Clasic Tesla


Tesla coils are  air core resonant-tuned transformers, but, what means exactly this definition?

Main advantage of alternate current is the capability of being transformed easily between different voltage levels using magneticaly coupled circuits, or, in other words, transformers.

But transformers, or at least classical ferromagnetic ones, present a great inconvenient when we try to reach very high voltages: magnetic saturation, which implies huge number of turns and, definitively, volume, weight and... of course, price.

Tesla coils are air core transformers so it's not present saturation problem, but magnetic  coupling is more dificult to get. Here is where resonance phenomena brings the solution. In a tesla coil, primary circuit is a LC circuit that works at its own frecuency (F0). secondary circuit is other LC, so,  if resonant frecuency of secondary  is equal to natural frecuency of primary (F0 = Fr), then electromagnetic energy transfer between both is very efficient, and energy can be easily transferred from primary to secondary.

In a tesla coil, voltage gain do not depends on number of turns. Here, it's a question of difference in the impedances of prymary and secondary circuits

How Tesla Coil works?

The followinng schematic shows a classic tesla coil which can be considered as the minimum system. Some values are given and must be considered as typical for a small - medium size tesla coil design.

AC input power (15A @ 220 V) is transformed to 12 kV by a high voltage transformer.

This high voltage is used to charge  capacitor C1, then the spark gap enter in conduction due to the high electric field present (in static gaps), or synchronized by a mechanical switching device such a rotary spark gap (RSG).

When the gap is "closed" all energy stored in the capacitor flows through the  primary coil (current peaks about 1.5 kA), generating a train of powerfull electromagnetic waves with a typical frecuency impossed by values of L1 and C1.

This strong electromagnetic field has the capability of generating by induction phenomena a very high electric potential in secondary circuit, with maximum energy transfer effieciency if resonance frecuency of secondary LC circuit equals to impossed primary frecuency.

Really, this way of working is more typical of a tuned radio emiter-receiver than a transformer. With this invention, Nikola Tesla estabilished the principles of radio communications and Wireless Transmmission of Power.

Tesla coil parts

HV power supply

The input voltage of a device such Tesla coil must be high enough if we want visible results. At least of many thousands of volts (>3-4kV) for a small size design, Although it is convenient to work with supply voltages greater than 10 kV in order to achieve optimum results.

High voltage power supply used in Tesla coil design is formed in essence by a conventional high voltage transformer, with a  ratio about 220:10000, and capable of giving a determinated max. current.

There are different types of high voltage transformers susceptibles of be used in Tesla coil design:

Neon Sign Transformers: are transformers used in neon signs. This deveices (neon signs) require high voltage to obtain light emission by neon gas excitation, being voltage required proportional to neon tube lenght.This type of transformers are manufacturated from 2500V to 12000V, although powers are generally small, cause currents supplied are low, about 35 mA. This transformer type is ideal for small Tesla coil construction, and is recommendable to use it in the first designs, because of its low current output which allows to eliminate power regulation element (variac), and gives some security in accident case. Usually this type of transformer is presented resin encapsulated.

Microwave Oven Transformers: microwave ovens utilizes a high voltage transformer to supply energy to the magnetron which generates the 2450 MHz which cook our foods. Voltage supplied by this transformer type is not high enough, usually about 2000V. This makes recommendable series conection of at least a couple of them. Although not much voltage is provided, is not possible to say same thing about current in the range of 0.5A that can be supplied to load, wich converts this type of transformer in a seriously dangerous device. Its appearance are the same of low voltage transformers, with nude coils and enameled paper and mica isolation.

Measurement Transformers: are used for voltage measurement in high voltage facilities. Low voltage winding usually works at 100V max. in most models. Use for which are designed (reducing voltage level) is the opposite to what is pretended here, but there is no problem cause transformers are completely reversible machines. There are models in a very wide range of voltages, being appropiated for Tesla coil use those between 10000 and 25000 V. Generally are presented in resin encapsulate, at least high voltage winding. Power available depends on model and can reach 4000 VA.

Distribution Transformers: used by electric companies to conditioning line voltage levels to give service to customers. They are big transformers, usually presented in oil filled case, being capables of high power handling. They are extremely dangerous and their use is reserved to advanced users, being necesaries for high power designs (> 10 kVA).

