I downloaded this from the ftp site. It was in the rqdocs.zip file.
April 8, 1994
High voltage capacitor construction, by Bert Pool.
First of all important warnings and disclaimers:
High voltage capacitors are very dangerous for several obvious
and some not-so-obvious reasons. Most experimenters know that a
capacitor can retain a significant voltage even after power has
Discharge all capacitors with a jumper cable before working on
them. When discharging large capacitors, the jumper cable
needs a high wattage 100 ohm resistor to limit discharge current.
EVERY CAPACITOR IN A "SERIES" DESIGN TESLA CIRCUIT MUST BE
DISCHARGED SEPARATELY! SHORTING ONE CAPACITOR WILL NOT DISCHARGE
OTHER CAPACITORS IN THE CIRCUIT!
Voltages and currents available on charged capacitors can be
lethal. Use common sense. The capacitors described here are
sealed and contain mineral oil. While mineral oil is not
especially flammable, a catastrophic capacitor failure can result
in pressure buildup and explosion if construction technique does
not allow for release of pressure.
Mineral oil will burn - a catastrophic capacitor failure can also
result in the release of oil which could ignite. It is highly
suggested that experimenters keep on hand a fire extinguisher
rated for oil fires.
Also, it is suggested that oil filled capacitor banks be stood in
a metal pan large enough to contain the oil in case a capacitor
container is breached or oil is released. This pan can prevent a
mess, as well as help contain a potentially nasty fire!
I have built over a dozen oil filled high voltage capacitors, and
I have had several failures due to construction shortcuts or
material failures and I have NEVER experienced a dangerous
buildup of pressure or a fire - but I know it CAN happen and I
take measures to be prepared for such an eventuality. Many high
power Tesla coils use "power pole" transformers; these too are
filled with oil and the same precautions for prevention of
explosion and fire apply here too.
As a high voltage experimenter, you take full responsibility for
safe construction and operation of your capacitors and other high
Be safe, be careful, use common sense!
Keeping all these safety ideas in mind, presented is a method for
building home-brew polyethylene capacitors which serve very well for
Tesla coils, ZPE devices, etc.
The original design is not mine, it was conceived by Richard Hull of
the Tesla Coil Builders of Richmond (TCBOR). TCBOR has an excellent
video showing step-by-step construction of this type of capacitor.
Other available video tapes provide excellent info on Tesla coil and
Tesla magnifier construction. Richard Hull may be contacted at:
7103 Hermitage Rd,
Richmond VA 23228.
A simple capacitor consists of two conductive plates separated by
an insulator. Capacitance is determined by the area of the
plates, the distance between them and the "dielectric constant"
of the insulator between the conducting plates.
This dielectric constant is represented by a number called "K".
On the next page is a table of materials and their "K" and their
puncture voltages taken from the "Radio Amateurs Handbook":
MATERIAL | "K" | PUNCTURE
| | VOLTAGE PER
| | MIL
AIR 1.0 240
BAKELITE 4.4 - 5.4 300
BAKELITE, MICA FILLED 4.7 325 - 375
FORMICA 4.6 - 4.9 450
WINDOW GLASS 7.6 - 8 200 - 250
PYREX GLASS 4.8 335
MICA 5.4 3800 - 5600
PLEXIGLASS 2.8 990
POLYETHYLENE 2.3 1200
POLYSTYRENE 2.6 500 - 700
PORCELAIN 5.1 - 5.9 40 - 100
QUARTZ 3.8 1000
TEFLON 2.1 1000 - 2000
A vacuum or air has a "K" of 1. If you put a piece of teflon
between your plates (same spacing), the capacitance will increase
2.1 times and it will handle at least 5 to 7 times as much voltage.
If you used a piece of good quality glass, the capacitance might
increase 8 times! (the voltage rating would increase only slightly).
The dielectric you choose will be determined by voltage, frequency
(if using a.c.), durability (glass breaks very easily), cost and
MATERIALS AND SOURCES:
The capacitors we will build use polyethylene. Poly is very easy to
work with, does not break, is inexpensive, readily available, has
very good a.c. characteristics (very low loss), and has good high
voltage properties. Mica would be even better, but it is VERY
expensive and is not readily available.
