Homemade stacked plate cap. Need a vacuum pump?

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Homemade stacked plate cap. *Need* a vacuum pump?
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 29 Oct 1997 23:15:49 -0600
Approved: tesla-at-stic-dot-net

From: 	Adam[SMTP:absmith-at-tiac-dot-net]
Sent: 	Wednesday, October 29, 1997 8:26 PM
To: 	tesla list
Subject: 	Homemade stacked plate cap.  *Need* a vacuum pump?


After reviewing several capacitor options for my 8" coil setup, I have 
found only one solution that I can realistically afford: A stacked plate 
capacitor.  I know a maxwell cap would be nice, but I can't justify to 
myself or to my family that it is fiscally responsible to pay $600-$800 
for a 0.025uF 50kV capacitor.  I have hand build every other aspect of my 
coil with the exception of the neon transformers ($30/each), so why not 
this part. 

I considered building the rolled LDPE and Flashing capacitor, but this 
too seemed cost prohibitive, primarly with regards to time and money 
spent finding the right materials.  So, I have settled on the following 
flat plate cap, to fit an existing rubbermaid container that I have 
already (15"W by 21"L by 5"H - $4.99):

Dielectric:    25 layers of LDPE sheet (14.75" by 19.2") Each layer is
               90 mils thick (3 * 30mils)  Total= 5 30mil sheets -at- 
$12/sheet

Plates:        26 (12" by 20.2") pieces of Reynolds Heavy Duty Aluminum 
foil,
               Three inches will stick out each end of the capacitor for
               connection to the buss wire.  $1.99 for one 50ft roll.

Fill:          2 gallons of Mineral Oil. $???

The plates will have a 1.375" border on each side, and 2" on the end that 
does not connect to the terminal.  Thus, there is always at least 2" 
distance from aluminum to aluminum without going through the PE sheet.  
Total overlap area of the plates will be 12" by 15.2".  I have choosen to 
use 90mil of PE, as I will be running at 12kV or 15kV input, and I only 
want to build this thing once.  

My calculations tell me this cap will measure between 0.022 and 0.025 uF, 
depending on whether I choose 2.0 or 2.2 for the dielectric constant of 
PE. Total cost should be under $100.

OK, here's the dilemma: I have no vacuum pump, and I don't intend to buy 
one.  Is this going to be a big problem? Is there a way do get the air 
bubbles out of the cap without one?  According to my Electromagnetics 
Textbook, it would take 90,000V peak to punch through 90mil of 
polyethylene.  Shouldn't I have enough of a saftey margin here that a 
little air bubble isn't going to break the thing? Also, wouldn't a vacuum 
pump just crush my rubbermaid box down and break the seal? This thing is 
going to have a good 2" of airspace above it when I'm done (I could not 
find a box shorter than 5").

Right now, I plan to just put as much weight as I can on the cap (to help 
squeeze air out), fill the box enough to cover the cap with 1" of oil, 
release the pressure and wait a few weeks for the oil penetrate as much 
as possible.  If anybody else has a better idea, please let me know!  

-Adam
 absmith-at-tiac-dot-net

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absmith-at-tiac-dot-net
Epoch, Inc. Digital Music Project

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