What is a Tesla coil? ---------------------------- A Tesla coil is a high voltage, high frequency resonant air core transformer. It was developed by the Serbian born inventor Nikola Tesla in the 1880s. It was originally created to serve as a transmitter for the wireless transmission of electrical power. The idea of wireless power transmission never caught on mainly due to lack of funds. The true Tesla coil had three coils, a primary, a secondary, and an extra coil which is called the Tesla magnifier. Today Tesla coils serve little practical purpose. There are a few practical purposes for them however. They are used to simulate lightning strikes to test aircraft hulls, and power distribution equipment. They are also used in the film industry wherever good lightning like discharges are needed. An example of this would be in the movie "Terminator 2". Most coils built today are 1/4 wave coils which do not make use of the Tesla magnifier coil. Tesla coils are fairly easy to build with readily available components. The normal hobby Tesla coil setup consists of a high voltage step-up transformer which has a secondary voltage of 5000-15000 volts AC. Neon sign and oil burner ignition transformers are commonly used. Almost all of the other components are made by the builder. The theory of operation of a Tesla coil is not too difficult to understand. The most basic Tesla coil system consists of a high voltage step- up transformer, sparkgap, capacitor, primary, secondary, and output terminal. First the capacitor is charged until it the air around the sparkgap breaks down and forms a conducting channel. The capacitor discharges rapidly into the primary coil and a magnetic flux is built up around the primary coil. The flux then collapses and charges up the capacitor again. The capacitor then discharges into the primary again. This forms what's called an LC (L stands for inductance and C stands for capacitance) oscillator. This cycle goes on until there is not enough power left to jump the gap. The capacitor then is charged by the high voltage transformer and the cycle repeats over and over again. You can think of the primary, sparkgap, and capacitor as a radio transmitter. The secondary can be thought of as a radio receiver. The resonant frequency of the secondary is determined by it's capacitance which is derived from the output terminal, the capacitance of the secondary winding, and the inductance of the secondary. If the resonant frequencies of both the primary, capacitor, and sparkgap circuit (also called a "tank" circuit) match with the resonant frequency of the secondary coil then high voltages will be induced in the secondary due to the effect of VSWR, or the standing waves in the coil. What happens is there is sparking that occurs at the output terminal. Understanding this is not required to build a sucessful Tesla coil, although it does help to know some of the theory. -- Safety concerns The skin effect Legal Concerns -- Types Of Tesla Coils ---------------------------- 1) Classic 1/4 wave monopolar Tesla coils Classic 1/4 wave Tesla coils are what most people think of when they hear "Tesla coil". They usually have a discharge terminal on top of the secondary coil, Primary coil near the base of the secondary coil. They require heavy RF grounding at the base of the secondary coil. 2) 1/2 Wave Bipolar Tesla coils Bipolar Tesla coils are Tesla Coils have 2 discharge terminals. They are connected to each end of the secondary. Discharges eminate from one terminal to the other. The primary coil is placed in the center of the secondary coil. The secondary coil is 1/2 wavelength long. The discharges from each terminal are 180 out of phase, so the discharges attract each other. They do not necessarily require grounding, the secondary coil can be left to work against itself. 3) The magnifier coil is the true Tesla coil. This is what Tesla experimented with in Colorado Spings. It consists of three coils, a primary, a secondary, and an extra coil or "magnifier". The extra coil is not in any way coupled to the primary's field flux. The primary and secondary make up a driver stage. The secondary is not like that of a 1/4 wave Tesla coil. It is normally very large and is wound with heavy wire ofnot many turns. The primary and secondary are coupled very closely. The magnifier is placed at a distance to prevent the primary's field from destructively interfering with the extra coil. The extra coil is base fed with a large conductor of decent surface area. This conductor is connected to the top of the secondary and to the bottom of the extra coil. What this does is allow the extra coil to develop very high voltages since it is allowed to oscillate freely. Magnifiers do not make a good first coil project. 4) The solid-state Tesla coil is driven by a solid-state driver at a frequency in which the secondary is excited at. Solid-state Tesla coils do not require a sparkgap, a high voltage transformer, or a high voltage capacitor. They normally use bipolar transistors or field effect transistors (FETs). They are beyond the scope of this FAQ for now, however. 