Glass is o.k. as far as availability and cost goes, but it has very
high internal losses for Tesla coil use, compared to poly. Glass is
also very fragile to work with. I've made glass capacitors weighing
over 600 pounds -NEVER AGAIN! You just can't beat making capacitors
out of polyethylene.
Purchase two sheets of polyethylene sheet, 48 inches wide by 96
inches long. Each sheet will be cut lengthwise into three sheets,
each 16 inches wide. All together, you will have 6 pieces of poly
16 inches wide by 96 inches long. This is enough material to make
THREE capacitors. (You will cut several more sheets if you choose to
use several thin layers of plastic instead of one single thick layer.)
The thickness of the poly sheets is determined by the voltage you
will be placing on the capacitor. You have to take into account
whether you are using d.c. or a.c.
If you are using d.c. voltage, calculate your poly thickness by
using dielectric voltage rating of 600 volts per mil (one mil = .001
inch). Some charts show that poly can handle 1200 volts per mil -
Tesla coil experience shows that this value is NOT conservative
enough, even taking peak-to-peak values into account!
If you are using a.c., and especially with a Tesla coil, you have to
de-rate the voltage ratings on the poly or you will have capacitor
failure. For Tesla coils using input transformers from 9 kilovolts
up to 15 kilovolts, use 90 mil thick polyethylene. If your input
transformer is 7200 volts or less, you can use 60 mil poly.
*** Special construction note! Once you have determined the
required capacitor dielectric thickness, it is MUCH better to make
your dielectric from several thin sheets of poly instead of one
single thick sheet. Example: If you need 90 mil thick poly, use
three 30 mil pieces stacked together to form a 90 mil dielectric. If
you use multiple layers the insulation will be MUCH more robust than
a single thick layer. The reason? If you were to use a single
thick layer of poly and it had a manufacturing defect, odds are that
the defect will extend all the way through the plastic. High
voltage would force its way through the single defect to blow up
your capacitor. If you have three or four thin layers stacked,
every sheet might conceivably have a defect, BUT it would be almost
impossible for all of the defects to be lined up to allow the high
voltage to punch through. Odds would be that while one sheet might
have a defect, you would still have several GOOD layers still
providing protection. Professionally manufactured capacitors
usually use this secret of layering.
*** From this point on, where the text describes using "a" piece of
poly, you will probably instead be substituting three or even four
thin sheets of poly for the reason described above. Keep this in
mind during the construction process!
The capacitor described, using .0625 inch thick polyethylene will
have a measured value of 0.0185 ufd with an a.c. working rating of
7500 volts r.m.s.*** A .090 inch thick polyethylene dielectric
capacitor will have a value closer to 0.010 ufd, and can be used in
Tesla coil circuits using a power source up to 15 kilovolts a.c.
*** TCBOR suggests that .060 thick poly will work -at- 15,000
volts. My experience shows this is not always true. I
recommend 90 mil poly for 12,000 to 15,000 volt operation.
As any coil builder knows, the resonant peak voltages go WAY above
the source voltages! You must design sufficiently thick poly
dielectric to handle this higher voltage.
Power LOSS in one of these caps in a Tesla coil configuration is
LESS than 0.5 watt per cap! Thus, they don't suffer from internal
r.f. losses which translate into heat failure.
A poly capacitor made with .060 poly should handle 36,000 volts d.c.
and the .090 thick poly job should easily handle 54,000 volts d.c.
These are very conservative ratings - good quality polyethylene
might handle twice these ratings. Tesla coils, of course, use
alternating current. The peak voltages generated in a resonant
Tesla circuit are incredibly hard on a capacitor. This is why you
must "over-engineer" the voltage capabilities of your capacitors.
Capacitors may be placed in series to achieve higher voltage
ratings, parallel for more capacitance. Combination series/parallel
combinations may be used to achieve any desired capacitance and
As an example, let's say you've built several 0.01 ufd capacitors
rated at 7,500 volts. You make the deal of your life and acquire a
terrific 15,000 volt power transformer....only your capacitors can't
handle this voltage. What to do? Assuming your coil requires a
capacitance of 0.01 ufd, what kind of connections can you make to use
the capacitors on hand?
