allon-size
wide-mouthed mayonnaise jar. The project only cost about $2.00 and is good to at least 10 KvDC at 2.5 nF. Units we've
tested at 15 KvDC did not fail; at that voltage, the capacitors stored just under 1/3 joule each.
First select a jar without bubbles, cracks, or blemishes and that has a mouth large enough to comfortably slip your
hand through. Next, carefully clean it out. You'll use aluminum foil inside and out as the conductive plates (see Fig.1).
Cut a foil disk 1-inch bigger that the bottom of the jar. Now coat the dull side of the foil and inside jar bottom
with a thin, even layer of rubber cement. Let both dry for 10-minutes, and press together. Smooth with firm hand
pressure. Avoid excess wrinkles. Do the rest of the inside except the top inch of the bottle using three of four
pieces of foil. (It is easiest to do the plate in pieces instead of all at once, since rubber cement "grabs" and it is
difficult to reposition the foil once contact has been made.) Now do the outside foil plate in pieces, leaving the
top inch bare. Check the foils with a continuity tester to determine if the pieces are in good electrical contact.
Areas of foil not in contact can be bridged with strips of foil or nickel-print paint.
For the top cover, cut two disks of clear plastic, one slightly smaller than the rim, the other 1/4-inch larger than
the rim. Glue the two pieces together to form a plug. Drill a 1/4-inch hole through the plug's center. Cut and
insert a length of 1/4-inch (outer diameter) metal rod or tubing through this hole. Attach a ball to its top, and
solder a wire or small-link chain to its bottom. The wire must make good electrical contact with the foil. Let the
assembly dry for a day with the cover off, to allow vapors from the rubber cement to dissipate, then cement the cover
on with silicone or Krazy Glue.
Some nifty low inductance capacitors can be made from pieces of copper-clad epoxy circuit board (see Fig.2). For a
simple two-plate capacitor, you can use one double-sided sheet. For multiple sections, use single-side board.
To prepare each board, start by etching away a 1-inch strip from around all its edges. That process can be simplified
by first masking off the strip, spraying the bare copper with an etch-resistant paint, removing the masking tape, and
Clean the board after etching, and rinse with de-ionized or distilled water. Thoroughly air-dry the sections, or use
a blow dryer. Attach strips of aluminum foil to each plate.
If you are building a multiple-section capacitor, connect the aluminum foil strips together as shown in Fig. 3 and
secure them using glue or nylon bolts at each corner. Spray the finished assembly with several coats of an insulating
product, or paraffin.
If you use the dimensions shown in Fig. 2and a 0.060-inch gap between plates, you can achieve a capacitance of 1.94nF
(1940pF) per section. When deciding on the gap width to use, keep in mind that the greater the space between
successive plates the lower the chance of arcing. For example, a 1-inch spacing gives you a 30% larger gap than a
20-Kv spark can jump. Insulation will further improve that margin.
The Stacked Sheet Design:
This type is virtually identical to our PC board capacitor, but it can be designed to handle considerably more voltage.
You simply substitute sheet plastic or glass dielectrics, and glue aluminum foil in place of the copper for each
section (refer to the PC board capacitor drawing in Fig. 3 as needed). All in all, it's an easier design to build,
as it does not involve the effort of etching copper, and you can continue to add sections to your original prototype
to increase its capacity as future demands require.
When building a large capacitor of this type, we suggest that you use nylon bolts at the corners to hold it all
together. The bolt holes should be pre-drilled before assembly, and all chips cleared away. Make sure the
plate-to-edge spacing is adequate for the voltage you will subject the capacitor to. Add extra spacing if you intend
to use bolts at the edges.
Glue foil carefully to the top of the fir