rmation 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 with the peak voltage--
not rms (Root Mean Square) voltage--that they will be exposed to. If you wish to convert an rms
voltage to its equivalent peak sinewave value, multiply it by 1.414
So, to roughly calculate the proper voltage rating needed for an AC capacitor, you first double its required rms
voltage rating then multiply by 1.414. To further simpli8fy this calculation, all one needs to do is multiply the AC
(rms) voltage in question by 2.828. Now divide the voltage by the puncture-voltage rating to get a preliminary
thickness value. Finally, you must add a safety margin of 50% to 100%. The actual percentage depends on the
characteristics of the applied AC voltage. For a pure sinewave AC, we suggest a 50% safety margin whereas high
frequency, non-sinusoidal applications such as Tesla coils require a full 100% extra thickness.
If one is available, equip an oscilloscope with a high voltage probe to visually observe exactly what the circuit is
doing so you can determine the proper safety margin. An oscilloscope will also enable you to detect destructive
voltage spikes and superimposed AC (also called AC ripple) so you can design a capacitor to handle those harmful
Of course physical size, weight, and fragility are also important characteristics of capacitor design. If you have
size limitations, Mylar is the best dielectric material to use since it has a very high puncture voltage per mil, and
thus makes a very compact capacitor. Plastics are light, so most capacitors will weigh less than ten pounds. The
toughest plastic is Lexan, which is difficult to crack even with a hammer and is often used for vandal proof windows.
Glass is the worst material for a lightweight, durable capacitor, and can even crack under its own weight when lifted.
Take all this into account when selecting your materials.
Of course, the overall cost in labor and materials should also be considered before constructing a capacitor.
Calculate beforehand the cost of your materials. Paper and polyethelene are the cheapest. Glass is the next hig