5) Vacuum tube Tesla coils are very much like the solid-state designs. Since tubes can handle higher powers they make better drivers for large Tesla coils. Tube types commonly used are 805s are 811s. They too, like their solid-state relatives will not be touched upon in this FAQ now anyways. What are common power supplies that are used for Tesla coils? ------------------------------------------------------------- 1. Neon sign transformers The most common power supply for powering a Tesla coil is the neon sign transformer. These step up the 120 VAC from the wall socket to the several thousands of volts necessary for spark gap operation. The most common neon transformer voltage ratings are 6000, 7500, 9000, 12000, and 15000 volts with current ratings of 30 or 60 ma. Occasionally you will find 120ma neon transformers but these are not very common. These transformers are ideal for Tesla coils as they are internally current limited with magnetic shunts. This means that neons can take direct short circuits with no ill effect. Generally speaking the higher the transformer voltage and current rating the more output you can expect from your Tesla coil. The most useful neons for Tesla coils are the 9000 and 12000 volt models. The 15000 volt ones are quite hard on capacitors. For a first coil project 9000 volt neons are recommended. 2. Where to obtain these transformers First thing to do is to get out the yellow pages. Look under signs and take note of all the sign companies in your area. Make special notice of any that advertise sales and service of neon signs. Now take your list and go to these sign companies and ask them for used neon sign transformers. Tell them what you want them for. This will often generate some interest. If you strike out at the first place go to the next one. Expect to pay about $20 for a used 30ma neon and $35 for a 60ma neon. If they want considerably more than this move on to the next place. Sometimes you can get lucky and find a place with a pile of transformers in unknown condition. Negotiate a price and take your chances or see if you can do some testing on the spot. You can check the secondary windings for continuity with an ohmeter. The resistance will vary considerably from transformer to transformer. Just check for opens or short circuits. Or take along an AC line cord and plug the units in. Attempt to draw an arc from each high voltage terminal to the case. Each side should show an equal arc. Careful you dont get shocked. The smaller neons can deliver painful shocks and the larger ones can kill. 3. Unpotting and repair of bad transformers How about getting a Tesla power supply for free? Check out those same sign companies in your area and ask for dead neon transformers. Any place the services signs will have bad transformers. Some places pile them up while others pitch them into the the trash dumpster. Check these places out and ask them to save their bad units for you. What good will dead transformers do? You can unpot these dead units and disassemble the transformers. Often just unpotting them will revive them. Neons transformers are potted in tar. Tar is a cheap and not very effective insulator. Over time the tar dries out. High voltage arcs within the tar block slowly carbonizes some of this tar. Once carbonized it becomes more conductive inviting more arcing and more carbonization. Eventually high voltage secondary winding shorts out. Because of the current limiting in neons the shorted secondary is normally not damaged. So removing the carbonized potting compound completely restores the neons to operation. Other problems like broken wires and connections can also be repaired once unpotted. New insulation like hot glue or silicone RTV is used to insulate the secondary windings from the core. Or the transformer can be put into a container of mineral or transformer oil. 4. Oil burner transformers Oil Burner Ignition Transformers (OBIT) are also useful as Tesla coil power supplies. These transformers are used to ignite the burners on old style oil burning furnaces. They are magnetically shunted like neons so may be short circuited without damage. They are not as rugged as neons not being designed for continous duty operation. However they do serve well. They are most commonly available in 10KV/23 ma and 6kv/23 ma ratings. The 10kv units are what would most interest us. You may find these at heating/airconditioning contractors. Often you may find such places may have a couple old furnaces ready to be scrapped. The transformers can be removed and Sometimes be had for free. Some places may have a couple old transformers on hand. Most heating contractors dont place much value on these used transformers so will usually give them away for the asking. New units can be ordered but are expensive. Before buying a new OBIT consider going to a neon transformer. > Power distribution transformers > Where to obtain these transformers > Typical price range for this transformer > Current limiting > Inductive > Arc welders > Variacs > Resistive > Heating elements > Water tanks > Microwave oven transformers > Where to obtain these transformers > Typical price range for this transformer > Minature Tesla coil power supplies > Flyback transformers > Ignition coils What type of primary is best? > Helical primary > What is a helical primary? > What are they suited for? > Inverted cone primary > What is an inverted cone primary? > What are they suited for? > What degree of rise should I use? > Flat pancake primary > What is a flat pancake primary? > What are they suited for? > How do I determine how many turns will I need? > What spacing should I use? > What type of wire works best? > What should the primary coil forms be made of preferably? > Should I insulate my primary wires? What type of capacitor will I need? > What the capacitor does in the Tesla tank circuit. > Determining capacitor size needed based on transformer characteristics. > Dielectric concerns > Why is polyethylene used in capacitor construction? > Why are glass and ceramic types undesirable? > Voltage ratings > Where to get commercially made capacitors > Saltwater types > What is a saltwater capacitor? > What does the saltwater do? > How are they made? > Materials needed > Flat plate > Rolled plate > Advantages/disadvantages of certain capacitors What is a spark gap? ------------------------------ The spark gap is the heart and soul of a Tesla coil. It is the single most important component and yet one that gets little attention from beginners. More coils have shown disappointing results due to poor gaps than any other single cause. Think of the spark gap as a high speed high power switch. It appears as an open circuit while the main capacitor is being charged by the power supply transformer. Once the cap is fully charged the gap ionizes the air between the electrodes and begins to conduct. Once conducting it becomes a short circuit and allows the cap to dump its charge into the primary coil. Once the cap discharges the gap must quench (go out) and become non-conductive once again so that the cap may begin charging once again. There are several type of gaps and each has its own advantages and disadtanges. 1. Static gaps. Static gaps can take several forms. The most basic is just two electrodes spaced a small distance apart. This can be the heads of two heavy bolts or two brass knobs from the hardware store. This is an easy gap to implement and will serve Ok for the beginner. The spacing between the electrodes depends on the power supply voltage from your transformer. 9kv neons do well with about .150 inch of gap while 12kv neons can use up to .200 inch. 15kv neons can use up to .250 inch. Wider gaps than this should be avoided as they put a great deal of stressn your capacitor and transformer. The single static gap while easy to build is a poor performer. All the heat is dissipated in one area and the gap doesnt want to quench. The hot ions in between the gap electrodes tend to keep the gap lit at lower voltages. Blowing air through the gap with a fan or blower will improve the operation of this type of gap by removing these hot ions. A much better approach is to break up the single large gap into a series of smaller gaps in series. This is called a series static gap. This divides the arc up into smaller parts that develop less heat and operate more efficiently. This type of gap also benefits from a little forced air to remove hot ions. One such gap popularized by Richard Quick and often referred to as the RQ cylinder gap consists of multiple electrodes made from lengths of hard copper tubing. The electrodes are usually made from 3/4 or 1 inch copper tubing cut from 2 to 4 inches long. Larger and longer elctrodes provide more thermal mass and handle the heat better. These electrodes are mounted inside a piece of PVC pipe and spaced about .025-.03 inch apart. 7 such electrodes providing 6 gaps between then are normally used for 9kv neons and 9 electrodes providing 8 gaps are used for 12kv neons. Up to 11 electrodes with 10 gaps can be used for 15kv neons. A small muffin fan is mounted on the end of the PVC pipe holding the electrodes to provide cooling airflow. 2. Air blast gaps Blowing a high speed column of air through the teeth of a static gap greatly improves the operation by stretching and cooling the arc and removing hot ions from between the electrodes. Playing the output of an air compressor between the electrode faces can produce some good effects. This type of gap is loud and requires a constant supply of high speed air to be effective. 