The High Voltage Capacitor

By
E. Benson Scott, II, M.D.

107 Contempo
West Monroe
Louisiana 71291

The high voltage capacitor in Tesla coil circuits is frequently neglected. It may seem more expeditious to adjust the coil windings, spacing, or voltage, than to begin with the capacitor of sufficient capacity. The capacitor may be placed in series or across the circuit in parallel. This was briefly discussed in TCBA SEWS of Volume II, #1, 1983, as well as in Colorado Notes, page 403 (see question 1 of this issue, “Input/Output” column). Without getting into resonant effects, I will briefly discuss the capacitor component.

Any two metal plates placed in close proximity to each other and connected to a battery will produce a charge between the plates. The plates remain charged even though no current flows through the circuit. With this in mind, we can think of the capacitor as a device to store electricity. The charge which a capacitor develops is proportional to the voltage and the ability of the capacitor to hold a charge. This ability to hold a charge is termed capacitance. The larger the plate area and the smaller the space in between the plates, the greater the capacitance. The capacitance also depends upon the kind of insulating material between the plates. The material between the plates is called dielectric and the relationship it has to holding a charge is termed the dielectric constant. Air has a reference dielectric constant of 1, glass a dielectric constant of 7.8, and titanium 140.

Plates can be stacked together alternating connections to every other plate to form a multiple plate capacitor - with additional capacity added with each pair of plates attached. The unit of capacitance is the farad. This is usually too large a measurement so the microfarad (uF) (1/1,000,000 farad) is used. One microfarad is equal to one picofarad.

In dealing with high voltage circuits, the breakdown voltage is important. We do not want the capacitor to spark and lose its charge. This depends upon the thickness as well as the character of the dielectric. This voltage is not directly proportional to the thickness because doubling the thickness does not quite double the breakdown voltage (but would be close for practical purposes). Simply put, to withstand high voltage, the dielectric must be thicker, but the thicker the dielectric, the smaller the capacitance for a given area. A high voltage capacitor must have more plate area than a low voltage capacitor of the same capacitance. This normally makes for a physically large device.

The formula for determining the capacitance is C=0.224 x K x A/d (n-1). C is the capacitance in picofarads, K is the dielectric constant of the material between the plates, A is the area of one side of one plate in square inches, d is a separation of plate surfaces in inches and n is the number of plates used. A table of dielectric constants /breakdown voltages per .001" is given at the end of this article.

Now that we understand what a capacitor is, how it physically is formed and the rule for putting it together, let's look at several dielectric materials with their dielectric constant and breakdown or puncture voltage. The puncture voltage is the number of volts required to arc across a 0.001 inch gap. Notice how much better some dielectrics are than others as well as the ability of some materials to withstand high voltages. For example, by using window glass instead of air, the capacitance can be increased 7.5 times. The puncture voltage is increased 10 times allowing one to decrease the space in between the plates, thus raising the capacity even more.

Getting down to the meat of the subject, the capacitor plates can be made up of almost any conductor, these can be copper plates, aluminum foil, one-sided printed circuit boards, etc. The dielectric can be air, glass, mineral oil, etc. The spacing can be adjusted to withstand the voltages involved, i.e., an air capacitor with a voltage rating of 20,000 volts would require spacing's in the order of one inch. By moving to glass, one could consider spacing of 0.1 to 0.2 inches.

Take, for example: we develop a capacitor using air (the dielectric constant is 1). We use a plate with an area of 36 square inches (6" x 6"). We used 20 plates separated one inch apart. We end up with 153 picofarads or 0.000153 microfarads. If we go to castor oil, we develop 720 picofarads because we have used oil instead of air which has a much, much better dielectric constant. Let's reduce the spacing to 1/2-inch (better breakdown voltage). We now develop 1,440 picofarads or .00144 microfarads (the closer spacing gives us 10 times more capacitance). We are talking about a capacitor in the order of 7"x7"x12. By using window glass, plates 10"x10" separated 3/10", which should be fairly safe, and 20 plates, we develop .011 microfarads or 11,000 picofarads. Moving to 40 plates, we develop .0227 microfarads which would be quite reasonable for most systems.