Hand-made Transformers: one of the possibilities is to build youself your high voltage transformer, strarting usually from a magnetic core from other transformer, which coils are removed in order to make apropiated ones which will work at required voltages. It requires a carefull previous study and design, and, of course, a precise manufacturing, specially in high voltage coil which must incorporate proper isolation materials in order to avoid dielectric failure. It is adviceable to start from a big enough core to be able to handling adecuated power.

It is also convenient to add some power control device in order to add output regulation capability and to avoid dangerously high currents. Tesla coil power levels do not allow, or at least do not recommend, resistive potentiometer or reostate use as power regulation system. More adviceable solution is the use of a variable autotransformer (Variac) which consists in a single coil wounded in a toroidal core, wich output tap can be placed in every turn, givin an adjustable output voltage between cero volts and input voltage. Size, weight and ... price deppends directly of power wich is able to handle, wich never be lower than whole design power requeriments.

Also, it is recommendable to install security elements in different points of power supply design, in order to protect it and electric line where is plugged on the equipement, avoiding perturbations, voltage peaks, high frecuency harmonics, etc. There are two types of security elements: Security gaps, destined to neutralize dangerous voltage levels and, Radiofrecuency filters, destined to protect all elements from high frecuency harmonics generated in device operation.

Finally, being the Tesla coil an inductive load, it is also adviceable to install a capacitor battery on input for power factor correction.

HV capacitor

The high voltage provided by power supply is used to charge primary circuit capacitor, usually referred as "Tank Capacitor". Its capacitance value and working voltage determinates the amount of energy wich can be handled by primary circuit, known as "Bang Size" and represents the amount of energy which can be transferred each time the spark gap conducts. Bang size can be expresed as E = 0.5 * C * V^2.

Value of capacitance is limited, of course, by transformer impedance, to assuring that load will not require currents higher than transformer nominal output, wich could pruduce overheating in continuous operation mode.

Capacitor is a very important element in a Tesla coil system, although all of its parts are. It must be capable of suportting properly the voltage applied in operation, having low dielectric losses as well as minimum inductance and parasistic resistance.

In Tesla coil design can be used comercial high voltage capacitors, but most usual thing in amateur coilers is the self-construction. This hand-made capacitors fall on two categories:

Bottle Capacitors: providing a small capacitance, a few nanofarads, but supporting very well the high voltage, each one is formed by a bottle with saline solution inside and covered by a layer of conductive material as aluminium foil on external surface. One terminal is introduced into saline soluttion, while other one is conected to external surface, in the way that glass works as dielectric element. Usually are used many of them conected in series to achieve adequated capacitance. This type of capacitor is easy to build and quite safe which makes it apropiated for first designs. Moreover, it allows adjust of capacitance, adding or taking off bottles. Its use is restricted to low power designs (< 500 VA).

Stacked Plate Capacitors: consist in a number of metalic plates separated by dielectrical material foils. Electrically are conected forming a paralel single capacitor battery. The whole system formed by conductive and dielectric plates must be inmersed in a non-conductive medium, with dielectric propierties better tha air, in order to avoid dielectric failure. Generally mineral oil is used succesfully. This capacitor type can be built in two geometrical ways: plain and cilindrical, although last one presents higher parasite inductance.

Independiently of what type of capacitor is used, in every design is convenient to install security elements which protect capacitor of dangerously high voltages that can be produced. Usually security gaps are attached to capacitor terminals.

Spark gap

This element is responsible of energy transfer between main capacitor and primary coil. It is a disruptive element which works like a switch, allowing or blocking current flow. Its most important characteristic is the quenching capability, or, in other words, ability to cut electric arc. It is desirable to extingish the spark in the instant in time when all capacitor energy has been completely transferred to primary coil, in order to avoid posible energy return to capacitor from primary. This is known as "first notch quenching".

There are many gap types, all of them can be clasified on two categories:

Static spark gap: In their simplest configuration are formed by two electrodes separated by a given distance, in order to allow arc forming when voltage diference between electrodes reach air dielectric break value, and extingishing electric arc when voltage level falls. Electrodes must support high temperatures produced, so they must be manufacturated using apropiated materials, if possible tungsten, although other ones could work properly, with an adequated design wich promotes thermal dissipation. Also it is possible to incorpore an air cooling system using an adequated fan.