First of all, you could connect two caps in series, as shown below:
c1 c2 0.005 ufd 15 kv capacitor
|| || made from two 0.01 ufd, 7.5 kv
You now have a capacitor rated at 15,000 volts...BUT the capacitance
is now only .005 ufd! You increased the voltage rating, but placing
capacitors in series reduces the capacitance. Now what? We connect
two more caps in series to create another 0.005 ufd capacitor, then
we take our two 0.005 ufd caps and put them in PARALLEL. We end up
with a 0.01 ufd capacitor rated at 15,000 volts:
|-------||----||--------| 0.01 ufd, 15 kv
| || || | capacitor made from
--------| |--------- four 0.01 ufd, 7.5 kv
| || || | capacitors
Because the transformer has an output of 15,000 volts, and we know
the PEAK voltages will be much higher than this, we might wonder
whether the capacitors we just wired together can handle the peak
spikes. To reduce the chance of failure, we would be better off
placing THREE of our capacitors in series to achieve a voltage
rating of 7,500 times 3 or a total of 22,500 volts. But what would
our capacitance be if we do this? Right! The capacitance will be
0.01 divided by three or 0.03333 ufd. Guess what?! We need to
parallel THREE sets of these caps to bring the capacitance back up
to 0.01 ufd:
c1 c2 c3
|| || ||
| || || || | 0.01 ufd 22.5 kv
| | capacitor made from
| || || || | nine 0.01 ufd, 7.5 kv
| || || || |
| c4 c5 c6 |
| || || || |
|| || ||
c7 c8 c9
Using this method, you may build a capacitor rated for any
capacitance and at any voltage rating! The down side is that you
can use up a LOT of capacitors!
We will make our conductive plates out of aluminum roof flashing.
You can buy a fifty foot roll of 14 inch wide aluminum roof flashing
here in the Dallas area from Home Depot for about $22 (4-93). This
is almost exactly the length you will need for three caps. The
aluminum flashing, by the way, is about 10 mils thick. My local
Home Depot charged me - $21.80 (4-93)
For a capacitor container, we use 6 inch diameter sdr 35 PVC pipe
cut into 19 inch lengths.
PVC pipe comes in 13 foot lengths, $1.25 per foot, or $16.25 + tax
(2-93). You will also need flat "glue on" pvc end caps for this
pipe; $4.40 each, or $26.40 for six (enough for 3 caps). Do NOT get
the rubber gasketed end caps - they are not flat on the end and the
seals will not hold up once immersed in oil.
You will also need pvc primer and cement to glue the end caps to the
You will also need some 1/4 inch thick clear plexiglass sheet
scraps, each piece large enough to cut a 6 inch diameter circle.
You will need plastic tie-wrap straps about 24 inches in length, or
enough 12 inch straps to link together to form nine 24 inch long
straps. Try to get straps that do not contain metal teeth inside
the lock of the strap.
To make connection to the aluminum plates you will need some #10
screws, washers, and nuts. You will need two split-bolt electrical
connectors to go on the top of each capacitor - Home Depot,
$3.33 each. You will also need 3 or 4 feet of high voltage wire
to connect the split-bolt output connectors to the aluminum
You will need two inexpensive radiator drain petcocks (valves) from
Chief Auto (#852079) for $2.99 each. We will use these valves to
fill the capacitor with oil and/or attach a vacuum pump.
Finally, you need a very high quality mineral oil to fill the
capacitor. I use SnapLube-100 from Tulco Oil in Arlington Texas,
It comes in a five gallon re-sealable pail for $43.10, including
tax. I've never had an oil related capacitor failure using this
oil. SnapLube 100 is a highly refined non-carcinogenic oil containig
no PCB's. Stay away from PCB oils. Any way, just about any good
quality, low moisture mineral oil will work in these capacitors.
BOTTOM PVC end caps:
Glue two 1/2 inch by 1/2 inch square pieces of plexiglass stock
three inches long across the inside bottom of the BOTTOM end
caps. Make sure these two strips are at least 1/4 inch away from
the walls of the plastic end cap! You have to allow space for
the wall of the pvc pipe to reach the bottom of the end cap.
The purpose of these spacers is to prevent the rolled capacitor
from sitting directly on the bottom pvc end cap. The space
allows any moisture in the oil to settle to the bottom of the
cap - the capacitor will sit on these plexiglass supports above
the moisture contaminated oil.
Cut three pieces of 6 inch diameter pvc pipe 19 inches long.
Clean the pieces with alcohol. Prime ONE end of each pipe with
pvc primer. Immediately swab bunches of pvc cement on the
bottom 3 inches of each pipe. Swab the inside walls of three
end caps which you prepared in the first step. IMMEDIATELY push
an end cap onto the glue coated end of each of the three pipes.
