The Tesla Coil

Nikola Tesla 

"If you want to find the secrets of the universe, think in terms of Energy, Frequency & Vibration" - Nikola Tesla

To investigate the electrical realm of high frequency and high voltage, Tesla invented an apparatus that pushed the limits of electrical understanding. None of the circuits typical components were unknown at that time, but its design and operation together achieved unique results - not the least because of Tesla's masterful refinements in construction of key elements, most particularly of a special transformer, or coil, which is at the heart of the circuit's performance.

Such a device first appeared in Nikola Tesla's US Patent No: 454,622 (1891), for use in new, more efficient lighting systems.

A Tesla Coil is an Electrical Resonant Transformer Circuit. It is used to produce High-Voltage, Low-Current, High-Frequency Alternating Current electricity.

(Tesla experimented with a number of different configurations consisting of two or sometimes three coupled resonant electric circuits).

Tesla used these circuits to conduct innovative experiments like:

• Electrical Lighting
• Phosphorescene
• X-Ray Generation
• High Frequency Alternating Current Phenomenon
• Electro-Therapy
  
• Transmission of Electrical Energy without wires

How it works?

The power source is hooked up to the primary coil. The primary coil's capacitor acts like a sponge and soaks up the charge. The primary coil itself must be able to withstand to massive charge and huge surges of current, so the coil is usually made out of copper, a good conductor of electricity. Eventually, the capacitor builds up so much charge that it breaks down the air resistance in the spark gap. Then, similar to squeezing out a soaked sponge, the current flows out of the capacitor down the primary coil and creates a magnetic field.

The massive amount of energy makes the magnetic field collapse quickly, and generates an electric current in the secondary coil. The voltage zipping through the air between between the two coils creates sparks in the spark gap. The energy sloshes back and forth between the two coils several hundred times per second, and the charge building up in the secondary capacitor gets so high that it breaks free in a spectacular burst of electric current.

The resulting High-Frequency Voltage can illuminate fluorescent bulbs several feet away with no electrical wire connection.

The Operation:

A Tesla Coil is a Radio Frequency Oscillator that drives an air-core Double-Tuned Resonant Transformer to produce high voltages at low currents. Tesla's original circuits as well as most modern coils use a simple spark gap to excite oscillations in the tuned transformer. More sophisticated designs use transistor or thyristor switches or vacuum tube electronic oscillators to drive the resonant 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 50kHz to 1 MHz. Although some oscillator driven coils generate a continuous alternating current, most Tesla coils have a pulsed output; the high voltage consists of a rapid string of pulses of radio frequency alternating current.

The common spark excited Tesla coil circuit consists of the following components:

• A high voltage supply transformer: to step the AC mains voltage up to a high enough voltage to jump the spark gap. Typical voltages are between 5 to 30 kilovolts.
• A capacitor: that forms a tuned circuit with the primary winding of the Tesla transformer.
• A spark gap: that acts as a switch in the primary circuit.
• An Air Core Double Tuned Resonant Transformer: which generates the high output voltage.
• A Capacitive Electrode: in the form of 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.

Note: A Torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis that is coplanar with the surface. For Example: a ring doughnut.

Q-Factor or Quality Factor is a dimensionless unit for the losses of a coil, quartz or resonator. For coils this refers to the ohmic losses of the coil wire. They are inversely proportional to the coil quality and are calculated from the ratio of reactance to ohmic resistance.

Current, Magnetic Fields and Induction: One of the Maxwell's equations and Ampere's Law tells us that the current flowing through a wire creates a magnetic field around it. If we want to use this magnetic field to our advantage (as we do in an electromagnet), we coil the wire. The magnetic fields from the individual turns add together in the center.

A constant current makes a static magnetic field. What happens when we put a changing current through wire? - Another of Maxwell's equations, Faraday's Law of Induction, tells us that a magnetic field changing in time induces a voltage across the wire proportional to the rate of change of the magnetic field.

If the current is abruptly shut off, Faraday's Laws tells us that there will be a sharp spike of voltage. If an oscillating current flows through the coil, it induces an oscillating field inside it. This, in turn, induces a voltage across the coil which tends to oppose the driving current.

Resonant Circuits: A Resonant Circuit is like a tuning fork. It has very strong amplitude response at one particular frequency, called the resonant or natural frequency. A resonant circuit achieves the highest voltages when driven at its natural frequency, which is determined by the value of its components.

Resonant Circuits use Capacitors and Inductors, and therefore are also known as LC circuits. They are also known as 'Tank Circuits', because of the energy storage elements present.

