"Build capacitors that really pack a punch for high voltage fun!"
Rewritten by Tony van Roon (VA3AVR)
As one who supplies parts to those who experiment with high voltage, I get
a lot of letters and phone calls from frustrated builders that go like: "Can you supply an inexpensive xxx microfarad
capacitor at a working voltage of YYY? My only source wants $249 for one."
Sometimes, a high price is justified; other times, a seller has the only capacitors of a special value available, and
will soak you for the maximum dollar.
It is feasible to build your own capacitors of any voltage and energy storage size for either AC or DC use. The process
involves a step-by-step logical approach that we'll present here. We'll explain how to plan and construct a capacitor,
where to get materials, safety considerations, tips and hints, and include a few simple projects.
A Capacitor's Description:
A capacitor consists of two or more plates of a conductive material separated by an insulating substance called a dielectric.
A dielectric may be solid, gel, liquid, or gas. A capacitor's ability to store energy is measured in either microfarads (uF),
nanofarads (nF), or picofarads (pF). Micro means one millionth, nano stands for one billionth, and pico for on trillionth
(farads are also used, but in high voltage work they are impractically large units). Several factors affect capacitance.
The formula for determining capacitance is: C=(0.224KA/d)(n-1)
Where C is the capacitance in picofarads, K is a constant that depends on the insulator (or dielectric)
between the plates (called the dielectric constant), A is the area of one conductive plate in square inches,
d is the separation between adjacent plates in inches, and n is the number of plates. As you may know,
different insulators have different dielectric constants. Table 1 shows the values of K for some common materials and
the peak voltage they can withstand per 1/1000th inch (called a mil) of thickness. This rating
is called the puncture or breakdown voltage.
The better the insulating 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 pharma