Rotary spark gap: They use a spinning disk with electrodes on its periphery, which pass near static electrodes strategically placed when spinnig, producing discharges between them. This type of gap has an excellent quenching capability, wich made it adviceable for medium and high power designs. During its design time must be studied both mechanical and electrical considerations. The spinning disk must be built from a non-conductive material wich can support also mechanical solicitations produced by centrifuge force, and must be moved by a proper electric motor, preferently using a speed regulation system, in order to adjust number of discharges per second (BPS Beats Per Second), which optimal values are usually between 100 and 200 BPS for a 50 Hz line frecuency. Rotary electrodes are cooled by air on its movement, but stationary ones require a proper design, with more mass and heat sinking structures to avoid risk of destruction by high temperatures achieved. The use of a synchronous AC motor with an adequated placement of electrodes make possible switching in maximal voltage instant, on 50 Hz wave peaks, wich maximizes energy stored in capacitor and transferred to primary coil.

Primary coil

Primary coil receives the discharge of all energy stored in main capacitor and must be capable of convert it into a powerfull electromagnetic field, for what must acomplish with certain geometrical and electrical requeriments. Electrically, it must provide adequated inductance which in combination with main tank capacitance determines oscilation frecuency of primary LC circuit. Geometrically, primary must be designed to reach highest coupling factor possible with secondary coil, so size and geometric shape depends in great amount on secondary characteristics.

Primary coil types are classified on three basic geometries:

Helical Primary: consists on a single layer helix shape winding with the same axis of secondary coil and placed in the bottom of it. Primary diameter must be long enough to ensure avoiding arc with secondary coil.

Flat Spiral Primary: in this case the winding forms a flat spiral perpendicular to secondary axis and placed in lower end of it. Distance between inner turn and secondary coil must be long enough.

Inverted Conical Primary : Its geometry responds to a inverted conical helix and can be considered as an hybrid model between two precedent ones. Angle formed with horizontal is usually between 30º and 45º. This is a design which provides and excellent coupling factor being used in most of designs.

Secondary Coil

The strong field created by primary coil generates, by electromagnetic induction, output voltage on secondary coil, wich must be capable of absorbing the whole electromagnetic field. In essence secondary coils consists on a single layer cilindrical winding, providing an inductance value which, with capacitance provided by discharge terminal forms secondary LC circuit with a resonance frecuency which must match primary LC working frecuency.

Resonance frecuency can be calculated as follows: Fr = 1 /[2*PI*SQRT(LC)], where L corresponds to secondary inductance and C express total circuit capacitance. This capacitance is formed by secondary coil self-capacitance (Cs) produced by short distance between turns, and capacitance added by discharge terminal (Ctop), so C = Cs + Ctop.

Secondary coil self-capacitance or parasite capacitance depends on secondary size and is calculated using the Medhurst formula: Cs = K*D, where D represents the secondary coil diameter and K is the Medhurst constant, determined by secondary H/D ratio. This formula gives self-capacitance value in picofarads, starting on following K values:






















Relationship between height and diameter of secondary coil is an important data, being the chosing of an adecuated value a critical question to reach a coupling factor high enough to ensure an optimal energy transfer between primary and secondary coils. Value needed for secondary diameter depends on the amount of power to handle, being adecuated the following values.

Power (kVA)

Diameter (cm)

Height (cm)

0.5 - 1

10 - 15

45 - 70

1 - 2

15 - 20

55 - 85

2 - 4

20 - 25

70 - 110

4 - 7

25 - 30

85 - 140

7 - 10

30 - 35

100 - 160

In other hand, the type of wire used to build it must have a section big enough to present a lower enough resistance. So it is recommendable to use 0.71 mm copper enameled wire or thicker for higer powers. About number of turns needed it is adviceable to maintain it between 500 and 1000 to achieve optimal results.

The tube wich supports the winding must be made of a non-conductive plastic material, usually PVC, being convenient to apply also an isolation layer over coil surface (epoxy or phenolic resine) to prevent possible dielectric failures.

