Tesla Coils Essay, Research Paper

PES Physics Information Search

A Tesla coil is a device that produces a high-frequency, high voltage current. When working at such high frequencies it can generate spectacular lightning-like discharges in to the air.

The most popular use for the Tesla coils has been in the film industry, where they are used whenever lightning or electrical arcs are required. Scaled down versions of the tesla coil are also used in television sets. The Tesla coil was invented over 100 years ago. By Nikola Tesla.

(The Tesla coil featured above is eight feet high with a toroid ring that is four feet in diameter. This Tesla coil is capable of pumping out quarter of a million volts out in to the air.)

The Tesla coil is a high frequency current generator, that uses the principles of electrical resonance to produce an antinode of high potential at the top of a large coil of wire. The Tesla coil is basically a high frequency transformer with the bottom end of the primary and secondary coils connected together and firmly grounded.

PES Physics Information Search

(The Illustration above is of a Tesla coil capable of producing half a million volts.)

A Tesla coil consists of three main parts. The first part is called the primary coil or simply primary.

The primary is simply a coil of thick wire wound around several times. The driver circuitry is connected directly to the primary coil, and the primary coil is wound on top of the secondary coil.

The second and largest part of a Tesla coil is called the secondary coil, or just secondary. The secondary consists of a hollow core. Around the core is wound magnet wire from the bottom to top. Secondary coils can be any length; however, a larger secondary always produces more power.

The last main part of the Tesla coil is called the Toroid. Toroids can be shaped in many different ways. The most common is a ring or ball that sits on top of the secondary coil. Toroids can be made out of any conductor of electricity. The electric arcs produced by the Tesla coil are emitted from the toroid.

How they work

Tesla coils are resonant transformers. This means that there is a specific frequency at which they operate. This frequency is called the resonant frequency.

What determines this resonant frequency is the secondary coil.

To get the secondary to resonate, pulses of energy have to be fed at just the right rate and frequency.

Energy pulses come from the primary circuit. This circuit is made up of (1) the high-voltage transformer, (2) the primary capacitor, (3) the spark gap and (4) the primary coil. Together, these parts form a crude type of oscillator. What happens is thus: the transformer charges the capacitor up until there is a high enough voltage across the spark gap to jump the air gap. When this spark occurs, the energy stored in the capacitor is ‘dumped’ into the primary inductor. The primary inductor then builds a magnetic field as the capacitor’s energy flows through it. The magnetic field will eventually collapse, and will in turn dump what energy is left back into the capacitor. This see-saw activity continues until there isn’t enough voltage left to jump the spark gap.

The oscillation frequency is determined by the value of the primary capacitor and the primary inductor. Together, they form what is called a parallel-resonant circuit. In typical Telsa coil designs, the frequency is adjusted by altering the primary coil’s inductance.

If the energy bursts are of the same frequency as the secondary, the energy transferred by the primary’s magnetic field will start to build up in the secondary coil. Much like a laser, this energy grows and amplifies itself until there is an incredible voltage built up at the top of the coil, which dissapates into the air in the form of electrical sparks.

The operation of the Tesla Coil is as follows:-

1. The spark gap initially appears as an open-circuit. Current from the HV power supply flows through a ballast inductor and charges the primary tank capacitor to a high voltage. The voltage across the capacitor increases steadily with time as more charge is being stored across its dielectric

Eventually the capacitor voltage becomes so high that the air in the spark gap is unable to hold-off the high electric field and breakdown occurs. The resistance of the air in the spark gap drops dramatically and the spark gap becomes a good conductor. The tank capacitor is now connected across the primary winding through the spark gap. This forms a parallel resonant circuit and the capacitor discharges its energy into the primary winding in the form of a damped high frequency oscillation. The natural resonant frequency of this circuit is determined by the values of the primary capacitor and primary winding, and is usually in the low hundreds of killohertz

The close proximity of the primary and secondary windings causes magnetic coupling between them. The high amplitude oscillating current flowing in the primary causes a similar oscillating current to be induced in the nearby secondary coil. The self capacitance of the secondary winding and the capacitance formed between the Toroid and ground result in another parallel resonant circuit being made with the secondary inductance. Its natural resonant frequency is determined by the values of the secondary inductance and its stray capacitances. The resonant frequency of the primary circuit is deliberately chosen to be the same as the resonant frequency of the secondary circuit so that the secondary is excited by the oscillating magnetic field of the primary.

Energy is gradually transferred from the primary resonant circuit to the secondary resonant circuit. Over several cycles the amplitude of the primary oscillation decreases and the amplitude of the secondary oscillation increases. The decay of the primary oscillation is called “Primary Ringdown” and the start of the secondary oscillation is called “Secondary Ringup”. When the secondary voltage becomes high enough, the Toroid is unable to prevent breakout, and sparks are formed as the surrounding air breaks down.

Each time energy is transferred from one resonant circuit to the other, some energy is lost in either the primary spark gap, RF radiation or due to the formation of sparks from the secondary. This means that the overall level of energy in the Tesla Coil system decays with time. Therefore both the primary and secondary amplitudes would eventually decay to zero.

This schematic diagram represents the Tesla coil distilled to its essence. A cursory examination will reveal that it consists of a spark-excited LC tank circuit magnetically coupled to an un-terminated resonator. L1 and L2 form a simple resonant, air-core transformer. High voltage current from any convenient supply is applied in parallel with the spark gap SG1. When the charge on capacitor C1 reaches the breakdown potential of SG1, the spark gap arcs violently, switching immense current through L1 at astonishing speed. (Spark gaps are still among the fastest electrical switches known to science).

With an electric arc established across SG1, the current surges rapidly between L1 and C1 at a fixed frequency determined by their respective values. The resulting intense magnetic field surrounding L1 couples some of the tank circuit energy to L2, exciting it to oscillation as well. The powerful oscillations developed in L2 cause very high potentials to develop at the un-terminated end, resulting in the formation of lightning-like electrical discharges into the air as well as to any nearby objects. At some point, power transfer to L2 steals so much energy from the tank circuit that the arc across SG1 can no longer be sustained. The extinguishing of the arc is called quenching, and marks the end of a charge/discharge cycle. This cycle can be a single pulse, or a series of charge/discharge cycles up to as many as several hundred per second, depending on the current available from the high voltage supply. If the train of charge/discharge cycles is fast enough, the Tesla coil appears to the eye to produce a steady output of electrical discharge, although it is actually a pulsed device.