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INTRODUCING: THE TESLA COIL

   A Tesla coil, in simple terms, is a high voltage resonant transformer. While there are several different types, this site will be focusing on the spark gap type. In this design, a high voltage transformer charges a capacitor bank, which discharges across a spark gap when fully charged, sending a burst of energy through a small, primary coil. This creates a magnetic field around the primary, which quickly breaks down when the capacitor is empty. The decomposing magnetic field induces a current back into the primary, which recharges the capacitor. The rate at which the primary coil and capacitor recharge each other is known as the resonant frequency. If the resonant frequency of the primary circuit matches the frequency that the larger secondary coil naturally resonates at, the primary coil's magnetic field induces a voltage in the secondary coil. The voltage  continues to climb until it breaks out of the topload (a metal sphere or toroid atop the secondary coil) in the form of lightning. However, the primary coil doesn't transfer all of its energy back to the capacitor, so the current induced back into the coil gradually (within a few microseconds) dies down. When this happens, the spark gap's discharge dissipates (quenches), and the transformer recharges the capacitor until it can jump the gap again and cause another burst of resonating (or oscillating) current. Please note that the gap will be firing hundreds of times per second, so you won't actually see the spark gap quenching and firing. Also note that Tesla Coils are among the more condition-sensitive electrical circuits, and things like connecting the secondary coil bottom wire to the ground or the topload size make all the difference in performance. To help in the design process, I suggest using a Tesla Coil designer program/app (JavaTC is a free-to-use website and download that is my personal favorite; however, others such as TeslaMap are also excellent choices).

TESLA COIL TYPES

   While all coils operate under the same underlying principles (i.e. matching the resonant frequency of the primary circuit to the secondary coil), there are several different forms they can take on. Although this site primarily covers the spark gap type, it seemed worthwhile to at least mention the different types.

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SPARK GAP TESLA COILS:

   Shown at left, spark gap coils utilize an inductor-capacitor (LC) oscillator circuit (the inductor being the primary coil and the capacitor being the capacitor bank) that is switched via a spark gap (see the section above for how this works). 

   Spark gap coils come in three forms:

  • Static gap (has two or more stationary electrodes placed close together).

  • Asynchronous rotary gap (has rotating electrodes that pass stationary electrodes several hundred times per second). 

  • Synchronous rotary gap (same as asynchronous, but the electrodes pass by each other when the AC waveform coming from the transformer reaches its peak).

   Rotary gap coils can produce almost any desired BPS, while static gaps are limited by the power supply's operating frequency (50/60Hz for typical iron-core transformers). The BPS also directly affects the coil's output. For example, a low-BPS coil (say, 100BPS) will usually yield slow, purple arcs that dance around the topload with a low buzzing sound, while a higher-BPS coil will generally fire off single, screaming, white bolts that move around at chaotic speeds.

   While spark gap coils are by far the easiest to build, they are generally less efficient, more hazardous, and louder than their solid state equivalents.

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SOLID STATE / VACUUM TUBE TESLA COILS:

   These types of coils, unlike their spark gap counterparts, lack a spark gap. Instead, they use high power solid state components (vacuum tubes, IGBT's, MOSFET's, etc.) to switch the current going to the primary coil. Many also lack a resonant primary capacitor (DRSSTC and VTTC models are exceptions), and thus rely on features such as secondary feedback to create the driving, resonant oscillations. Among the different forms of solid state coils, there are:

  • Slayer exciters/normal solid state coils (SSTC)

  • Vacuum tube (VTTC)

  • Phase loop-locked (PLLSSTC)

  • Dual resonant (DRSSTC)

  • Class-E SSTC

Each of these coil types can also run in various arc interruption modes, which determine the output spark shape and appearance:​

  • Continuous wave (CW): arcs are hot, bright, and often have little noise or sound. Their shape is often described as fuzzy or, in higher power models, flame-like.

  • Quasi-continuous wave (QCW) or ramped: arcs in this mode of interruption are typically very long and straight (described as "sword sparks"). CW coils running on unfilter, rectified AC (from a wall socket) will also tend to produce such sword sparks.

