From: chip (Chip Atkinson)
Date: Tue, 1 Nov 1994 13:17:06 +0700
The High Voltage, Pulse Discharge, CAPACITOR,
Many high voltage projects require a high voltage pulse discharge
capacitor. Whether your project is a laser, Tesla coil, rail gun,
high power taser, or particle accelerator, you will most likely
need a high voltage rated pulse discharging capacitor in your
Commercial units are expensive. Manufacturers of these units do
not stock them. Each and every unit is built to order. "Off the
shelf" units do not offer the performance required for most high
voltage projects. Off the shelf capacitors get hot, have high
loss to output ratios, and/or will break down in spark excited
tank circuits. Some types are potential explosion hazards.
You can build your own capacitors for these projects from
polyethylene plastic and aluminum flashing.
The following instructions are for a pulse discharging capacitor
with a .02 uf at 35-40 kvdc rating. The unit is rated for work in
spark excited tank circuits with up to 15 kvac rms inputs, 12
kvac is the recommended maximum rms voltage rating, and they will
run all day pulse discharging 10 kvac without getting warm. This
is an ideal unit for small Tesla coils, and may be run in series/
parallel combinations for Tesla coils from desktop size up to 4
kva megavolt coils. The material cost is around $100.00 per unit
as opposed to $150 - $300 for a similar commercial capacitor.
Materials for this unit are as follows:
Three yards of low density, 60 mil, polyethylene plastic.
This plastic is available from any good sized plastics dealer.
One ten inch by twelve inch sheet of 1/4" plexiglas.
One fifty foot roll of 14 inch wide aluminum flashing. This is
available from any hardware store.
Eighteen inches of 1 inch schedule 40 pvc pipe.
Twenty inches of 6 inch PVC DRAIN PIPE. DO NOT USE SCHEDULE 40!
Six inch pvc DRAIN PIPE is available at any good plumbing supply
in ten foot lengths.
One: six inch pvc DRAIN PIPE END CAP.
Two: end caps for the 1 inch schedule 40 pvc pipe. The end caps
must have flat bottoms (not rounded) or you will need to cut
One gallon of pure U.S.P. Mineral Oil.
Two: 1/4 x 20 brass machine screws and four nuts.
Two: #8 Pan Head Machine screws with washers and nuts. Screws
should be equal to or less than 3/8ths of an inch long.
Loctite thread fastener (medium strength)
Six or eight: 12" long nylon wire ties
PVC cement (medium body, clear, works best)
PrepSol (Dupont paint store) or U.S.P. alcohol
Four inch stack or clean newspaper or BUTCHERS PAPER
Lint free wipes or good quality paper towels. Don't use the cheap
BUILDING THE CAPACITOR TANK
Start out by cutting the PVC drain cap in half. You want to cut a
ring 1-1/2" high off of the end cap. The bottom of the end cap
should be saved intact with a 1-1/2" high side wall.
Lay the ring cut from the 6" PVC drain cap on the sheet of 1/4"
plexi and scribe a circle. Cut the circle out and glue it to the
ring with PVC cement. This forms the clear, see through, lid for
the capacitor tank. When the PVC cement has dried, drill two
holes through the plexi for terminals. The holes should be on
opposite sides of the lid. A small hole is drilled dead center
Cut some strips of plexiglas, 3/4" wide by 2" long, out of the
Glue one of the 1" PVC end caps to the inside center of the 6"
PVC drain cap section. Glue at least four of the plexiglas strips
around the 1" end cap. The strips are placed so that they are
flush with the 1" PVC end cap. They should form a "star" pattern
radiating out from the center to form a shelf, 3/4" high, for the
capacitor roll to sit on. This shelf prevents the roll from
resting on the very bottom of the tank, and allows cool oil to
circulate. It is important that there is sufficient room between
the edges of this shelf and the side wall of the 6" drain cap to
allow the 20" section of 6" PVC drain pipe to seat all the way to
the bottom of the end cap.
When the end cap assembly is dried, glue and seat the 6" PVC
drain pipe in place. Use plenty of PVC cement to prevent leaks.
Once the end cap is firmly seated in the 6" PVC pipe, then cement
the 18" length of 1" PVC pipe down into the center ring. This
pipe saves oil, as well as providing a center post for the
capacitor roll. Glue the second 1" PVC end cap onto the top of
the 1" pipe to seal it. Let the PVC cement dry thoroughly, then
wash the tank out with strong detergent, and allow to dry.
This completes the capacitor tank construction.
THE CAPACITOR ROLL
The capacitor roll is made from the polyethylene sheet and the
aluminum flashing. It is important that all of these materials
are absolutely clean and free from defects.
Clean and vacuum up a work area large enough to lay all of your
plates and dielectric out. If things are dusty you may want to
mop. When the work area is clean; lay down fresh newspaper, or
even better, butcher paper, over the entire work area. You will
need a long, hard, smooth, flat surface to roll your capacitor up
on. A clean, paper covered concrete floor works well, as does a
couple of paper covered buffet or serving tables.
Cut the poly sheet lengthwise into three equal strips. The
standard material width for this sheet is 48 inches. You will get
three 16" wide strips from the sheet, though only two strips will
be required to make one capacitor roll. The strips must be washed
and wiped on both sides with PrepSol or alcohol and lint free
wipes or high quality paper towels. Then they must be wiped dry.
