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TESLA COIL

The tesla coil is a high voltage secondary tank circuit fed by a resonant primary circuit. The primary is tuned to the self resonant frequency of the secondary coil by changing a tap location on the primary coil. A typical primary circuit is fed by 15000 volt "pole pig" or neon sign transformer. A high voltage capacitor across the transformer charges up many times during each line cycle. A rotary spark gap run by a 3600 RPM motor, repetitively connects the primary coil across the capacitor, discharging the capacitor through the primary, thereby exciting the secondary circuit. The secondary discharge is actually radio frequency high voltage at the resonant frequency of the secondary coil. A rule of thumb for predicting the length of spark discharge is 1 foot of discharge per kilowatt of input power.

Powerful RF discharges can disrupt radio and television reception. High voltage discharges can disrupt and destroy even electronics located nearby.  Therefore, high power coils are usually run outdoors away from buildings and electronic devices.
 
 

My input power starts with a 10 KVA pole pig transformer, like many that you see up on telephone poles along the street. The transformer is connected in reverse so 240 vac input is transformed to 14400 vac. The high voltage charges a capacitor bank, which is repetitively dumped into the primary coil through a rotary spark gap into the tesla coil itself. The primary coil is made from 1/2 inch copper water pipe wound around the base of the secondary coil drum. The secondary is approximately 400 turns of 18 gauge wire, wound around a hollow wooden form that is 24 inches in diameter and 48 inches tall. The top end of the secondary coil feeds a toroidal capacitor top hat that acts as a energy storage device. A million or more volts of RF energy breaks free from the toroid into the open air, or anything within reach. Sparks are emitted in a 10 foot radius in all directions when the tuned circuit primary is tuned to the secondary resonance. Different sized top capacity hats give secondary resonances, so the primary tap point needs to be adjusted to match. My coil resonates at about 56 Khz, as checked with a signal generator and oscilloscope.