# Capacitors

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From: mconway-at-deepthnk.kiwi.gen.nz (Mark Conway)

MC> Just another wee question. I am trying to make the capacitors
MC> for my tesla coil but cannot find the thick ( 60-90 mil ) poly-
MC> ethylene sheet anywhere in this country. Do you think that it
MC> will be ok to use about 8 sheets of normal thickness (250 micron
MC> = .25 mm) plastic sheet for the dielectric instead?

Just a reminder for those of you overseas.

1 mil = .001 inches = .0254 millimeters

Yes, you can use multiple layers, but re-read this from my archives:

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Date: 16 Nov 93  21:48:37
From: Richard Quick
To: All
Subj: 10KVA Tesla Coil
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(cont from previous post)

Higher Qs, higher voltage, and additional capacitance in stacked
plate capacitors can be easily obtained. The trick is to use
thinner dielectric.

Now the dielectric strength of polyethylene is given as
1000 volts per mil, but this is not the case in Tesla coils.
The standard breakdown voltages of a dielectric are calculated
using DC voltage. When you run AC across the dielectric, the
breakdown voltage must be divided by two. Then you must figure
that the peak voltage from a AC sine wave is higher than the
rms voltage most people go by. You meter won't see it, but your
dielectric will. Then you have resonate rise in the Tesla tank
circuit. To give you an idea of resonate rise in a tank, think
about the tidal forces that can be created with timed pushes in a
bathtub. It don't take much energy to push water over the side.
The same principal operates in the tank circuit in a coil,
especially with a synchronous gap system. The current pulsing
back and forth from capacitor plate to capacitor plate causes a
voltage rise that appears on the dielectric in the capacitors.
The standard 60 mil poly is supposed to hold up to 60,000 volts
per the book. I have blown holes through 60 mil poly with a 12 kv
neon sign xfrmr in a Tesla tank circuit and my gap wide open. My
pinky finger fit inside the hole.

One of the neatest homemade stack plate caps I have seen was
built by Bill Richards of T.C.B.O.R., the cost was pretty low,
the materials came from his laundry room, the grocery store, and
the drugstore. The only thing required was 56 hours of time in
arranging the plates according to Bill. But he did end up with
.03 uf 15 kVAC pulse capacitor in a five gallon bucket. It was
quite a performer on his coil at 3600 watts!

He shopped around for one gallon ziplock freezer bags with a 3
mil thickness. With a sharp scissors he cut the ziplocks off of
the tops of the bags. Then he cut aluminum foil squares that fit
inside the bag leaving a 1/2" of space around all four sides of
the plate. So the plate had dielectric borders 1/2" on all sides.

When two bags were stacked on top of one another, there were two
layers of dielectric, for a total of 6 mils. Being practical,
Bill figured correctly that the stacked bags would hold up to at
least 1000 volts rms input in the Tesla tank. He built up stacks
that had a value of about .45 uf each, with each stack rated at
1000 volts. Then he wired stacks in series.

By squeezing fifteen stacks vertically into a bucket, and
covering the whole thing in about three gallons of mineral oil,
he got the required capacitance at the required voltage. Since
the electrical forces are so well distributed among hundreds of
dielectrics, he had plenty of breakdown safety margin. He gave
the unit a couple of days to rest after construction, topping it
up with oil as required, and gave her the works at 15 kv on a big
coil. The heavy buss wiring never even got warm, and even though
it bubbled out enough air to displace a few more pints of oil, it
did not break down.

It turns out that this is a homemade version of commercial pulse
discharging capacitors. Stacked capacitor sections of very high
value are placed in series until the proper voltage requirement
is met. The cap has a very high Q because all of the plates are
very close together, with a minimum of connections and bussing
required. They deliver a very sharp pulse discharge.

Bill's cap was pretty cramped in the bucket. Because of the
square shape of the bags, a rectangular tank would have made
things easier to fit and wire. But he ran his buss bars through
the side of the bucket (sealed with hot glue) and by snapping on
the lid, he could pick it up by the handle and move it around
with ease.

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12

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