Date:   Fri, 13 Jun 1997 17:03:24 -0400
  From:  "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:  "Owen Lawrence" <owen-at-iosphere-dot-net>
    CC:   <tesla-at-pupman-dot-com>

> From: Owen Lawrence <owen-at-iosphere-dot-net>
> To: tom_mcgahee-at-sigmais-dot-com
> Date: Friday, June 13, 1997 9:49 AM
> Without getting into thoughts about how difficult it would be to
> why not use epoxy rather than spar varnish?  Get the kind that
takes at
> least a day to set.  You wouldn't have to worry about it drying,
> setting.  I'm sure it would be difficult, but if a process could be
> and mastered it would make a very convenient capacitor.  Is there
> drawback to using epoxy (i.e. RF losses)?  Talk to you later.
>   - Owen -
> owen-at-iosphere-dot-net
> http://www.iosphere-dot-net/~owen

First, if properly made there would be NO epoxy at all between the
poly layers since the pressure would have squeezed it out. The trick
is to make it so that no air is ever ably to sneak back IN!

This gets us into the topic of encapsulated dry capacitors. Yes, I
have some definite ideas along these lines. I haven't shared them yet
because I am still mulling over the ramifications of some of the
design elements. (So MANY thoughts and not enough time to try hardly
any of them. It is *so* frustrating to me not to be able to actually
try these ideas out immediately!!

OK, imagine that you want to manufacture say a .01 mfd 15KV AC RMS
poly capacitor, and you have access to a press. (or a reasonable
facsimile thereof). Let's say that you determine you can build the
basic capacitor itself so that the size is 8" x 10" x 4".

Further assume that the poly plates are 8" x 8" (the ends of the
metal plates accounts for the other 2"). Build a box from 1/4"
plexiglass that has INSIDE dimensions of 8.5" x 10.5" x 5". This box
will have four sides and a bottom, but NO TOP (yet). Assemble the DRY
capacitor inside the case such that it is centered. Place a 8" x 8"
1/4" thick piece of plexiglass over the top of the capacitor such
that it lines up with the underlying poly. On top of this place
several strips of 8" x 1" x 3/4" thick plywood such that there are
spaces at least 1" wide. On top of this place  a sheet of 8" x 8" x
3/4" plywood. Place this entire assembly into an overhead press.
Bring the press to bear and increase the force to a value such that
the pressure is high but not enough to fracture the plexiglass. (This
has to be determined experimentally.) 

This will squeeze the poly and plates flat and drive out all air from
the capacitor itself. Now fill the plexiglass box with epoxy up to
the point where it fills the spaces between the strips of plywood do
a depth of almost 3/4 inches.

At this point the capacitor is air-free. After the epoxy sets you can
remove the tension slowly, and if we are lucky the assembly will
hold. Now you can pour in some more epoxy if you want, so that the
box is totally filled with epoxy at the top. 

Oh yeah, I assume that the electrical connections are coming out via
heavy wires or in some other way, such as via bolts through the side.

Not having every attempted to build a capacitor under pressure, I
don't know what all the "gotcha's" will be. That is where the actual
experimentation comes into play. I would begin by trying the idea out
on a small scale first. Maybe make a .005 mfd at 12KV AC RMS or
something like that.

The key elements in the design above are that pressure is not applied
to the walls of the case, but only to the base via the press. Once
pressure is applied a liquid is introduced. (Keep bubbles to a
minimum around the edges of the capacitor.) Once the liquid hardens
the pressure is released and a final topping off of the structure can
be executed if desired. No oil is required, as PRESSURE is used to
drive all air out, and this pressure is not taken away until the
capacitor is completely surrounded by an encapsulating medium that
also provides structural rigidity.

RTV Silicone and other such things may ALSO be appropriate materials,
but the details of manufacture will only be known once actual
experimentation can be performed.

Please note that the purpose of the thin strips of wood are to allow
pressure to be transmitted to the top plastic plate (and therefore to
the cap) and still get the encapsulating medium to engulf enough of
the TOP section such that when pressure is removed there are
encircling sections of the epoxy in place to keep the assembly

When pressure is released as the press is removed, there may be major
distortion of the plexiglass. I just don't know exactly what will
actually happen without experimentation. And I can't do any such
thing until probably early September. So maybe someone else can take
these ideas and run with them.

Hope this helps someone out there.
Fr. Tom McGahee