3. Vacuum gaps Instead of blowing high speed air through a static gap the vacuum gap sucks air through the gap. Use a shop vac or purchase a used or surplus cannister type vacuum cleaner motor for the purpose. This type of gap is very effective. In fact the vacuum cleaner motor can replace the muffin fan on a RQ style cylinder gap and increase the performance of that type of gap. 4. Quench gap. A special form of static gap is known as the quench gap. In this gap the electrodes are wide flat plates with their faces parallel and spaced very close. The electrodes are spaced from each other with mica rings that enclose the gap area. The gap areas between electrodes become sealed chambers and the oxygen in these chambers is used up quickly when the gap is first put into operation. This type of gap works fairly well at lower voltages than other gaps and provide a nearly continous wave (CW) output. These were used in spark radio transmitters until spark transmitters became obsolete with the invention of the triode vacuum tube in the 1910s. This gap design is difficult to implement due to the sealed nature of the gaps and lack of suitable material for the spacers. The mica rings normally used are difficult to find and expensive 5. Rotary gaps A rotary gap is a motor driven gap that has a solid disk with electrodes mounted around its periphery. These electrodes come into line with stationary electrodes to form the gap. As the disk rotates each set of electrodes comes into play in sucession. With a large number of spinning electrodes and a high rotational speed the number of gap presentations can become quite high. And quenching is quite good with a rotary as the electrodes are in firing position for only a short time. Rotary gaps work well with higher power systems but are unnecessary with small to medium power systems running from neon sign transformers. In fact neons work better with one of the multiple static gap systems and can actually be damaged by a rotary gap. What type of spark gap will I need? ------------------------------------- The type of gap you should use depends a great deal on the kind of transformers you are using and the power level you are operating at. Low power operation of less than 500 watts can often do well with a single static gap using a pair of bolts or brass knobs. Even here though a series string of gaps like the RQ style cylinder gap will improve efficiency. Medium power systems up to 1500 watts using neon sign transformers do very well with a RQ style cylinder gap. Above this power level plenty of forced air should be used for cooling or you can go to a vacuum gap. The vacuum gap works quite well up to 2500 watts. Above this power level rotary gaps become necessary and are often used in series with static gaps. Rotary gaps should generally not be used with neon sign powered coils as the forced break causes a great deal of strain that neons dont handle well. What is Quenching? ----------------------- No discussion of spark gaps is complete without at least a rough definition of "quenching". This term is commonly thrown around when talking about spark gaps. Quenching refers to the ability of an established arc to be extinguished. It is much easier to start a gap firing than it is to put it out once it has fired. In Tesla coils quenching the arc is critical to good coil performance. A cold, non-firing, spark gap is "clean". It contains no plasma or hot ions. On applying voltage to the gap, a tension is esta- blished between the electrode faces. Once the tension becomes high enough to ionize the air between the gap the resistance drops rapidly. This breakdown ionizes the gas between the electrodes and the arc begins to ablate and ionize the metal electrodes themselves. This mixture of ions forms a highly conductive plasma between the gap electrodes. Without this highly conductive channel through the gap, efficient tank circuit oscillation would not be possible. But this plasma also shorts the gap out. A gap choked with hot ions does not want to open and allow the capacitors to recharge for the next pulse. The gap is "dirty" with hot ionized gases and must be cleaned so it can be extinguished. To quench the hot gap the built up ions must be removed. This can be done with a flow of air from a fan, blower or vacuum cleaner motor. Once the excess ions are removed and the gap is cooled by the airflow it can quench much easier. Anything that reduces the amount of heat in the gap will aid quenching. Series static gaps reduce the heat in each gap by breaking up the gap in smaller segments. Quenching becomes easier. What is the secondary coil for? > Aspect ratios > Determining needed turns for a specific frequency > Commonly used form materials > Form preparation > Winding > Ground contacts > Can I feed my wire throught holes? > How should I plug the ends? > Why do the ends have to be plugged? > Instead of using epoxy to glue the end plugs in, Can I use silicone sealant? What is the purpose of the discharge terminal? > How do I connect the discharge terminal to the secondary? > What is the advangtage of a toroid over a sphere? > What does adding a terminal do to the secondary's resonant frequency and why? > How is a discharge terminal constructed? > Materials needed Power supply protection circuits and devices. > Bypass caps > What is their purpose? > Do I need them on my coil? > Safety gaps > What do they do? > How does it work? > Do I need it on my coil? > RF chokes > What do they do? > How do they