Discharge Terminal

Discharge terminal is placed on top of the secondary coil, having a double function: In the first way, provides capacitance (Ctop) which, united to secondary selfcapacitance (Cs) forms secondary LC circuit total capacitance. In second way, discharge terminal radiates radiofrecuency energy generated. Really, discharge terminal forms a plate of a capacitor, while the other plate is formed by earth itself (floor, walls, etc.), being produced between them the electric discharge.

Geometry of this element is important because it is designed to modulate the shape of a very high electric field generated in system operation. It is convenient to give the apropiated orientation to electric field, as well as give it adequated geometrical shape, the most uniform possible, avoiding sharp edges and irregular surfaces on discharge terminal design.

There are two geometries used frecuently in discharge terminal designing:

Spherical terminal: formed by a metalic surface sphere. This type of geometry generates a fairly uniform electric field in all space directions. It presents the inconvenience of easily arc production in all directions, including those wich represents a risk ofimpact with primary or other parts.

Toroidal terminal: it is the most utilized, because it generates a electric field concentrated into horizontal directions and quite attenuated in vertical axis, generating sparks which grows horizontally, avoiding risk of primary impact.

Usually are constructed from materials such aluminium flexible ventilation conduit. Toroid surface must be smooth enough to avoid field deformation, but, in experimentation and operation testing it is usefull to install a metalic spike to locally concentrating electric field, forcing arcs to be produced only in a certain direction.

Simulation - Typical Waveforms

A great way to aproach to Tesla Coil working mode, even without the need of build one, is to make a software simulation study, using an adecuated software and circuit model.

For this task results very usefull PSPICE simulation software, in the past from MICROSIM and actually as a component of ORCAD (Capture, Pspice, Layout).

The next model has been designed using Pspice 8.0 from Microsim. This model is based on information obtained from other pages on the net, with adecuated parameter settings in order to obtain an aproximated results of my design Coil #4.

Output Voltage

Output voltage value in a Tesla coil has a dependence from too many factors, such as an adecuated switching, firing the gap when the capacitor voltage reaches the maximum value.As we can see, the Tesla operates in a pulsed mode, presenting in each pulse an voltage output of about many thousands of kilovolts.

These pulses are really trains of sinusoidal waves with an exponential atenuation, detailed in next graph

Primary Current

This voltage generatión in secondary needs very high current flowing through primary circuit, produced by capacitor discharge across the gap. In the propossed model for Coil #4 are registered current peaks up to 1700 Amperes, as shown in next two graphs.

Capacitor Voltage

Primary capacitor is one of the most stressed elements of the design due to high frecuency voltage sweep supported, as followed pics shown-

The sinusoidal charge / discharge cycle imposed by transformer voltage is truncated by each "break"or gap switching wich closes the circuit, preferred when capacitor reach its maximum voltage, descharging it trough primary coil, wich generates an enormous magnetic field that, in part induces output voltage in secondary circuit, and the rest generates new high voltage trough primary (selfinduction), in the capacitor bornes, repeating the cycle, with an exponential attenuation due to energy transferred to secondary and to different looses (termically in gap arc, magnetic looses, joule effect in primary and secondary and isulation faults.)

AC Input Current

Left pic showns instantn and RMS values of current demand of whole system. And as we can see current waveform is severally modified , so it is recomended to use RC filters to prevent AC line perturbations by introduced harmonics. Current demand of  Coil #4 design, operating at 100% reach 20 Amp RMS @ 220 V.

Right figure shows instant and RMS values of AC power demand wich reach 4kVA.

Safety Information


Tesla Coils are extremely dangerous devices and must be designed, built and operated by persons with adequated knowledge in electricity and safety in high voltage devices.

Hazards inherents to this kind of machines are numerous and of diverse nature and, while here are exposed many of them, other not mentioned potential ones may exist. Each part of a Tesla Coil presents his own risks, so this document studies separately each constructive element and its asociated hazards.