  • Interrupted: SGTC's, DRSSTC's, and common SSTC's typically run using an unspecialized interrupter that turns the coil on and off at a given frequency (spark gap coils do this automatically with their spark gap). Sparks from this mode are lightning-like and jagged.

Although solid state coils have more capabilities (modulating a solid state coil's interruption rate at audio frequencies allows the coil's arcs to actually play music), they are more sensitive and harder to build than spark gap coils. For more information on solid state coils, click here.

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TESLA COIL JARGON

   Before I go any further, there is something very important I need to discuss: the language. For a person new to Tesla Coil building, there are several terms used by Tesla Coil building community that most people trying to do their own research won't understand. Here is a list of some of the more difficult terms and abbreviations used:

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AC: alternating current (i.e. a current that periodically changes voltage or amperage).

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Arcs: the electrical bolts of lightning coming from the secondary coil. Also a term for most electrical discharges.

 

Ballast: something placed in series with a power supply that limits its current intake in order to prevent overcurrent damage or breaker trips.

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BPS: breaks per second (i.e. the number of times the spark gap fires per second).

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Cap: capacitor; a circuit component composed of two or more conductors separated by a non-conductive dielectric. Their job is to store electricity as a static charge.

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Coiler: one who builds Tesla Coils for work/hobby purposes.

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Corona: ionized air in proximity to very high voltage. It appears as a faint, purple-blue glowing haze of plasma.

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Dielectric: the electrical insulator found in a capacitor.

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Dielectric strength: the voltage required to break down a given dielectric.

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Electrode: a metal strip, rod, or other shape that, in a spark gap, is where the electricity jumps across.

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Farad (F): the unit of measure for capacitance (i.e. the energy storage space of a capacitor). Low values (typically <0.1uF) are typical in Tesla coils.

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H/D (aspect) ratio: the ratio of height to width, usually used in reference to the secondary coil.

 

Henry (H): the unit of measure for inductance. 

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Inductor: practically any wire coil that generates a magnetic field by passing electricity through it or visa versa.

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Line/mains: the electric outlet (wall socket) and any wiring connected to it with the same voltage.

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MMC: multiple mini capacitors (i.e. a bank of smaller capacitors connected to yield a higher voltage tolerance or capacitance value).

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MOT: microwave oven transformer.

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NST: neon sign transformer.

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OBIT: oil (or gas) burner ignition transformer.

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Oscillation: a cycle in which the voltage or current change one way (say, from positive to negative) and then back again.

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Pole pig: a powerline (pole-mount) distribution transformer.

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Peak voltage: the RMS voltage multiplied by the square root of 2 (or 1.414). For instance, the peak voltage from a 120VAC wall socket will be 120*1.414 = ~170V.

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Quenching: the extinguishing of the spark gap's discharge (usually for a few milliseconds).

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Resonant frequency: the frequency at which an electric current naturally "wants" to alternate at in a given coil or circuit.

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RMS voltage: in AC voltage, this is the voltage as it is generally labeled. Also known as the average voltage.

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Tank: the primary circuit.

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Topload/terminal: the metal object found at the secondary coil top (i.e. where the arcs come from).

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mA = milliamp   kVA = kilovolt-amp   kW = kilowatt   kV = kilovolt   W = watt  

uF = microfarad   nF = nanofarad   pF = picofarad   kHz = kilohertz   uH = microhenry

mH = millihenry   Hz = hertz  

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TESLA COIL SIZE

   Immediately, one of the biggest questions faced by anyone building a coil is "how big?" This actually determines a lot about the coil, such as its power intake, required parts, etc. Here are some general specifications for different size coils:

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Small coil:

  • The secondary coil on a small coil is less than 16" (~40cm) tall and 4" (~10cm) wide. The H/D ratio is anywhere between 8:1 and 4:1.

  • The primary capacitance (the amount of energy stored in the cap bank) should be between 0.001uF and 0.02uF (1nF-20nF).

  • The transformer should be between 40W and 400W of power at 10mA-40mA of current. NST's, OBIT's, and flyback transformers are all good choices, as well as car ignition coils and bug zappers for really small coils.

  • Static spark gaps are most commonly used, but a rotary gap may be used if the transformer can handle the higher BPS and power demands.