work? What type grounding do I need for my coil? > Why is grounding is needed? > Do I really need a good ground for my bipolar Tesla coil? > What type of ground strap is preferable? > Can I use my house ground? Tuning a Tesla Coil > How do you tune a tesla coil? > What is the purpose of tuning a tesla coil? > What is the best method of tuning? > My alligator clip is heating up when I use it as a tap, What should I do? How can I help eliminate RFI/EMI? > Line filters > Grounding > Faraday Cages Tesla coil schematic: _______ (_______) OT T1 RFC1 | ||(-----+--------******---+---------+ ( ||( | | | ( +---+ ||( o | | ( <---| |------)||( o SG1 | | ( | | )||( | o | L1 ( 120VAC |LF1| )||(-----+ | ) ( | | )||( | o MG +-->) ( L2 <---| |------)||( o | ) ( +---+ ||( o SG2 | ) ( | ||( | RFC2 | | | C1 ) ( | ||(-----+--------******---+---| |------) ( --+-- | | | | --- --+-- --+-- - --- --- - - LF1 is a commercial line filter wired in reverse. Input (120VAC) is connected to the "load" side and T1 is connected to the "line" side. The case is grounded to the normal electrical ground. This helps keep RF energy out of the house electrical system. T1 is a shunted transformer. Neon sign transformers and oil burner ignition fit this description. The core is grounded to the dedicated RF ground. SG1/SG2 are safety gaps to protect T1 from RF "kickbacks". They must fire occasionally for them to be effective. Their spacing is slightly wider than the main spark gap (MG). RFC1/RFC2 are RF chokes. They should be in the vincinity of 5-10mH each. MG is the main spark gap. It should consist of multiple gaps made of a heavy material such as hard copper water pipe. The number of gaps required is determined by your power supply. C1 is the high voltage, pulse discharge rated tank capacitor. It should be a good match with your transformer for best results. L1 is the Tesla primary coil. It should be constructed of a heavy material. Soft copper tubing is typically used, however heavy litz wire or stranded wire without a twist in it can be used. It should have a large surface area to minimize the skin effect. It is typically around 10-15 turns and has an adjustable tap for tuning. L2 is the Tesla secondary coil. It is normally around 500-1000 turns of enameled magnet wire closewound on a sealed PVC or low loss coil form. OT is the ouput terminal. Toroid (torus shaped) terminals should be used, but a sphere will also work, but not as well. Formulas ------------------------------------- Ohm's law ------------------------------------- E = I * R I = E / R R = E / I E = voltage (volts) I = current (amperes) R = resistance (ohms) Finding a transformer's impedance ------------------------------------- This formula is useful for finding the input transformer's impedance so a capacitor can be matched with it. Z = E / I Z = impedance in ohms E = secondary voltage output I = secondary current ouput in amps (divide milliamps by 1000 to get amps) Matching capacitor size to transformer -------------------------------------- 1 C = ------------------- 2 x pi x Z x .00006 C = capacitance in microfarads needed for primary capacitor. Z = Transformer Impedence pi = 3.141592654 Note: The .00006 is the 60 Hz AC, if you live outside the US then substitute your cycle rate. Watt's law ------------------------------------- This formula is useful to find a transformer's power output based on it's output voltage and current or to find other values such as current or voltage output based on it's power rating. P = E * I E = P / I I = P / E P = power in watts or volt-amperes (VA) E = voltage in volts I = current in amps Inductive reactance ------------------------------------- This formula finds the reactance (AC impedance) of a given inductor at a given frequency. X(l) = 2 * pi * f * L X(l) = AC impedance (reactance) in ohms pi = 3.1415 f = frequency in hertz L = inductance in henries (divide by 1,000,000 to convert microhenries to henries) Capacitive reactance ------------------------------------- This formula is the same as the inductive reactance formula except for one point, the value is inverted. It determines the reactance (AC impedance) of a capacitor at a given frequency. 1 X(c) = -------------- 2 * pi * f * C X(c) = AC impedance (reactance) in ohms pi = 3.1415 f = frequency in hertz C = capacitance in farads (divide by 1,000,000 to convert microfarads to Farads) Frequecy of LC circuit ------------------------------------- This formula determines the resonant frequency of an LC tank circuit. 1 -------------- ______________ f = \/2 * pi * L * C f = frequency in hertz pi = 3.1415 L = inductance in henries (divide by 1,000,000 to convert microhenries to henries) C = capacitance in farads (divide by 1,000,000 to convert microfarads to farads) Q (quality) of an inductor ------------------------------------- This finds the quality (how good it is) of an inductor. It's based on it's resistance and it's reactance. Q = X(l) / R Q = quality (there is no unit for quality) X(l) = inductor's reactance in ohms R = the inductor's resistance in ohms