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 Free terminal and circuit of large surface with supporting structure and generating apparatus A Tesla coil is an air-core, dual-tuned resonant transformer that generates very high high-voltages at radio frequencies. It is named after its inventor, Nikola Tesla. When Tesla patented the device (US1119732 - Apparatus for Transmitting Electrical Energy), he called it a self-regenerative resonant transformer. However, this term is no longer in conventional use. The coil achieves a great gain in voltage by loosely coupling two resonant LC circuits together, using an air-core (ironless) transformer. Unlike a conventional transformer, whose gain is limited to the ratio of the numbers of turns in the windings, a Tesla Coil's voltage gain is proportional to the square root of the ratio of secondary and primary inductances. The Tesla coil's outer conducting surfaces, which are charged to a high potential, have large radii of curvature to minimise leakage of the oscillating charges through corona discharges or sparks. Although modern Tesla Coils are designed to generate long sparks, Tesla's large systems were originally designed for wireless communication or power transmission, so he used these techniques to prevent corona and streamer losses.
DescriptionThe intensity of the voltage gain of the circuit with a free, or elevated, toroid is proportional to the quantity of charge displaced, which is determined by the product of the capacitance of the circuit, the voltage (which Tesla called "pressure"), and the frequency of the currents employed. Tesla coils are composed of two coupled resonant circuits: the primary tank circuit, which is a series LC circuit composed of a high voltage capacitor, spark gap, and primary coil; and the secondary LC circuit, a series resonant circuit, which is the secondary coil and the toroid. The toroid is one terminal of a capacitor, the other terminal being the Earth (or "ground"). The primary LC circuit is "tuned" so that it will resonate at the same frequency as the secondary LC circuit. The primary and secondary coils are magnetically coupled, creating a dual-tuned resonant air-core transformer. The terminal consists of a metallic frame, sometimes a circular ring covered with smooth half spherical metal plates (constituting a very large conducting surface). The terminal has relatively small capacitance, charged to as high a voltage as practicable. The outer surface of the elevated conductor is where the electrical charge chiefly accumulates. It has a large radius of curvature, or is composed of separate elements which, irrespective of their own radii of curvature, are arranged close to each other so that the outside ideal surface enveloping them has a large radius. The frame is carried by a strong platform and rests on insulating supports. The circuit consists of a coil in close inductive relation with a primary, and one end of which is connected to a ground-plate, while its other end is led through a separate self-induction coil (whose connection should always be made at, or near, the center in order to secure a symmetrical distribution of the current) and a metallic cylinder to the terminal. The primary coil may be excited by any desired source, such as a high frequency alternator or a capacitor discharge. The important requirement is that a resonant condition be established. The conductor of the shaft to the terminal is in the form of a cylinder with smooth surface of a radius much larger than that of the spherical metal plates, and widens out at the bottom into a hood (which is slotted to avoid loss by eddy currents and for safety). The secondary coil is wound on a drum of insulating material, with its turns close together. When the effect of the small radius of curvature of the wire itself is overcome, the lower secondary coil behaves as a conductor of large radius of curvature, corresponding to that of the drum (this effect is applicable elsewhere). The lower end of the upper secondary coil, if desired, may be extended up to the terminal and should be somewhat below the uppermost turn of the primary coil. This lessens the tendency of the charge to break out from the wire connecting both and to pass along the support. Utilization and production This circuit is designed to be driven with alternating current. A large Tesla coil can operate at very high peak power levels, up to many megawatts. It should therefore be adjusted and operated carefully, not only for efficiency, but also for safety. If, due to improper tuning, the maximum voltage point occurs below the terminal, along the secondary coil, a discharge (spark), or possibly a ball of plasma, may break out and damage or destroy the coil wire, supports, or other nearby objects. It is advisable to begin the tuning (i.e. setting the primary coil's resonant frequency to the same value of the secondary coil's) under