A Tesla coil is an electrical resonant transformer device designed by inventor Nikola Tesla in 1891. It is used to produce high voltage, low current, high frequency alternating current. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits. Tesla used these devices to conduct innovative experiments in electrical lighting, phosphorescence, X-ray generation, high-frequency alternating current phenomena, electrotherapy, and the wireless transmission of electrical energy. Tesla coil circuits were used commercially in spark-gap transmitters for wireless telegraphy until the 1920s, and in medical equipment such as electrotherapy and violet ray devices. Today, their main usage is for entertainment and educational displays, although small coils are still used as leak detectors for high-vacuum systems. Originally, Tesla coils used fixed spark gaps or rotary spark gaps to provide intermittent excitation of the resonant circuit; more recently, electronic devices are used to provide the switching action required.
Operation
A Tesla coil is a high-frequency, air-cored resonant transformer, which can be used to produce very high voltages. In Tesla’s early designs, a simple spark gap was used to set up the oscillations in a tuned, radio-frequency transformer. Tesla coils can produce output voltages from 50 kilovolts to several million volts for large coils. The alternating current output is in the low radio frequency range, usually between 50 kHz and 1 MHz. The original, spark-excited Tesla coil circuit, shown below, consists of these components:
A high-voltage supply transformer (T), to step the AC mains voltage up to a high enough voltage to jump the spark gap. Typical voltages are between 5 and 30 kilovolts (kV). A capacitor (C1) that forms a tuned circuit with the primary winding L1 of the Tesla transformer A spark gap (SG) that acts as a switch in the primary circuit The Tesla coil (L1, L2), an air-core double-tuned resonant transformer, that generates the high output voltage. Optionally, a capacitive electrode (top load) (E) in the form of a smooth metal sphere or torus attached to the secondary terminal of the coil. Its large surface area suppresses premature air breakdown and arc discharges, increasing the Q factor and output voltage.
Resonant transformer
The specialized transformer used in the Tesla coil circuit (L1,L2), called a resonant transformer or radio-frequency (RF) transformer, functions on different principles to transformers used in AC power circuits. While an iron-cored transformer is designed to transfer energy efficiently from primary to secondary winding, the resonant transformer is designed to temporarily store and transfer high frequency currents. A primary winding is in parallel with a capacitor, while the secondary winding has a "parasitic' capacitance with its inductor, as illustrated by the diagram. The inductors and capacitors together function as two LC circuits, "tuned" so they have the same resonant frequency, and each spark briefly completes a circuit on the primary side, producing oscillation back and forth between the primary and secondary. This is analogous to the way a tuning fork stores vibrational mechanical energy. The primary coil (L1) consists of relatively few turns of heavy copper wire or tubing, and is wired in parallel to a capacitor (C1). Current flows through the spark gap (SG) with each spark, and repetitively energizes the primary side LC circuit. The secondary coil (L2) consists of hundreds to thousands of turns of wire on a hollow cylindrical form, and the primary is coupled to it by being wound on an insulating former outside it. The inductance of (L2) resonates with stray capacitance (C2) and the capacitance of the toroid's metal electrode attached to the high-voltage terminal. The peculiar design of the coil is dictated by the need to achieve low resistive energy losses (high Q factor) at high frequencies, which results in the largest secondary voltages:
The coils in a Tesla transformer are loosely coupled. The primary winding is larger in diameter and spaced apart from the secondary, so the mutual inductance is low and the coupling coefficient is typically 0.05 to 0.2; this means only 5% to 20% of the magnetic field of the primary passes through the secondary when open-circuited. The loose coupling slows the exchange of energy between the primary and secondary coils, which allows the oscillating energy to stay in the secondary circuit longer before it returns to the primary and begins dissipating in the spark. Each winding is also usually limited to a single layer of wire, which reduces proximity effect losses. The primary carries very high currents. Since high-frequency current mostly flows on the surface of conductors due to skin effect, it is often made of thick wire with a large surface area to reduce resistance, and its turns are spaced apart, which reduces proximity effect losses and arcing between turns.
The output circuit can have two forms:
Unipolar: One end of the secondary winding is connected to a single high-voltage terminal, the other end is grounded. This type is used in modern coils designed for entertainment. The primary winding is located near the bottom, low potential end of the secondary, to minimize arcs between the windings. Since the ground (Earth) serves as the return path for the high voltage, streamer arcs from the terminal tend to jump to any nearby grounded object. Bipolar: Neither end of the secondary winding is grounded, and both are brought out to high-voltage terminals. The primary winding is located at the center of the secondary coil, equidistant between the two high potential terminals, to discourage arcing.
Operation cycle The circuit operates in a rapidly repeating cycle in which the supply transformer (T) charges the primary capacitor (C1) up, which then discharges in a spark through the spark gap, creating a brief pulse of oscillating current in the primary circuit which excites a high oscillating voltage across the secondary:
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