"We are whirling through endless space, with an inconceivable speed, all around us everything is spinning, everything is moving, everywhere there is energy. There must be some way of availing ourselves of this energy more directly. Then, with the light obtained from the medium, with the power derived from it, with every form of energy obtained without effort, from the store forever inexhaustible, humanity will advance with giant strides. The mere contemplation of these magnificent possibilities expands our minds, strengthens our hopes and fills our hearts with supreme delight."Nikola Tesla, 1891Tesla CoilsNikola Tesla (1856-1943) is still surrounded by controversy and mystery fifty years after his death. He is perhaps the most controversial and mysterious inventor of the technical age, and is also a cult icon of alternative science followers. A Serbian immigrant, he arrived in America a pauper, earned fame and fortune as an eccentric but flamboyant genius, and died in poverty. Nikola Tesla's patents still today are the cores of our electrical power distribution and use. Royalties from his many inventions founded empires of industry and technology for those shrewd enough to have obtained them cheaply, such as Morgan, Westinghouse and Edison. The celebrated Tesla Coil even to this day presents the most awesome electrical display in the world. Cult followers believe Tesla was centuries ahead of today's technology, inventing antimatter, and antigravity. No matter whom you ask, they will tell you Tesla was eccentric and brilliant. In fact, his patents founded empires of industry and technology.Tesla coils are an air-core, resonant transformers. It is basically a high frequency RF oscillator. The Tesla coil formed the basis of all radio transmitters before to 1915. These spark gap transmitters were banned from use by the Communications Act of 1934. They work at high frequencies, and generate tremendous voltages with spectacular lightning-like discharges. The most popular use for them has been in the movies, where they are used whenever lightning or electrical arcs are required. In fact Nikola Tesla built his famous Tesla Coil in Colorado over 100 years ago. The only thing that is a problem is the lack of solid information about them.How They WorkAs I mentioned, Tesla coils are resonant transformers. This means there is a specific frequency that they operate at - the resonant frequency. There is no special universal Tesla coil frequency, you either target a frequency in the design, or tune a coil into whatever frequency it happens to be happy with.What determines this resonant frequency is the secondary coil - a complex LCR network. The inductive (L) component is the physical coil itself, and is based upon the number of turns, the diameter and length of the coil. The capacitive (C) component is comprised of several isotropic values; the surface of the secondary wire and the terminal electrode. (Isotropic capacitance is virtual capacitance, there is a capacitive effect even though it appears like there isn't any physical plates to create the capacitor.) The resistive (R) component consists of the wire itself, and identifies the physical resistance of the secondary coil at the resonant frequency.To get the secondary to resonate, pulses of energy have to be fed at just the right rate and frequency. A good analogy is a bell. To get the bell to ring, you need to tap it with a hammer. If you tap too hard, you can crack the bell. If you tap and hold the hammer on the bell too long, you don't get a clean, pure tone out of the bell.Energy pulses come from the primary circuit. This circuit is made up of the high-voltage transformer, the primary capacitor, the spark gap, and the primary coil. Together, these parts form a crude type of oscillator. 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 seesaw 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 most Tesla 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 dissipates into the air in the form of electrical sparks.Whew, that was a lot to explain. Though it seems like there is way too much to know in order to build a Tesla coil, it is surprisingly made of a few simple pieces.The Components of a Tesla CoilA Tesla coil has 6 basic components. The first is the primary transformer, which is a high-voltage iron-core transformer. The second is the tank capacitor, which is a high-voltage capacitor that is usually homemade, but can be purchased for a high price from commercial suppliers. The third is the spark gap, basically two wires separated by a small gap of air. The fourth is the primary coil consisting of about 10 to 15 turns of thick heavy gauge wire wound around the base of the secondary coil. The fifth is the secondary coil, and it consists of many hundreds of turns of relatively thin, small gauge enameled wire. The primary and secondary coils make up an air-core transformer. That means that there is no iron core inside of the coils. The sixth basic component is the toroid. It is usually an aluminum doughnut-shaped object, and placed on top of the secondary coil. The high-voltage sparks radiate in all directions from the toroid out into the air.How the Components OperateThe primary transformer converts the AC line voltage (120/240 volts AC) to over 10,000 volts. This energy is used to charge the high-voltage capacitor. The capacitor is wired in series with primary coil to the output of the transformer. When the voltage builds up enough to break down the spark gap, a spark will bridge the spark gap and complete the circuit between the capacitor and the primary coil by shorting out the transformer. A spark ionizes the air, causing it to become much more conductive. All the energy stored in the capacitor will be forced through the primary coil. The process of charging the capacitor and firing the spark gap occurs very rapidly. The spark gap may fire from 120 to over 1000 hertz, but 120-hertz delivers the largest energy bursts. When the energy is transferred to the primary coil, an electromagnetic field is generated and surrounds the secondary coil. The secondary coil absorbs this energy and magnifies the voltage further. The resulting voltage could be a couple hundred thousand volts for small coils, or millions for very large ones. It is very important to efficiently ground the coil. The energy released comes in the form of large arcs, much like lightning.Quite similar to the Tesla coil are the Van de Graaff generators. There is no denying the similarity in appearance of Van de Graaff generators and Tesla coils. Both produce high voltage electric arcs. The differences from there are tremendous.Van de Graaff generators are electrostatic devices. A potential is gradually built up through friction from a rubber belt and collects on the accumulator until it discharges to a grounded object, i.e., a ground rod or you. The rate of discharge is inversely proportional to the distance the arc has to jump. The spark is thin and makes a mild snap when it flies. It carries almost no current and is harmless (unless it's twenty feet high). Typical generators might produce 200,000 to 400,000 volts depending on the relative humidity. They are safe to touch while operating.The Tesla coil is a dynamic device. High voltage radio-frequency current is produced at the rate the spark gap fires. 120 times a second would be typical. Maximum arcs are produced in profusion immediately and continue for as long as the coil is left running. The sparks while barely visible in strong light, appear quite strong in low light conditions. The arcs are purplish to whitish depending on whether they are allowed to go to the open air or are drawn to a nearby grounded rod or wire. A ground wire held close produces very intense sparks.Tesla coils are not quiet, humming generators like Van de Graaff machines. The spark gap is extraordinarily powerful and sounds like an unmuffled chain saw. Hearing protection must always be used when operating one. The gap must also be shielded from the eyes. Not to mention the ozone build-up near the gap generator and the toroid.Tesla coils are not harmless toys. One must constantly be watchful of what is touched. It is said that the RF current from the coil flows over the surface of the skin, and is therefore not capable of killing. However, it is also known that contacting a high-powered discharge can cause burns and is extremely painful. The primary circuit of the coil is where the true danger lies; plenty of voltage and amperage there. An inadvertent touch of a fully charged primary capacitor, even with the coil off, can kill you. Primary voltages run up to 15,000 volts, but the discharger puts out anywhere from a quarter-million to a couple million volts depending on the input power.In short, Nikola Tesla's magnificent "toy" has given us something exciting to watch, teaming with raw power. It helps us open our minds to something that is difficult to comprehend but shows endless possibilities for what might be possible in the future.