YOU WILL NOT GLUE A CAP ONTO THE TOP OF EACH PIPE YET - JUST THE
BOTTOMS! As you push the bottom end caps on, twist them to help
insure a good seal.
Take the remaining three end caps which will be used for the
TOPS of the capacitors. Using a saber saw cut a five inch
diameter hole in the end of each cap. This will leave a 1/2 inch
pvc border surrounding the hole. Cut three 6 inch diameter
circles out of your 1/4 thick plexiglass stock.
Next, we will score the TOP of the plexiglass with a sharp metal
scribe in the shape of a big X, going from edge to edge. Below
is a crude picture of these two cuts. Make the scribe marks at
LEAST 1/32 inch deep.
Cut two scribe / \ / \
marks at right | \ / |
angles to each | \ / | <-- TOP of plexiglass
other. | / \ | capacitor cover
\ / \ /
\ / \/
*************** Important Safety Note *******************
The purpose of the scribe marks is to provide starting
places for CRACKS to occur should the capacitor fail and
excess pressure occur. The plexiglass will bow outward
and crack at the scribe marks, releasing pressure. MAKE
SURE THAT THE SCRIBE MARKS ARE ON TOP WHEN YOU INSTALL
Use pvc cement and glue a 6 inch plexiglass disk over the
hole you just cut in the end of the top cap. MAKE SURE
THE SCRIBE MARKS ARE ON TOP! Do this for all three top
end caps. You just created three TOP end caps with clear
windows in the ends!
*************** Safety Notes *************************
The plexiglass will seal to the pvc, but pvc cement does
not effectively bond plexiglass to pvc. This is good!
Should the capacitor experience a pressure buildup due to
electrical failure we WANT the plexiglass to crack and
come loose and let the gasses out! The plexiglass cover
also allows us to SEE a capacitor failure - the light
from the arc shines out! The plexiglass cover serves as
at least two important safety needs! Do NOT build a
capacitor without this or some other form of over-
pressure release mechanism!
*** Now let all pvc cement dry for 24 hours before doing
any additional work on the pipes or end caps! See "PVC
CONTINUED" below for completion of the TOP end caps
after the glue has dried.
Cut your aluminum flashing into six pieces, 14 inches
wide (natural width) by 93 inches long. Use large
scissors to make cuts. Cut all corners round (use a
fifty cent piece to mark the curves on the corners) to
reduce corona. Using a hand operated paper hole punch
(any office supply has these), punch a hole in ONE end
of EACH sheet, about 1/2 inch from the end, midway between
the end corners. Take wet/dry sandpaper and sand all
burrs and rough edges off the flashing. Preparation of
the aluminum edges and corners is VERY important! Any
burrs or roughness will result in capacitor failure!
Attach a length of high voltage wire to the hole in the
aluminum using a wire terminal and # 10 hardware. The
wire should be about 16 inches long. Do not do anything
with the other end of the wire yet. Cut your screw off
as short as possible, very close to the nut. DO NOT GET
METAL FILINGS OR METAL DUST ON YOUR ALUMINUM FLASHING OR
POLY! METAL DEBRIS MEANS CAPACITOR DEATH!
Place sheets of newspaper on a carpeted foor. Put one of
your poly sheets down on the newspaper and clean both
sides with paper towels and alcohol. Measure a line 16
inches from the edge. Take a magic marker and draw a
line down the length of your sheet, 16 inches from the
edge. Go to the opposite edge and do the same, drawing
another line down the length of the sheet. Your poly
sheet should now be marked into THREE equal pieces, each
16" by 96". Using a box knife, carefully cut along these
lines. You will have three pieces 16" by 96" long. When
you complete these two cuts, carefully set the three
pieces aside on a clean piece of paper. Next, clean,
mark and cut your second piece of poly stock into three
pieces. It is important that the poly you use is
scratch-free and umblemished! Any imperfections can
cause capacitor failure. You now have six 16" by 96"
pieces of poly (again, if you are layering your poly, you
will have several times this number of sheets.)
Place a 16" by 96" length of poly on your newspaper.