  • The arcs from a small coil are typically no longer than 12" (1ft.)

Medium coil:

  • The secondary coil is between 16" and 36" tall with a diameter of 3"-12". The H/D ratio is usually between 6:1 and 3:1.

  • The transformer should be between 400W and 3000W of power at 30mA to 600mA. Large NST's, NST's or OBIT's in parallel (to increase current output), MOT banks (2-4 in series for 4kV-8kV), small pole pigs, or flyback transformers powered by ZVS drivers all make good choices.

  • The primary capacitance should be between 0.02uF and 0.15uF (20nF-150nF).

  • Either static or rotary gap designs may be used. Coils with higher current transformer (+100mA) work better with rotary gaps, while lower current coils work fine with a decently quenched static gap (e.g. one with a fan or blower).

  • Arc output is typically between 12"-60" (1ft.-5ft.).

Large coil:

  • The secondary coil is over 36" tall and 6" wide. H/D ratio is between 6:1 and 3:1.

  • The transformer is over 3000W and over 300mA. Pole pigs, large MOT banks, or several paralleled NST's are all good options.

  • The primary capacitance is over 0.05uF (50nF).

  • Rotary gaps are preferred, but a heavily quenched static gap will still work.

  • Arcs are over 60" (5ft.) long.

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SAFETY

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   Tesla Coils, while incredibly beautiful machines, can be extremely dangerous. Here are some general "do's" and "do not's" to follow when operating or working on a Tesla Coil:

  • Never place iron or steel material near the primary coil, since the magnetic field created by it can induce eddy currents that will cause the metal to grow incredibly hot (if you want a demo, look up an induction heater).

  • Don't make the base/circuitry housing out of metal (the high voltage circuitry inside may arc to it and short out. It may also capacitively alter the secondary coil's resonant frequency).

  • NEVER TOUCH THE TANK CIRCUIT OF AN OPERATING TESLA COIL!!! The currents and voltages found within an operating coil can be lethal if contacted in the right way. Although the high frequencies and low currents found in the secondary bolts prevent them from penetrating your body far enough to cause instant death, they can still cause a dangerous shock and burns if the coil is medium or large in size. Avoid contacting the bolts longer than a foot coming from a Tesla Coil unless you know what you're doing or have proper protection.

  • Don't stare at an operating spark gap for extended periods of time, as it emits invisible UV radiation that can damage your vision.

  • If possible, wear hearing protection. The sound levels produced by a large operating coil are comparable to a jet engine and can cause permanent hearing damage.

  • The electromagnetic (EM) fields generated by a Tesla Coil can damage electronic equipment (i.e. phones, computers, etc.). Keep them shielded or at an appropriate distance.

  • Even when off, the capacitor bank may still hold a dangerous charge. Avoid contact unless you are certain the bank is discharged.

  • For more safety information, go to http://scipp.ucsc.edu/edu/tesla/teslacoil/safety.html.

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COIL OPTIMIZATIONS

   Like all hobbies, coil building has some "tricks to the trade" worth knowing. Here are several that often help make the difference between an awesome coil and a waste of time and money:

  • Make the secondary coil wider and shorter (decrease the H/D ratio).

  • Use less primary coil inductance (e.g. use fewer turns) and increase the primary capacitance. Using more primary capacitance increases the peak primary currents, and is one of the best, most effective ways to get bigger arcs from your coil. Just be sure the transformer can handle the amount of capacitance you plan to use (the maximum usable capacitance in farads can be found by dividing the transformer current in amps by the transformer voltage in volts, then dividing the resulting number by the BPS. More about this can be found in the capacitor section).

  • Increase the spark gap spacing to its widest setting that still fires reliably (just be sure the capacitors can handle the increased voltage stress).

  • Use larger surface-area electrodes in the spark gap. Needle-point electrodes tend to give somewhat poor performance compared to flat or rounded electrodes!

  • Use thicker (20-26AWG) magnet wire in the secondary coil. 

  • If possible, use a lower resonant frequency (in the primary circuit, this means more capacitance/inductance. In the secondary coil, this means longer wire length and/or a bigger topload). This many help the capacitors last longer by decreasing the number of oscillations per second.

