What is a Tesla Coil?

For the uninitiated, a Tesla Coil is a device that generates very loud, very impressive, and very DANGEROUS lightning-like discharges that are several feet long. Invented by Nikola Tesla in 1891, the Tesla Coil is technically considered to be a dual-tuned resonant transformer (more on that later), and has a widespread following amongst hobbyists around the world. With the advent of the World Wide Web, a mass of information can be found regarding it… some useful, and some downright worthless and potentially dangerous… which is where my safety warning comes in…

Safety

Although the Tesla Coil is a fascinating device, one should never forget that many dangers are involved in operating such a device. At the very least, extremely high voltages and currents are present in the driving circuitry of a Tesla Coil… enough to cause electrocution if contacted by part of the body. UNDER NO SITUATION SHOULD PERSONS INEXPERIENCED WITH HIGH VOLTAGE SAFETY ATTEMPT TO BUILD OR OPERATE A TESLA COIL. My coil is the product of over two years of research, design, and consultation with others more experienced than me who have successfully built such devices. Bypassing these steps is likely to lead to a coil that doesn't work, or worse still, to a fatal error brought about by a gap in one's knowledge of high voltage safety.

OK, now that the heavy stuff's out of the way… :-)

What parts make up a Tesla Coil?

Transformer: The first component in a Tesla Coil is a step-up transformer that steps up the 240V (or 110V) mains to a much higher voltage of several kilovolts. Small coils typically use a neon sign transformer (NST), which are commonly found with ratings of 9kV, 12kV and 15kV with secondary currents of 30mA and 60mA. Sometimes, when NSTs are not available, other power supplies are improvised using oil burner ignition transformers (OBITs), microwave oven transformers (MOTs), or for very small coils, automotive ignition coils or flyback transformers from TVs or computer monitors. For very BIG coils, power distribution transformers (aka pole pigs) are often utilized (warning: whilst you might survive being shocked by an NST, if you get near the output of a pole pig or even a MOT, you will not get a second chance…)

Protection circuitry: As NSTs are fairly fragile transformers and don't like excessive back-EMF, many people use a filter network of resistors and capacitors (and sometimes chokes) to protect them. Whether or not you decide to use a filter network, a "safety gap" adjusted to shunt excessive voltage to ground is usually considered a must…

Spark gap / capacitor / primary coil: Together, these three components comprise the "tank circuit"… an LC oscillator designed to oscillate at a certain frequency in the 100-500 kHz range. The simplest spark gap (but also the worst performing) is a pair of electrodes separated by an air gap (most practical spark gaps use multiple gaps arranged in series and/or airflow through the gap to improve performance… other more advanced variants such as the rotary spark gap exist but I won't attempt to cover them here :). The capacitor can be as simple as an array of beer bottles filled with saltwater and wrapped with aluminium foil, ranging to arrays of commercial pulse-rated polypropylene capacitors (known as the MMC - Multiple Miniature Capacitor). Lastly, the primary is usually a flat or slightly inclined spiral of copper refrigeration tubing or heavy wire.

Secondary coil / topload: Just as the tank circuit has a natural frequency at which it oscillates when excited, so too does the secondary coil / topload system. The secondary coil, usually a 3" or bigger diameter PVC pipe wound with 22AWG or thinner magnet wire for at least 18" or so, stands upright in the centre of the primary coil. The topload is usually a metallic object of a toroidal (i.e. donut-like) shape that sits on top on the secondary (from which the sparks jump off). The topload, by virtue of its capacitance to ground, helps to lower the natural frequency of the secondary coil.

In brief, how does it work?

When powered up, the 50/60Hz AC from the transformer charges the capacitor. When the voltage across the capacitor exceeds the breakdown voltage of the spark gap, the spark gap fires, acting like a high-speed switch that dumps the energy stored in the capacitor into the primary. This causes the tank circuit to oscillate at its natural frequency… a frequency designed by the builder of the coil to be the same as that of the secondary / topload system. Through the phenomena of resonance, this causes the secondary to see a massive rise in voltage, which eventually causes the air surrounding the topload to ionize and form long streamers that discharge either to ground or air.

This is by no means an exhaustive discussion of Tesla Coil theory… it is really only scratching the surface of how a Tesla coil operates. Many factors influence how well a particular coil operates, and even today no-one fully comprehends all of the nuances of this apparently simple device (I certainly don't!) However, in recent years (in the case of MMC development, as recent as three years ago!), great advances have been made in the construction technique of the Tesla Coil, and it is certainly true that the coils of today far outperform coils that were commonplace earlier in the 20^th century.