A Tesla Coil is a high voltage device that makes big sparks. It was pioneered by Nikola Tesla over 100 years ago. This is an introduction to my many Tesla coils and their effects.

More technical explanation: In its simplest form it is an air cored transformer driven by discharging a high voltage capacitor (eg 10 kV) through a few of the primary turns. A resonance occurs with this capacitor and the few primary turns (eg 100 kHz).  This induces current in the tall secondary with many turns (eg 1000). A high frequency resonance of the same frequency occurs between the inductance of the tall secondary coil and the capacitance of a rounded metal object (typically a donut shaped toroid). A very high voltage (eg 200 kV) results in electricity (streamers) directly into the air or sparks to nearby objects. Best results occur when the number of primary turns makes both resonances equal in frequency.

A Tesla coil such as the one above with sparks of 8 feet equates to perhaps 250,000 Volts. It is very hard to measure this voltage directly.

A simplified circuit diagram of one of  my Tesla coils is shown below with various filter circuitry and safety spark gaps to prevent the very high voltages from wreaking havoc with the lower voltage side. It uses 4 microwave oven transformers (MOT’s) driven by a variac. These take 240 V AC mains and supply about 8 kV to the primary coil, capacitor and spark gap. This circuit is resonant at about 100 kHz and drives the secondary coil and toroid which are resonant at the same frequency.