low-power, increasing the power gradually, until the apparatus has been brought under perfect control. While tuning, a small projection (called a "breakout bump") may be added to the top terminal in order to stimulate corona and spark discharges (sometimes called streamers) into the surrounding air. Tuning is then adjusted so as to get the longest streamers at a given power level. For a variety of technical reasons, toroids provide the best overall shape for top terminals of modern Tesla Coils. Since Tesla Coils can produce currents or discharges of very high frequency and voltage, they are useful for various purposes including classroom demonstration, theater and movie special-effects, and product/technology safety testing. In typical operation, long, branching high-voltage sparks may strike out in all directions from the toroid into the air, producing a dangerous, yet strangely beautiful, lightning-like display of electricity "in action". This device was an early predecessor (along with the induction coil) to a more modern device called a flyback transformer, which provides the voltage needed to power the cathode ray tube used in televisions and computer monitors. ("CRT" displays are now beginning to become obsolete due to the technological advancement and proliferation of liquid crystal displays and related technologies.) A modern variant of the Tesla coil is also used to power plasma globe sculptures and similar devices. Theoretically, a variation of the Tesla coil could utilize the phantom loop effect to form a circuit to induct energy from the earth's magnetic field. This concept is part of Tesla's wireless transmission of electric power distribution system (US1119732 - Apparatus for Transmitting Electrical Energy - 1902 January 18). This type of coil may have been used as the basis for the Wardenclyffe Tower project. Why do Tesla Coil Sparks jump to open air?While generating discharges, electrical energy from the secondary and toroid is transferred to the surrounding air as electrical charge, heat, light, and sound. The electric currents that flow through these discharges are actually due to the rapid shifting of quantities of charge from one place (the top terminal) to other places (nearby regions of air). The process is similar to charging or discharging a capacitor. The current that arise from shifting charges within a capacitor is called a displacement current. Tesla Coil discharges are formed as a result of displacement currents as pulses of electrical charge are rapidly transferred between the high voltage toroid and nearby regions within the air (called space charge regions). Although the space charge regions around the toroid are invisible, they play a profound role in the appearance and location of Tesla Coil discharges. Refer to the schematic above. When the spark gap fires, the charged capacitor discharges into the primary winding, causing the primary circuit to oscillate. The oscillating primary current creates a magnetic field that couples to the secondary winding, transferring energy into the secondary side of the transformer and causing it to oscillate with the toroid capacitance. The energy transfer occurs over a number of cycles, and most of the energy that was originally in the primary side is transferred into the secondary side. The greater the magnetic coupling between windings, the shorter the time required to complete the energy transfer. As energy builds within the oscillating secondary circuit, the amplitude of the toroid's RF voltage rapidly increases, and the air surrounding toroid begins to undergo dielectric breakdown, forming a corona discharge. As the secondary's energy (and output voltage) continue to increase, larger pulses of displacement current further ionize and heat the air at the point of initial breakdown. This forms a very conductive "root" of hotter plasma, called a leader, that projects outward from the toroid. The plasma within the leader is considerably hotter than a corona discharge, and is considerably more conductive. In fact, it has properties that are similar to an electric arc. The leader tapers and branches into thousands of thinner, cooler, hairlike discharges (called streamers). The streamers look like a bluish "haze" at the ends of the more luminous leaders, and it's the streamers that actually transfer charge between the leaders and toroid to nearby space charge regions. The displacement currents from countless streamers all feed into the leader, helping to keep it hot and electrically conductive. In a spark gap Tesla Coil the primary-to-secondary energy transfer process happens repetitively at typical pulsing rates of 50 - 500 times/second, and previously formed leader channels don't get a chance to fully cool down between pulses. So, on successive pulses, newer discharges can build upon the hot pathways left by their predecessors. This causes incremental growth of the leader