Place one of your aluminum sheets on top of this first
poly sheet. Align the aluminum so that you have an even
1 inch border of poly showing all the way down it's 93
inch length. Now scoot the aluminum towards the screw
end so that the aluminum hangs over the END of the poly
Note the 1 inch overlap of
the aluminum end over poly! | |
| | |
Poly--->| | Aluminum sheet #1 O|<--hole and
| | | screw for
| | | wire
| | |
Left |--------------------------------------| Right
Place poly sheet #2 on top of the first aluminum sheet.
Align this poly exactly on top of the first poly sheet.'
Place the final aluminum sheet on top of the second poly
sheet. The screw end should be on the far end of the
capacitor, OPPOSITE the first screw.
| | Note the 1 inch overlap of the
end of the aluminum over the poly!
| | |<- Poly sheet
Hole & -> |O Aluminum sheet #2 | |
screw | | | (poly and alum
for wire | | | sheets #1
| | | omitted
|======================================= | for clarity)
Left |--------------------------------------| Right
You should have a capacitor sandwich consisting of, from the
bottom of the stack up, poly, aluminum, poly, and aluminum. Make
sure that the screws on the two sheets of aluminum are NOT ON THE
SAME END OF THE CAPACITOR!
Now carefully roll the capacitor up, starting on the left end.
Make the core hole in the center of the roll about 2 inches
across. Roll the capacitor up as TIGHT as possible! KEEP THE
BORDERS ON EACH SIDE OF THE ALUMINUM STRAIGHT! If your aluminum
gets too close to the edge of the capacitor, high voltage can arc
around the edge of the poly, causing catastrophic failure. I
really suggest getting a helper to help keep things lined up
straight as you roll!
Once the capacitor is rolled up, you should have a cylinder 16
inches tall and about 4.5 to 5 inches in diameter. If your
capacitor is much over 5 inches in diameter, then it won't fit
inside the PVC pipe.
Take three plastic tie-wrap straps and tie the rolled capacitor
up so that it can't unroll. The two wires should come out of the
top of the capacitor - one from the inside of the roll, one on
the outside. Place a scrap piece of poly between the head of the
screw and the underlying poly sheet so that the screw won't
puncture the underlying poly sheet.
PCV PREPERATION CONTINUED:
Carefully slide your rolled capacitor into one of the pvc
pipes, making sure the wires are "up". You should have
about a 1/2 inch space between the outside of your capacitor
and the inside wall of your pvc pipe. The capacitor will
sit on the two plexiglass spacers which keep the capacitor
1/2 inch above the bottom of the end cap (see step one).
Take two of the split-bolt connectors and set them on top of
the plexiglass window on the TOP end cap. The head of the
bolt will sit on top of the plexiglass. Place the two bolts
opposite each other, about 5 inches apart.
Split bolt ----> / O \
/ \ <-- 6 inch round cap with
| | 5 inch plexi window
Split bolt ----> \ O /
Mark the plexiglass with a marker right around the edges of
the bolt where it sits on the plexiglass. Next, clamp the
bolt in a vise. Drill two holes through the head of the
bolts. We will use two 1.5 inch long # 10 screws through
these holes to attach the bolts to the plexiglass.
Take the plexiglass window and prepare to drill holes in it.
Place a drilled split-bolt on the plexiglass where we marked
it earlier, and mark the new hole positions on the plexi.
Drill the two holes. Coat the bottom of the split bolt with
epoxy and attach the split-bolt connectors to the plexiglass
window with 1.5 inch # 10 hardware as shown below. The
epoxy is important to prevent oil leaks. Place epoxy on the
screw threads where they pass through the split bolt and
where they pass through the plexiglass:
|| || <-- SPLIT BOLTS --> || ||
|| || || ||
|| === || || === ||
====|==== < --- glop epoxy! -----> ====|====
nuts ->=== ===
| <-- SCREWS --> |
Note: each split-bolt is held on with TWO sets of screws and nuts!
Next, we drill and tap two holes in the plexiglass window for
our two valves. Locate the holes for the valves as shown
Split bolt ----> / O \
/ \ <-- 6 inch round cap with
| | 5 inch plexi window
Valve holes --->| O O |
Split bolt ----> \ O /
This arrangement keeps the split-bolts far apart, and the
two valves will be far enough away to prevent problems.
The holes for the valves should be slightly smaller than
the threaded portion of the valve. Be very careful tapping
the plexiglass else it will split!
