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Help with Custom Cap thoughts?? Construction Details




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From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Sunday, April 05, 1998 3:48 PM
To:  Tesla List
Cc:  djQuecke-at-worldnet.att-dot-net
Subject:  Re: Help with Custom Cap thoughts?? Construction Details

> ----------
> From:  djQuecke [SMTP:djQuecke-at-worldnet.att-dot-net]
> Sent:  Wednesday, April 01, 1998 4:53 AM
> To:  Tesla List
> Subject:  Help with Custom Cap thoughts??
> 
> Hello Coilers,
> 
> I wrote to the list several months ago about a material I want to try for
> building custom capacitors.  As usual, I have taken what seems to be a
> fairly simple task and turned it over in my mind for so long that I'm now
> thoroughly confused about the basics.
> 
> If you would bear with me I'll try to explain myself and perhaps get a
> couple of rather simple questions answered.
> 
> The material is called ALPOLIC.  It is maybe a 3/16" thick piece of
> polyethylene sheet with perhaps a 1/16" thick sheet of aluminum bonded to
> each side.  It comes in 4' x 8' sheets and you can use wood working
> equipment to cut it, etc.
> 
> Pluses: zero air gap between poly and AL.  Poly is thick enough to withstand
> high voltage.  Works fairly easily.
> 
> Minuses: The AL sheets have a special paint bound to their surface which
> doesn't remove easily.  Poly perhaps too thick.  Electrical connection
> between plates has me confused.
> 

BIG SNIP of construction details and questions....

DJ,
If *I* decided to use ALPOLIC to construct a capacitor, here is how I would
go about it:

1) Remove all paint/printing from the aluminum surfaces. A solvent
bath and rub-down should be sufficient. Use a fine sandpaper if necessary.

2) Cut the big sheet down into smaller sheets that can fit in a 
reasonably sized container. (What is reasonable for one coiler
is ridiculous for another coiler). It is useful to first find a 
suitable container and then determine sheet size and number of
sheets required from *there*. Yeah, you can make custom enclosures,
but I like to go for simple, quick, easy, and cheap whenever possible.

The larger the plates, the less waste, but really large size plates
make the container difficult to move. Two smaller caps is often
better in the end than one huge cap that is very unwieldy.

3) Each sheet now has to have a strip of aluminum removed from THREE
of the four edges on BOTH sides. For the sake of sanity, we need to be
to talk about the sheets in a consistent way. Plop a sheet in front of
you. We will call the Top edge the "TOP", the Bottom edge the "BOTTOM",
the Left edge the LEFT, and the Right edge the RIGHT. Remove a strip
of aluminum from the TOP, RIGHT and BOTTOM of the side facing you. Leave
the LEFT side intact. Grab the right hand side and flip the right side
over so that you are now looking at the other side of the sheet.
Remove a strip of aluminum from the TOP, RIGHT and BOTTOM of the side 
facing you. Leave the LEFT side intact.

The size of the strip to be removed is important. I personally always use
at least 1". So let's say remove 1" from the TOP and BOTTOM, but double
that (2") from the RIGHT. That is for BOTH "faces" of the material.

Repeat this procedure for every ALPOLIC section to be used in the capacitor.

4) Cut N+1 aluminum foil strips that are just a little less wide than the
width of the aluminum from the TOP to the BOTTOM. The length of each strip
should be at least 3" plus the height of the full stack of plates, plus
2".

5) These foil strips are going to come out the left/right sides of the
cap as it is assenbled. They will alternate, coming out first from one
side and then from the other side. 

Note that foil strips will be sandwiched between opposing aluminum plates...
The bottom and top plates must have the foil strips attached by having 
them compressed between the outer plate and an appropriate insulating
plate (or in some other fashion if desired)

The key to building up the plates is to first begin with  the ALPOLIC
plates in a pile, ALL ORIENETED IDENTICALLY. Remember that first and last
plates require an insulating plate to compress the uluminum foil strip.

Begin by placing the initial insulating plate down. Lay a foil strip down
such that it comes out the RIGHT side. It should over-hang 3" onto the
insulating plate. Grab an ALPOLIC sheet and orient it so that when held in 
front of you the LEFT side FACING you has NOT had aluminum removed.

Carefully place the ALPOLIC sheet down so that the foil strip is sandwiched
under the RIGHT side.

Lay a foil strip down such that it is on top of the LEFT side. 
It should over-hang 3" onto the facing plate. Grab another ALPOLIC sheet
and orient it so that when held in front of you the RIGHT side FACING you 
has NOT had aluminum removed.

Carefully place the ALPOLIC sheet down so that the foil strip is sandwiched
under the LEFT side.

Lay a foil strip down such that it is on top of the RIGHT side. 
It should over-hang 3" onto the facing plate. Grab another ALPOLIC sheet
and orient it so that when held in front of you the LEFT side FACING you 
has NOT had aluminum removed.

Carefully place the ALPOLIC sheet down so that the foil strip is sandwiched
under the RIGHT side.

... Continue this process of placing foil sheets on alternating sides and
orienting the ALPOLIC sheets so that the ALPOLIC sheet applied has the
side facing you with the unremoved aluminum on the side opposite the
LAST applied strip of foil.

When you get to the FINAL ALPOLIC sheet, apply the foil and then apply
a cover sheet of insulating material (or affix the foil to the 
aluminum in some other way).

Starting with the topmost sheet of ALPOLIC, bend the foil strips up one
at a time such that the ones on the same side are all neat and touching
one another. You can attach to these aluminum foil strips in several ways.
Sometimes I clamp the ends of the foil between two thick aluminum strips
about 1/16" thick and then punch a couple of holes and secure the assembly
with a series of bolts and nuts. Heavy wire is then attached via
terminal rings.

A quicker/cheaper way is to wrap the foil ends tightly around a 1/2" wide
strip of flashing or other reasonably thick metal flat stock to which
I then attach a ring terminal.

6) Immerse in oil. 

7) Do bubble removal.

8) Have fun!


The above method connects the sections in parallel to increase
available capacitance.  Connect several of these capacitors in series
for increased breakdown voltage but lower capacitance.

*****
My own feeling is that the ALOPLIC requires much more work than a regular
poly flat plate foil-n-oil cap. The thickness is too high, resulting in
lower capacitance, and the removal of aluminum around the edges in not
a trivial matter. While it may be of use to someone who cannot come
up with another source of poly, I can guarantee you it will NOT be better
than a regular rolled or flat plate poly cap.

As regards the question about running this or any other cap dry...
***NOT*** recommended, as the resultant corona will degrade the
insulation. I am refering to the corona around the edges particularly.
I have done many post-mortems on caps and find that the failure mechanism
is often corona breakdown around the edges, especially where the
foil comes out. That is why I DOUBLE the insulation length at these 
ends.


Hope this helps.
Fr. Tom McGahee