from one pulse to the next, lengthenning the entire discharge on each successive pulse. Repetitive pulsing causes the discharges to grow until the average energy that's available from the Tesla Coil during each pulse balances the average energy being lost in the discharges (mostly as heat). At this point, dynamic equilibrium is reached, and the discharges have reached their maximum length for the Tesla Coil's output power level. The unique combination of a rising high voltage Radio Frequency envelope and repetitive pulsing seem to be ideally suited to creating long, branching discharges that are considerably longer than would otherwise be expected by output voltage considerations alone. However, even 100 years later, there are many aspects of Tesla Coil discharges and the energy transfer process that are still not completely understood. The Skin Effect MythThe dangers of high frequency electrical current are sometimes perceived as being less than at lower frequencies. This is often, but mistakenly, interpreted as being due to skin effect, which tends to exclude penetration into conducting media. Although skin effect is effective for good electrical conductors (i.e., metals), skin depth of human flesh at typical Tesla Coil frequencies is still of the order of 60 inches or more. This means that most high frequency currents will still preferentially flow through the deeper, better conducting, portions of an experimenter's body such as the circulatory and nervous systems. In reality, a human being's nervous system does not directly sense the flow of potentially dangerous currents above 15-20 kHz. And, since the body no longer provides a warning "shock", novices may touch the output streamers of small Tesla Coils without feeling painful shocks. However, there is anecdotal evidence among Tesla Coil experimenters that temporary tissue damage may still occur as muscle, joint pain, or tingling hours afterwards. This is believed to be caused by caused by the effects of internal current flow, and is especially common with continuous wave (CW) solid state or vacuum tube type Tesla Coils. Large Tesla Coils and Magnifiers can deliver dangerous levels of high frequency current, and develop significantly higher voltages (often 250,000-500,000 volts, or more). Because of the higher voltages, large systems can deliver higher energy, potentially lethal, high voltage capacitor discharges from their top terminals. Doubling the output voltage quadruples the electrostatic energy stored in the same top terminal. If an unwary experimenter accidentally places himself in path of a high voltage discharge to ground, the resulting high current electric shock can cause involuntary spasms of major muscle groups, and even induce life-threatening ventricular fibrillation and cardiac arrest. Even lower power vacuum tube or solid state Tesla Coils can deliver RF currents that are capable of causing temporary internal tissue, nerve, or joint damage through Joule heating. In addition, an RF arc can carbonize flesh, causing a painful and dangerous bone-deep RF burn that make take months to heal. Because of these risks, knowledgable experimenters avoid contact with streamers from all but the smallest systems. Professionals usually use other means of protection such as a Faraday Cage or a chainmail suit to prevent dangerous currents from entering their body. InstancesMissing image Operating_tesla_coil.jpg This is a small tesla coil in operation. The Colorado Springs laboratory of Tesla possessed one of the largest Tesla Coil ever built, known as the "Magnifying Transmitter". The Magnifying Transmitter is somewhat different than classic 2-coil Tesla Coils. A Magnifier uses a 2-coil "driver" to excite the base of a third coil ("Resonator") that's located some distance from the driver. The operating principles of both systems are similar. The world's largest currently existing 2-coil Tesla coil was made by Greg Leyh. It is a 130,000 watt Tesla coil, part of a 38 foot tall sculpture. It currently resides at a farm near Auckland, New Zealand. [1] (http://www.lod.org/Projects/electrum/) PopularityTesla coils are very popular devices among certain electrical engineers and electronics enthusiasts. Someone who builds Tesla coils as a hobby is called a "Tesla coiler", or simply "coiler". There are even "coiling" conventions where people attend with their home made Tesla coils and other electrical devices of interest. Low power Tesla Coils are also sometimes used as a high voltage source for Kirlian photography. [2] (http://www.cebunet.com/kirlian/) It should be noted that there are rather significant safety issues regarding coil assembly and operation by hobbyists (including professional engineers). [3] (http://www.pupman.com/safety.htm) See alsoExternal linksPatents
Other Information
de:Tesla-Transformator ja:テスラコイル pl:Transformator Tesli
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