluminum foil itself in the process. 

Commercial extended foil caps usually have the electrical 
connections coming out the opposite ENDS of the container.
If you decide to go this route, then connect the electrical
strap/wire to a brass bolt. The thicker the better. This
can be soldered direct to the bolt head, or attached via a
ring terminal. In the case of flashing, just punch a hole
direct through the flashing. Use a flat washer on each side 
of the flashing to allow a good surface area contact.
One endcap has just one hole in it, sized for the bolt. The bolt
wiring is first attached/soldered. If a ring terminal is used, 
then a nut should be tightened on at this time to secure the 
electrical connection. I recommend a flat metal washer now be
placed on the bolt so that a large flat area will be presented to
the inside of the big PVC end cap that will go on the container 
PVC pipe. The bolt assembly as described is now inserted from
the INSIDE of the end cap. Use clear bathtub sealer all UNDER
and around the internal part of the assembly. Apply a small amount
of sealer also at the outside, where the bolt is sticking through.
Apply a large flat washer on the outside, followed by a nut.
Assemble tight. Because this first assembly has been done
with the assembly outside the PVC case, we were able to use
quite short electrical connections. Carefully insert the 
cap assembly inside the big PVC pipe. When ready, swab generous
amounts of PVC cement on the inside of the endcap and the outside
of the PVC pipe. QUICKLY assemble the end cap to the PVC pipe.

Do the same for the other end. The wire on this side
will have to be a few inches longer, since the wire must be long
enough to allow connection to the end cap while the cap assembly 
is inside the PVC pipe. Other than that, assembly for the last
side is pretty much identical to the first side. 

Allow assembly to stand for several hours before continuing.
This is to allow the cement time to do its thing.

The end cap on the last side should also have provisions for
attaching at least one hose. The cap is oriented with the
end with the hose connector facing straight up. If using a
single hose connector, fit it with a short section (2 or 3 inches)
of hose, and attach a "T" connector. One hose goes to the
vacuum pump, and one hose goes into your bucket of
transformer oil. EACH hose should have its own hose clamp or
valve. (There may be one of these already on your pump).
Make sure that the hose in the oil is at the bottom of the
bucket. I generally do this by using a piece of bare wire to
attach a small but heavy object such as  a hunk of metal
to the end of the hose. Failure to do this may result in the 
hose rising to the surface during pumping. Then it will suck
in air.

Close the clamp that goes to the oil. Open the pump clamp.
Turn on the pump. Wait for the vacuum to stabilize. Close
the vacuum clamp. SLOWLY open the  oil clamp. The oil will
enter the big PVC pipe. When the vacuum is almost gone,
close the oil clamp. Now open the vacuum clamp SLOWLY and pump
down again. If you see oil going up the vacuum line, be
careful, as this oil will then go through the vacuum pump.
(Some people use an oil trap in the vacuum line).

Once the container is pumped down, close the vacuum clamp
and SLOWLY open the oil clamp. More oil will flow into the
PVC pipe.

Continue this process until the PVC pipe is filled with oil.
Actually, it is USEFUL to have a small air space in the PVC 
pipe to allow for any future expansion of hot oil. Without
this expansion space the expanding oil can force the PVC pipe
to burst. First, of course, we have to remove all the air
that is inside the cap assembly.

Close the oil hose clamp and SLOWLY open the vacuum hose
clamp. Allow the vacuum pump to continue pumping for a
reasonable time. close the vacuum clamp and turn off the
vacuum pump. Wait a few minutes and then bleed air into
the system SLOWLY via the air hose side. At this time
there is no more vacuum in the PVC pipe. Whack the side of
the PVC pipe a couple of dozen times, really hard to dislodge
air bubbles. Close the vacuum clamp, turn on the vacuum pump,
and SLOWLY open the vacuum clamp to continue sucking air.
Repeat this procedure of removing vacuum, whacking, and 
re-applying vacuum three or four times.

Your cap should now be ready. If you used thick end caps
and threaded nipples for your evacuation lines, then you can 
unthread these now and replace them with threaded brass 
studs. I like to use a small "O" ring to prevent oil leaks.

This form of capacitor is normally mounted horizontally. For
this reason you should ensure that any air in the PVC pipe
does not get inside the poly/aluminum cap section. It is 
useful to have a point marked as being the "top" of the
cap, so it is oriented consistently the same way.

Electrical connections are made via the bolts that come out
each end. I find it useful to have the electrode arrangement
as follows: PVC end cap, large flat washer, nut, lock washer,
nut, flat washer, ring terminal, flat washer, lock washer,
nut. This may seem like hardware overkill to some, but I
want my caps to be robust physically as well as electrically.

Hope this helps.
Fr. Tom McGahee