Static may become a problem here, and the dielectric may collect
dust. A ground strap run to a water pipe may be wired to a copper
or brass brush. The plates and dielectric may be swiped lightly
to ground out static, but do not scratch the material.
Cut two lengths of aluminum flashing 102" long. The flashing must
be six inches shorter than the polyethylene strips. The material
is already two inches narrower. Use a good pair of heavy duty
scissors to cut the aluminum. The strips of flashing (plates)
must have the corners well rounded, and have all sharp edges
smoothed. Trim the corners off with the scissors, then sand all
edges you cut it #150 emery cloth. Drill a hole, 1/2" from one
end of each flashing strip for the terminal mount. Inspect your
plate. It should have no dents, sharp points, "ruffles" along the
edges, etc. Many flaws can be carefully worked out.
The aluminum capacitor plates must be washed and dried. Fill a
five gallon bucket with very hot water and a good squirt of
liquid detergent. Roll the plate up and "dip, swish, and swirl"
until all the sanding grit, manufacturing oil, and dirt wash off.
Rinse the plate well and stand it on its edges on clean newspaper
until it is dried. Don't worry if the plates oxidize a little.
Lay out your meticulously clean plates and dielectric sheets.
Lay one strip of plastic dielectric down first. Then lay a plate
on top and center it. The plate is centered so that there is a
one inch border of dielectric plastic evenly along the long
sides. Line up the end of the plate with the terminal hole flush
(even) with one END of the plastic. The far end of the plate will
be six inches short of flush with the bottom dielectric sheet.
Lay a second sheet of plastic on top so that it is exactly lined
up the bottom strip of plastic.
Lay the last plate down on the stack and center it. The plate is
centered so that there is a one inch border of dielectric plastic
evenly on both of the long sides. Now, the first plate you laid
will have the terminal end flush with one end of the bottom di-
electric, it makes no difference which end; line up the second
plate so that the terminal end is flush with the end of the
second dielectric sheet, but it must be at the opposite end from
the bottom plate terminal.
Cut two 1" strips of aluminum flashing 14" long. Tape them
together into a 1" strap. Round it and sand it. Then untape it
and wipe or wash the strips. Reassemble and punch a hole in each
end. One hole for a 1/4" or larger screw (tank terminal), the
other for the #8 pan head machine screw (plate terminal). Using a
#8 pan head machine screw, mount this strap into the terminal
hole on the top plate. Use a flat washer, a tiny drop of loctite
thread fastener, and then a nut. Snug the connection down firmly.
This strap serves as a high current lead from the plate to the
terminal mount on the capacitor lid. Make sure that it is the
smooth pan head of the screw pressing into the plastic capacitor
dielectric as the capacitor is rolled up; not the sharp screw
shaft. Do not allow the sharp threaded end to press into the
capacitor. It is a good idea to have a couple of spare patches of
60 or 30 mil plastic to place under the pressure points of the
terminal connector screws. This will help prevent breakdown.
Starting from the terminal end of the top plate on the stack, the
end with the terminal strap already mounted, roll the capacitor
up as tightly as possible. Make sure that the top plate does not
curl around to touch back on itself on the first turn. A strip of
extra plastic here can be helpful. If the first turn of the roll
looks poor, then unroll, line everything up, and try again.
When the capacitor is tightly rolled, do not loosen your grip.
Have an assistant put two wire ties together and slip them over
the roll. When the wire ties are cinched, you may loosen up.
As you rolled the capacitor up, the first plate in the stack
worked its way out of the roll a few inches. This plate should
present you with a terminal hole to mount a second 1x14" strap of
aluminum to complete the second high current lead.
Mount the second lead, making sure the smooth screw head is
against the capacitor, not the sharp threaded end. You will have
one lead coming up from inside the roll, and the other coming up
from the outside. Put at least three wire tie strips around the
roll. Two 12" wire ties connected together will give enough
Set the capacitor roll into the tank. Fill with one gallon of
mineral oil. The roll must be covered by at least a quarter inch
of oil to suppress corona and prevent flashover. Note that the
oil soaks into the roll. The level will drop after filling, and
may drop again after use. Check on it occasionally until the
capacitor is fully broken in, a period of about six months.
Connect the leads from the capacitor roll to the tank lid. For
the tank lid terminals use at least 1/4 inch brass machine screws
and tighten down well. The head of the machine screw should be
inside the lid, the first nut on top will hold the connection
tight, the second nut is removable for connection to your
circuit. Do not seal or glue the lid in place.
Do not apply the full rated voltage to these units until they
have set for at least three days, and the oil has had a chance to
soak in to the roll. It is best to start them out at about half
voltage, or less, and run them for short periods for the first
few days on a smaller coil. These units run on the ragged edge of
their voltage ratings, yet they are quite serviceable. On larger
coils it is best to put these units in series/parallel to back
them up against kickback.
Because the material width of the polyethylene is 48", you get
three 16" strips of dielectric from cutting a length. You will
have one strip left over. Because of this, it is perhaps better
to plan on building at least two units at a time. This makes more
efficient use of material, but more so for the use of time. Once
a temporary "clean room" has been established it makes sense to
use it to fullest advantage.
The effort in building a first class cap is worth the extra time
and expense to do it right. The unit will last longer, withstand
more abuse, and give you more capacitance if it is well con-
structed. Once this effort is expended, and the unit is in
service, don't blow it. Rather than risk the investment you
should build more caps, "backing up" your existing caps and
increasing power by adding caps as you go.