Resonant Transformer: The specialized transformer used in the Tesla coil circuit is called a Resonant Transformer or Radio-Frequency (RF) Transformer. It functions differently from an ordinary transformer used in AC power circuits. While an ordinary transformer is designed to transfer energy efficiently from primary to secondary winding, the resonant transformer is designed to temporarily store electrical energy. Each winding has a capacitance across it and functions as an LC circuit, storing oscillating electrical energy, analogously to the way tuning fork stores vibrational mechanical energy. The primary coil consisting of a relatively few turns of heavy copper wire or tubing, is connected to a capacitor through the spark gap. The secondary coil consists of many turns of fine wire on a hollow cylindrical form inside the primary. The secondary is not connected to an actual capacitor but it also functions as an LC circuit, the inductance resonates with stray capacitance, the sum of the stray parasitic capacitance between the windings of the coil, and the capacitance of the toroidal metal electrode attached to the high voltage terminal. The primary and secondary circuits are tuned so that they have the same resonant frequency, so they exchange energy, acting like a coupled oscillator, during each spark the stored energy rapidly oscillates back and forth between the primary and secondary.

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 for 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 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 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.

Cycle of Operation:

1. Current from the supply transformer charges the capacitor to a high value.
2. When the voltage across the capacitor reaches the breakdown voltage of the spark gap, a spark takes place, reducing the spark gap resistance to a very low value. This completes the primary circuit and current from the capacitor flows through the primary coil. The current flows rapidly back and forth between the plates of the capacitor through the coil, generating radio frequency oscillating current in the primary circuit at the circuit's resonant frequency.
3. The oscillating magnetic field of the primary winding induces an oscillating current in the secondary winding, by Faraday's Law of Electromagnetic Induction. Over a number of cycles, the energy in the primary circuit is transferred to the secondary. The total energy in the tuned circuits is limited to the energy originally stored in the capacitor, so as the oscillating voltage in the secondary increases in amplitude the oscillations in the primary decreases to zero. Current flows rapidly back and forth through the secondary coil between its ends.
4. The secondary current creates a magnetic field that induces voltage back in the primary coil, and over a number of additional cycles the energy is transferred back to primary, causing the oscillating voltage in the secondary to decrease. This process repeats, the energy shifting rapidly back and forth between the primary and secondary tuned circuits. The oscillating currents in the primary and secondary gradually die out due to energy dissipated as heat in the spark gap and resistance of the coil.
5. When the current through the spark gap no longer sufficient to keep the air in the gap ionized, the spark stops ('quenches'), terminating the current in the primary circuit. 
6. The current from the supply transformer begins charging the capacitor again and the cycle repeats.

Health Issues regarding exposure:

Tesla Coil used for entertainment purposes with specialized suits

The high voltage radio frequency discharges from the output terminal of a Tesla coil pose a unique hazard not found in other high voltage equipment; when passed through the body they often do not cause the painful sensation and muscle contraction of electric shock, as lower frequency AC and DC do. The nervous system is insensitive to currents with frequencies over 10 - 20 kHz. The reason for this is that a certain minimum number of ions must be driven across a nerve's cell's membrane by the imposed voltage to trigger the nerve cell to depolarize and transmit an impulse. At radio frequencies, there is insufficient time during a half cycle for enough ions to cross the membrane the alternating voltage reverses. The danger is that since no pain is felt, experimenters often assume the currents are harmless. Teachers and hobbyists demonstrating small Tesla coils often impress their audience by touching the high voltage terminal or allowing the streamer arcs to pass through their body.

If the arcs from the high voltage terminal strikes the bare skin, they can cause deep seated burns called RF burns. This is often avoided by allowing the arcs to strike a piece of metal held in the hand, or a thimble on a finger, instead. The current passes from the metal into the person's hand through a wide enough surface area to avoid causing burns. Often no sensation is felt, or just a warmth or tingling.

However this does not mean that the current is harmless. Even a small tesla coil produces many times the electrical energy necessary to stop the heart, if the frequency happens to be low enough to cause ventricular fibrillation. A minor mis-adjustment of the coil could result in electrocution.

This sole idea of the Tesla coil was used by Nikola Tesla to develop the idea about the transmission of wireless electricity. He even constructed the Wardenclyffe Tower, an experimental wireless electricity transmission station during the period of 1901-1902, located in the village of Shoreham, New York.

But more about Nikola Tesla in the next blog.

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- Ron

Electrical Engineer & Technology Enthusiast

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