Coat the threads of the threaded end of the valves with
epoxy and screw the valves into the holes. The epoxy will
seal the threads closed to the plexiglass and will prevent
oil leaks. Do not get epoxy inside the valve itself!
Give the expoy a several minutes to dry. Hold the end cap
over the top of the capacitor. Test it to see how it fits
down over the 6 inch pvc pipe. Make sure the screws holding
the split-bolts do not come too close to the top of the
capacitor. Lift the end cap off the capacitor. The wire from
the inside of the capacitor will connect to a screw on one
split-bolt connector; the other wire from the outside of the
capacitor will connect to the OTHER split-bolt connector.
Measure and cut the wires so that they are as short as
possible, YET STILL LONG ENOUGH TO REACH THE SCREWS HOLDING
THE SPLIT-BOLTS. Crimp and solder a round terminal connector
on the end of the two capacitor wires. Attach each wire to
one of the split-bolts using another couple of #10 nuts and
Again, temporarily slide the top end cap on and check your
wires for fit. If all looks ok, it is time to glue this
First, open both vlaves so air can go through them. If you
forget this step, you won't be able to push the TOP down
over the pipe because of the trapped air!
Slather the outside top three inches of your pvc pipe with
primer. Coat the inside walls of the end cap with primer
too. Next, coat over the primer with lots of pvc cement.
Quickly push the cap on the pipe, twisting back and forth
to make a good seal.
Connect a vacuum pump to one of the valves with a length of
5/16 hose. While running the pump, apply additional glue
around the lip of the cap where it touches the pipe. Listen
for the hiss of air getting sucked in. If you find a leak,
add pvc cement until it stops. Now is the time to find and
fix any leaks! Once oil is introduced into the capacitor,
leaks cannot be sealed with cement - the oil will prevent
Once you are satisfied the capacitor is leak-tight, let it
sit for at least 24 hours to allow the pvc cement to
completely dry. DO NOT PUT OIL IN A FRESHLY GLUED PIPE,
OR YOU WILL HAVE ONE HELL OF A MESS!
After the glue has completely dried, test the end caps for
complete seal one more time. DO NOT TEST FOR LEAKS BY
PRESSURIZING THE PIPE! PRESSURIZING THE PIPE CAN RESULT IN
A VERY DANGEROUS EXPLOSION SHOULD THE PLASTIC FAIL!
Once you are satisfied the plastic-to-plastic seals are good
we can charge the capacitor with oil. Run a 5/16 rubber hose
from one valve on the cap to a vacuum pump. Pump the cap down
for one hour. This will help boil out any residual moisture
and will help remove air from between the layers of poly and
aluminum. Once it has been pumped down, run another 5/16 inch
rubber hose from the other valve down into the 5 gallon
pail of oil - but do NOT open this valve yet. Turn your
vacuum pump on. This will help to continue to produce a
condition of reduced pressure in the capacitor. VERY slowly
open the oil valve just a tiny bit. This will cause the oil
to flow up the hose from the pail through the oil valve into
WARNING: Do NOT allow oil to enter the vacuum pump or
permanent damage to the pump can result!
*** Be sure to slowly open the oil valve - the oil will want
to foam up and enter the vacuum pump. Regulate the flow of
oil so that the oil does not foam. Stop the flow of oil once
the poly capacitor is under about 3/4 inch of oil.
Close the oil valve completely and let the vacuum pump suck
air out between the plates of the capacitor. Again, the
oil will want to foam up from all the entrapped air, so you
will have to monitor the vacuum valve very closely to
prevent oil from foaming up and entering the vacuum pump.
After a half hour or so, most of the air will be out of the
capacitor - at least enough where you can open the vacuum
valve so that the vacuum pump is going wide open. Be aware
that you will get air bubbles coming up out of the
capacitor even if you pump from now on 'til doomsday!
You will actually suck air THROUGH microscopic channels in the
pvc pipe and end caps! Your goal is to get as much air out
of the capacitor as is practicable - usually a couple of
hours of vacuum pumping will do the job.
After the capacitor has been thoroughly pumped down, shut off
the pump, disconnect the hoses. Slowly open one of the valves
and let air into the space above the capacitor to relieve the
vacuum. Close both valves on the capacitor to prevent
additional air and moisture from entering the capacitor.
Next comes the really hard part: set the capacitor aside
for at least one week, preferrable two weeks. Two or three
times a day, gently rock the capacitor from side to side.
