cies. 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 information provided elsewhere in this article. Try one of the designs described later. Or perhaps you
prefer experimenting instead. Either way, when building for the first time, we suggest making small designs first to
get used to techniques and quirks before you invest lost time and money.
Your must also take into consideration the voltage that will be applied to the capacitor. That will affect your
choice of a dielectric and thus its required thickness. Should you use an inadequate dielectric or thickness, sparks
or arcs can result. A spark is a temporary breakdown that a lot of capacitors will survive, but an arc is serious:
it is a path burned into the dielectric or other component. Arcs carbonize materials, producing a highly constructive
channel that often renders an apparatus useless and very likely dangerous. Except in special cases where the
insulator is a "self-healing" type (like air, oil, and some plastics), a single arc will ruin the capacitor.
To compensate for the impurities that often appear in materials that are not highly refined for capacitor use, we must
add a safety margin to the thickness of the dielectric. In the case of DC, a good rule of thumb is a 50% margin. For
example, say you need a 500Volt DC capacitor using polystyrene. Consulting Table 1, note polystyrene's breakdown is
500 volts per mil, thus 1 mil is required. Adding 50% gives you 1.5 mils, which is adequate for pure DC. You can
always use a thicker dielectric it if's expedient, providing that you adjust the number of plates or their size to
accommodate the wider plate separation. It should be mentioned that when making a paper capacitor, you should use a
healthy safety margin since paper is not always uniform in thickness.
In comparison to AC, DC puts relatively little stress on a capacitor. By contrast, AC reverses the dielectrics'
polarity every cycle. So the dielectric in an AC capacitor must have twice the thickness required in an equivalent
DC capacitor. Further, when considering dielectrics in AC applications, you must deal 