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Re: Homemade stacked plate cap. *Need* a vacuum pump?
From: Alfred A. Skrocki[SMTP:alfred.skrocki-at-cybernetworking-dot-com]
Sent: Sunday, November 02, 1997 1:20 AM
To: Tesla List
Subject: Re: Homemade stacked plate cap. *Need* a vacuum pump?
On Wednesday, October 29, 1997 8:26 PM Adam
[SMTP:absmith-at-tiac-dot-net] wrote;
> ... 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.
Adam, unless you've already purchased the polyethylene, you may want to
look into plexiglass as a dialectric - it has double the dielectric
strength and a dielectric constant of slightly over 3, the big deciding
factor would be it's power factor - if it is greater than that of
polyethylene then it's a looser otherwise it may be cheaper and make a more
compact capacitor.
> OK, here's the dilemma: I have no vacuum pump, and I don't intend to buy
> one.
A good source for a vacuum pump is to wait untill a neighbor throws out
a refrigerator or an airconditioner and cut the two 1/4 inch copper pipes
on it an remove all the motor electronics ie. compressor, starter
capacitor, thermal switch and power cord.
> 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?
No, the ozone will slowly break down the polyethylene then arcing will
occur!
> Also, wouldn't a vacuum pump just crush my rubbermaid box down and break the seal?
Yes,it probably would. Rubbermaid containers are too flexible to try
pumping a vacuum in them.
> 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").
A simple way to eliminate bubbles without a vacuum pump is to mount the
capacitor on it's side and then put it on a vibrator to shake the bubbles
to the surface. BTW a 12 inch or bigger speaker driven at 50 watts or
more from a signal generator feeding an amplifier with 50 to 400 hz.
works great!
Sincerely
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Alfred A. Skrocki
Alfred.Skrocki-at-CyberNetworking-dot-com
.ooo0 0ooo.
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