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 higher
price. Labor time is about the same with Plexiglas, Lexan, and glass sheet capacitors. Exotic plastics such as
Teflon are not needed unless your application demands extreme chemical and thermal deterioration resistance.
Polyethelene has excellent chemical resistance, but breaks down gradually upon exposure to ozone gas (always present
around high voltage) becoming brittle and less resistant to arc puncture.
That brings us to another important consideration: the capacitor's useful life. To enhance a capacitor's life keep
the working voltage at or below the rated specification in both DC and AC applications. We discovered that charging
at no more than 70% of a capacitor's working voltage resulted in an amazing 10-fold increase in life time for one type
of commercial capacitor. Also, for DC capacitors, watch out for voltage reversals. If your system has a lot of
inductance, reverse voltage swings are always produced. Increase the safety margin if a lot of inductance is in the
circuit. Furthermore, the temperature should be kept below 120°F. As mentioned earlier, watch out for
superimposed AC, voltage spikes, and ringing. These types of AC waves can drastically shorten lifetime. Tesla coils
have notorious ringing. To repeat: if feasible, use an oscilloscope to visually analyze your circuit. Often a power
resistor inserted in the current path to the capacitor quenches ringing. With this criteria under our belts, let's
look at some problems your design and construction methods should prevent.
Signs of Trouble:
Your assembly techniques should seek to minimize the likelihood of a few possible problems. Luckily, all of them can
be prevent3ed at least in part by using ample amounts of insulating material such as No-arc or Corona Dope and/or high
voltage putty on all exposed areas. A plastic case to enclose the apparatus is also recommended (more on that later).
Still and all, you should know what problem insulation is preventing. The first problem insulation relieves is the
possibility of electrical shock.
Insulation a