property of the dielectric, the higher its resistance, and the less dielectric leakage loss
present. In low current, high voltage power supplies, minimizing all sources of loss is important to prevent undue
power-supply loading. For that reason, plastics are by far the best materials for large capacitors. A serious project
should involve one of the plastics.
Lexan, Polystyrene, and Plexiglas in particular are easy to glue, and can be cut with a table saw using a plastics blade,
or a carborundum impregnated all-purpose cutting blade like Zippity-Do (which is cheaper). A saber saw (also called
Jig Saw) with a really coarse wood blade will also work (other blade types clog or chip). Such plastics may be drilled
with high quality steel drill bits or special plastic bits. They must be drilled at 300 RPM or slower to prevent
chipping and melting, and be sure to leave the protective film or paper on the plastic when working with it.
Mylar, Polyethylene, Nylon, and especially Teflon are difficult to work with as they are very slippery. The best way to
attach plates to any of those materials is to use a glue specifically designed for the material. Polyvinyl chloride (or
just PVC) is moderately slippery. It can be glued with a PVC cement, or foil plates can be attached using silicone RTV.
Glass is, in principle, an even better dielectric. It also has th advantage of being easy to glue to with Silicone
RTV or Krazy Glue(Cyanocrylate Ester, or just 'Cyano' for short), and is readily available and cheap. However, it is
fragile, and may contain impurities that allow conductive paths for destructive arcs. Contradictorily, for your first
capacitor or two, we suggest that you try a type made with glass to gain experience, since they go together easily and
Many industrial capacitors are oil filled. Oil has an extremely high resistance, so it does not measurably increase
leakage. Silicone transformer oil is the best liquid insulator, but is rather hard to obtain. Mineral oil, on the
other hand, is readily available from most pharmacies. Although it has a low dielectric constant, it can be used in a
variety of simple ways to make very good high voltage capacitors.
For example, a dandy variable DC capacitor can be made by immersing a junked AM-radio tuning capacitor of the
movable-plate type in mineral oil so its shaft and connection leads come out of the container's top. If you wish to
try this idea, make absolutely certain the "cold" plates of the capacitor (the moving plates) are at ground potential.
Use a good, large non-metal knob for adjustment. A 100 to 365pF variable capacitor with a 1KvDC breakdown voltage
(i.e. a plate spacing of 1mm) becomes a 270- to 985pF unit wit 7500 VDC breakdown rating. Try pricing a 7500 volt
variable capacitor sometime, and you'll see the advantage to this approach!
You can use mineral oil in designs of your own, too. Immersion of a homemade capacitor in mineral oil will greatly
improve its voltage rating and lifetime.
Paper is an excellent dielectric when saturated with mineral oil. Try 20lb bon computer paper which has a 4 mil
thickness. Prepare this inexpensive capacitor by interleaving layers of dry paper with aluminum foil, and then
immerse the capacitor in oil until the paper gets saturate.
One disadvantage to using oil in home-made capacitors is that the tape or glue used to bond the assembly must be
oil-resistant. Silicone RTV is the best glue for these purposes.
There are several things to consider when designing and constructing your own capacitor. Let's point out each one
before moving to the construction details. The first and most important thing to concern yourself with is safety.
Despite the romance of high voltage, it' foolish to needlessly risk your life. Since you will probably be working
with lethal voltages, observance of all safety practices for high voltage (or HV) is absolutely essential.
For some guidelines, see the boxed text entitled "High Voltage Safety".
The next aspect to consider is capacity. If you have a specific capacitance in mind, you can design a capacitor
using the info