# Plastic soda bottles for bypass caps?

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Sent: 	Saturday, December 06, 1997 8:24 PM
To: 	tesla list
Subject: 	Plastic soda bottles for bypass caps?

I know that plastic soda bottles make for poor tesla tank capacitors, but
it seems to me that that you could make some decent bypass caps from the
lossy material (PET).  I was flipping through surplus catalogs looking
for doorknob caps for my bypass caps, and found that the size I am
looking for (200-300pF -at- 20kV) would run my around \$40 each, and I need
2!!! \$80, I don't think so.  With kitchen Aluminum foil and a few pepsi
bottles, I should be able to make 250pF caps that are quite small and
practically free! Anything I am missing?

I tested out some pepsi bottle material, and a single thickness (10-12mil
I think) doesn't puncture with 25,000 VDC on it- good, since I will only
have 4.5kVAC on it in use.  Plus, with a thickness of 10 mils, and k=3.0,
I work out that the plates have to have only 4 square inches of overlap.
Here is my calculation:

e0 = 8.85418781762 * 10^-12 (dielectric constant of free space)
k = 3.0 (for polyethylene terphthalate)
1 inch = 0.0254 meters
Plates = two Al foil plates with 2" by 2" overlap area = 4.0 sq. in.

(e0 * k * 4.0 * 0.0254) / 0.010" = 270 pF (approx)

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