  • Use a transformer (or transformer bank) with more output current. This determines how much the capacitor bank is charged each cycle, which ultimately effects the secondary spark length. While a higher voltage is beneficial, higher current power supplies can easily out-perform a higher voltage supply any time!

  • If possible, use a rotary gap. Just be sure the transformer can handle the BPS produced by it (the maximum BPS can be found by dividing the transformer current in amps by the transformer voltage in volts, then dividing the resulting number by the capacitance in farads).

  • While its important to have a higher current transformer, a higher voltage is also beneficial, especially for spark gap firing (however, using a higher voltage generally means buying a more expensive capacitor/capacitor bank to reach a given capacitance/voltage combination).

  • Raise the primary coil as close to the center of the secondary coil as possible without overcoupling (which results in primary-to-secondary arcing, corona, etc.).

  • Use thicker gauge wire, or even copper pipe, to wire the primary tank circuit.

TROUBLESHOOTING

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   Here are some helpful troubleshooting steps that'll get nearly any coil running:

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Coil draws power, but no gap firing or secondary output result:

  • The transformer is dead. Check the unit's secondary coil for continuity and/or output (i.e. try to draw an arc from it). If the transformer seem functional, continue to the next step.

  • The spark gap is over-spaced. Try decreasing the spacing to see if the gap will fire. If the coil still won't run and the gap still won't fire, continue to the next step.

  • There is a short or bad connection in the circuitry. Check all connections and retest. If nothing results, continue to the next step.

  • The primary capacitor is dead (i.e. has an internal short). Test the bank's continuity (infinite resistance is desired) with a multimeter.

  • If all else fails, the transformer's voltage may simply be too low to fire the spark gap.

Coil circuitry operates, but secondary output is weak or non-existent:

  • The primary and secondary circuits are out of tune. To fix this, try adjusting the primary coil size and number of turns (if possible, you may also try adjusting the capacitance). Using a designer program like JavaTC can be very helpful in this step, however, if you have access to the right tools, you can determine the resonant frequency (and then adjust it accordingly) yourself. If the circuits seem in tune, continue to the next step.

  • The topload is too large. Try adding a metal point to the topload or place a metal object nearby to try and coax out an arc. If this fails, try removing the topload altogether and retuning. If this fails, continue to the next step.

  • The primary-to-secondary coupling is too low (i.e. the coil centers are too far apart). To fix this, raise the primary (or lower the secondary) so the centers are closer together. If this fails, the coils might be over-coupled, and making the centers farther apart might prove beneficial. If this fails, continue to the next step.

  • The spark gap electrodes need to be farther apart, have a larger surface area, or (if the gap isn't rotary) have better quenching. Try altering the electrode spacing and/or electrode size. If the gap is static, try adding a powerful fan (blower or sucker) or blow compressed air over the gap to help with quenching. If this fails, continue to the next step.

  • The secondary has a bad connection (either to the ground or topload) or a short. Check all connections and retest. If nothing results, try connecting the coil to a higher surface area ground (connect the bottom wire of the secondary to something metal with a lot of contact with the earth). If this fails, continue to the next step

  • The primary circuit wiring needs to be a thicker. In many cases, the coil's spark length can be increased just by altering the wire gauge.

Coil circuitry operates, but experiences corona between the coils and/or has frequent primary-to-secondary or secondary-to-strike rail arcing:

  • The primary-to-secondary coupling is too high (i.e. the coil centers are too close together). To fix this, lower the primary (or raise the secondary) so the centers are farther apart. If this fails, continue to the next step.

  • The topload shape or size could be optimized. A toroidal (donut-like) topload shape is usually best, while spherical shapes tend to be slightly more problematic. A diameter larger than the primary coil diameter is usually best. If changing toploads make little to no difference, continue to the next step.

  • The coil is being overloaded (aka: being fed too much power for its size).

Spark output is choppy (not smooth-sounding, too crackly, dim or otherwise weak):

  • This usually occurs with rotary spark gaps as a result of lower voltage (<6kV) operation and/or inconsistent gap spacing. Consider revising the spark gap's design or increasing the input voltage until smooth gap firing is established.

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