Principles of Tesla Coil Construction

Part II - The Power Source

(continued)

High Voltage D.C. or Pulse Excitation

Although the Tesla coil is an alternating current operated device, it can also function if the electric current supply is direct current. Generally, the method used is to rectify the output of a high voltage transformer and operate it in conjunction with a rotary gap. Surplus houses often supply high voltage transformers with the output rectified. Such transformers were originally applied to ionizers and other electrostatic machines. One advantage of using a direct current supply is that it operates at a low enough current so that power arcs at the spark gap no longer present a problem.

Consideration When Choosing a High Voltage Source

by Richard G. Aurandt *

The KVA rating of a transformer is a product of its voltage times its current. Therefore, it is well to point out that you only need enough voltage to break down the spark gap and drive as much current as possible through the primary inductance. Since a capacitor's volume goes up by the square of the increase in voltage, it is desirable to hold the transformer between 7-10 KV and increase the current to increase the KVA for greater spark length.

A good “rule of thumb” in Tesla coil design is to use 500 VA (.5KVA) for every foot of spark needed. The .5 KVA/foot ratio is based on how bright the spark (current) against how long the spark (potential). The ratio indicated produces a “good looking” spark for whatever length is chosen. This number applies to well designed coils where good electrical considerations as well as good construction materials are employed. All phases of the circuit, especially the high frequency circuitry must have the highest “Q” possible.

Because a transformer being used to excite a spark gap oscillator is subjected to short circuit conditions each time the spark gap breaks down or when the capacitor is completely discharged, it must either be a high reactance (self limiting) type or have its input or output limited by chokes (reactors) or resistances, the chokes being preferable.

* (Editor's note) Richard G. Aurandt has had considerable experience in the field of high voltage-high frequencies. He has served as a special effects artist in movie productions and toured with the Sermons In Science program which featured demonstrations with high voltages at high frequencies. Mr. Aurandt has consented to contribute to future issues of TCBA NEWS when time permits.