Principles of Tesla Coil Construction

The Capacitor

The capacitor of an LC arrangement is one of the basic units of a resonant circuit. It is secondary to no other component. The inductance (L), and the capacitance (C), can be considered to be an electrical brain made up of two parts. They both have a strong control over the actions of the circuit. The values “L” and “C” determine, among other factors, the resonant frequency of a circuit. Vary either one, or both, and the oscillating wave is changed.

The underlying principle of the capacitor is that it acts as a storage facility for electrical energy. In so doing, it is said to possess potential energy. The quantity (Q) of the electrical charge is dependent upon both the capacitance (C) and the applied voltage (V). This is explained by the equation: Q = CV.

There is a relationship between the physical size of a capacitor and its capacitance. The larger the capacitor, the greater ability it has to store energy. The voltage handling capacity of a capacitor is generally a matter of the distance between its plates and the insulating material (dielectric) filling the spaces.

The best dielectrics are those that possess a high specific inductive capacitance (or dielectric constant) and a high resistance to rupture. Except for mica and some plastics (polystyrene and polyethylene), most solids are not suited for use in capacitors applied to Tesla coil circuits. The reason for this is that solids have a poor polarization time. That is, solids cannot charge and discharge as fast as the high voltage transformer that powers them. In addition, solid dielectrics do not fully discharge their energy into the circuit. Some of the total energy remains in the capacitor. The effect is a reduction in the quality factor of the capacitor.

Be that as it may, there are several reasons why Tesla coil builders continue to construct capacitors with glass as the dielectric. For one, commercial capacitors are very costly. Secondly, glass is an inexpensive and readily available substance. Thirdly, glass possesses a high dielectric constant and a high resistance to rupture. Lastly, they work.

However, after having built a number of glass dielectric capacitors, I have come to the conclusion that just about any oil filled commercial unit of proper value is better than a home built glass unit. My objections to glass capacitors arises from the fact that they become physically large, heavy, and are only workable with high voltages when immersed in oil. An improvement over glass dielectric units is one which only oil is used as a dielectric. This may result in the reduction of the total capacitance for a given number of plates but the results are quite unlike the units made with glass. Another advantage of the glassless capacitor is that there need be no disassembling for purposes of replacing a broken dielectric.

Generally, a capacitance of .001 microfarad to .01 microfarad is suitable for small table top Tesla coil projects. Medium sized coils throwing sparks from 2-3 feet in length run on capacitances of from .02 to .03 microfarad. Larger units require progressively higher capacitances beyond .05 microfarad.

Commercially applied capacitors should have a voltage rating capable of handling the transformer potential. Since most capacitors are rated in direct current potentials, the capacitor should have a voltage rating that is at least twice the voltage output potential of the power transformer. Basic tenants of capacitor principles allow increasing the voltage handling ability of a capacitor by mounting two or more units in series. It is always best to use capacitors of the same capacitance value and voltage rating when so doing. Placing capacitors in series, however, has a decreasing effect upon the total capacitance. Therefore, it is necessary to begin with units of high capacitance. For example, two .01, 10,000 volt units placed in series will result in a capacitance of .005 microfarad and a voltage rating of 20,000 volts. That is, .01 ÷ 2 = .005 mfd. Since this arrangement has produced a decrease in the total capacitance of the two units, laws of physics dictate a gain in other values that are inversely proportionate. That is, a decrease in capacitance by 1/2 will result in a voltage rating that is twice the original value. The original voltage rating of 10,000 ÷ 1/2 = 20,000 volts.

A capacitor with a low capacitance can be altered to have a higher capacitance by placing two or more units in parallel. Two .01 mfd., 20,000 volt capacitors placed in parallel arrangement will result in a capacitance of .02 mfd. at 20,000 volts. That is, .01 + .01 = .02 mfd. When capacitors are connected in parallel circuit, the voltage ratings remain equal to the capacitor having the lowest voltage rating. Here, again, it is best to work with capacitors whose capacitance and voltage ratings are alike. Proper capacitances and voltage ratings may also be obtained by placing series connected units and parallel connected units together.

The type of capacitor chosen boils down to whatever resources are held by the individual building a project. While high Q factor capacitors are most desirable, they are most often beyond the means of the average experimenter. Although oil filled commercial capacitors can be obtained at reasonable prices in the surplus market, glass dielectric capacitors will continue to be the basis of many amateur constructed projects. Even Tesla made use of them.

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