This will help air bubbles to rise. DO NOT AGITATE THE OIL!
The idea to remove air bubbles, not introduce more!
DO NOT ATTEMPT TO USE THE CAPACITOR IMMEDIATELY AFTER
PUMPING IT DOWN! If you do, entrapped air will cause it to
fail. I have learned this the hard way! The capacitor
needs to sit for one to two weeks to allow air to rise out
of the capacitor. The number one reason for premature
failure of a new capacitor is due to running the capacitor
with pockets of air trapped between the layers.
To break in your capacitor do the following:
Wait the required week or two weeks (see above).
*** Open one or both of the valves to act as additional
pressure relief ports in case the capacitor does fail.
************ Important Safety Note **********************
Set up the capacitor in a protected outdoor area. Make
sure kids, pets, nosey neighbors, etc. cannot get shocked
from the capacitor or test transformer. It is important
that the chosen test sight be outdoors because the capacitor
will be on high voltage for an extended period of time, and
you probably won't be around should the capacitor fail. If
the capacitor experiences a failure, and if power is not
IMMEDIATELY disconnected, there is a very real chance for
an oil fire! Better the fire burn a hole in your lawn than
to burn your house or business to the ground!
Attach a neon transformer controlled by a Variac (variable 115
volt transformer) to the capacitor. You may also use a
variable high voltage d.c. power supply, but the "hum" from
a.c. helps shake bubbles loose. Slowly bring the high voltage
up to about 50% of the capacitor's rated voltage. Let sit for
eight hours. During this time, you will see additional air
bubble up out of the capacitor.
Air is a very real enemy! After this 8 hour time, slowly
bring the voltage up to 75% of the rated voltage. Let sit
another 8 hours. Finally bring up to rated voltage and let
sit for 30 minutes. If the capacitor is going to fail, it
will usually do so by this time.
Failure can result from poor quality or imperfect
polyethylene, insufficient poly thickness, burred aluminum
edges, oil containing moisture (1 part water in 10,000 parts
oil reduces the oil's insulation factor by 50% !!!) or
entrapped air. Insufficient border spacing around the
aluminum can also cause capacitor failure.
It is highly suggested that you open one of the valves on
the capacitor when using with a Tesla coil or other high
peak power source in case the capacitor should fail.
Having a valve open will vent a sudden build-up of pressure
from the gases which come from the vaporized plastic which
occurs when a capacitor fails. Close the valves whenever
the cap is NOT in use to prevent moisture in the air from
entering the capacitor.
The capacitor should be stored and transported standing
upright. If the capacitor gets turned on its side, the air
in the space above the capacitor will get back into the
layers of poly and aluminum - and you don't want to have to
go through the pump/wait process again!
Theoretically you could completely fill the capacitor with oil
- providing you left a way for the oil to expand and contract
with temperature changes, AND if you could build a 100% sealed
PVC container. I assure you that doing this is harder than
one would suspect! The pvc end caps may seep a tiny amount
of oil THROUGH the plastic - pvc is not nearly as impermeable
as we would be led to believe! Stand the caps upright in a
large metal pan to contain any oil leakage.
The pan should be large enough to contain all the oil stored
in one of the capacitors. The pan will contain leakage, and
should you have a capacitor container failure and oil fire,
the pan will contain the burning oil - definitely a plus!
Again, remember one reason why we put a clear plexiglass
window on the top of the capacitor: it lets you see when
you have a capacitor failure! If the capacitor fails, the
high voltage arc will be visible through the clear window.
If you do have a capacitor failure, DISCONNECT POWER
IMMEDIATELY! FAILURE TO DISCONNECT POWER CAN RESULT IN A
CATASTROPHIC CAPACITOR FAILURE! DANGERS INCLUDE EXPLOSION
AND AN INTENSE OIL FIRE!
************** LEARN FROM MY MISTAKES! ******************
Due to impatience, I did not let some of my capacitors sit for
a sufficient time before use, so they had air trapped in the
layers. They failed. I built several capacitors with
poly which was not thick enough to withstand the voltage of my
transformer. They failed. In every case I quickly
disconnected power. Not once have I had a "catastrophic"
failure resulting in container failure or fire. But I ALWAYS
keep a fire extinguisher rated for oil fires handy, just in
case! So should you.
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