Discharge Terminal

Most evident discharge terminal effect are very high voltage electric sparks produced (from many hundred of thousands volts to millions of them). Initially, we could think that this represents the highest risk of death by electrocution of all design parts, however this is not true, cause this very high voltage is produced at a frecuency of 100 KHz or more, so the skin effect begins to be important. Skin effect becomes noticeable when an alternate current pass through a conductor, making this current to flow through the peripheral zone of it, with more important effect at higher frecuencies.This effect is caused due to difference of inductance presented by the outer zone of conductor, lower than inner zone inductance. This is a strictly geometric cuestion, so it is able to be applied to every conductor material, included human body. This way, if desafortunately a person is shocked by terminal discharge sparks, current tends to pass through external parts of his body, diminishing greately cardiac or respiratory arrest risk, but presenting, of course, other damages like burns on impact zones.

Other Tesla Coil noticeable effect, which can be assigned to discharge terminal sparks is ozone (O3) production, as is probed by characteristic lightning storm smell that remains after indoor tesla coil operation. Although small quantities of ozone can be even healthy, the amount of this gas generated by tesla coils (specially at medium-high power) represent an important risk of intoxication by ozone.

Finally is present the risk of X-ray production, inherent to high voltages operation. X-rays are high energy electromagnetic waves generated when a charged particle moving at determinated speed is forced to stop movement, for example by colliding whit heavy materials. Part of its energy is released in the medium and other part escapes as X radiation. Exposition to X-rays is a great risk for health, cause as X-rays are ionizing radiations, are capable of producing an important amount of chemical reactive free radicals associated with diseases like cancer, ADN mutation, etc.

Some amateur coilers with the resources needed for X-ray measuring (Geiger-Muller counters, thermoluminiscent dosimetry monitors, etc.) have monitored their equipments with the result of no measurable radiation levels, which is normal as is explained now: To Generate X-rays energetic enough to be measured, or which is the same, dangerous, charged particles (electrons) must be braked suddenly (against a heavy metal) from a very high speed, wich only can be achieved in vacuum conditions. So, arc impacts against different objects like walls do not present a high risk of X-ray production, but, all this hollow objects which can be evacuated in vacuum, like cathode ray tubes (TV), vacuum tubes and this kind of devices must be avoided near an operating tesla coil, cause joining high voltage produced with vacuum vessels risk of X-ray production is high, and represents a great health danger.


Secondary Coil

Secondary coil generates by induction phenomena coil output voltage, with bottom terminal conected to ground and upper to discharge terminal. Voltage difference between neighbouring turns is high enough to require a proper isolation to avoid risk of producing sparks between secondary coil turns, or even between primary and secondary, wich quickly damages it.

In other hand, secondary coil has a high parasite capacitance (self-capacitance) between contiguous turns, and, moreover, isolation material is capable of maintainig an important static charge, so risk of static discharge is present if secondary coil is handled inmediately after equipement operation.Although this type of discharge do not represent an important risk of electrocution is not a funny experience. As caution procedure before handling secondary coil is convenient to short-circuiting both terminals of coil and pass them along coil surface, sometimes producing typicall static discharge crackling sound.


Primary Coil

Primary coil receives all energy released by capacitor discharge through the spark gap, causing very high instantaneous currents (peaks of thousands of amperes). It generates powerfull trains of electromagnetic pulses (EMP), which are capable of generating induced currents in metal structures or other electric conductors.This currents could be strong enough to introducting a potential risk of fire if inflamable materials are stored near.

This EMP's are also able to inducing fatal voltages and currents in electronic devices placed near the coil, like measure and test equipement, computers, etc.So, never operate tesla coils near any type of electronics !

Another undesirable effect of primary operation, also caused by gap and terminal discharges, most if radiofrecuency ground is not efficient, are radiofrecuency interferences, wich can affect radio and TV receiving. As correction measures is recommended to improve RF ground and installing a line conditioner fitre in series in equipement power input.

Finally about primary coil, obviously is an element with a very high risk of electrocution by contact or aproximation, cause it usually operates at 15-20 kV is able to supply very high currents, so risk of death by electrocution is severe !!!



Primary circuit capacitor is an extremely dangerous device for many reasons. Capacitor presence gives primary circuit capability of discharging  huge energies in a very short time, which can be translated into DANGER, in fact, contact with any part of equipement associated directly to capacitor means a very high risk of death by electrocution, even with equipement turned off, cause capacitor can store a lethal charge long time. So, is convenient to shortcircuit in a secure way capacitor terminals before proceding to manipulate any element of design, specially primary circuit.

Even after been discharged, capacitor is capable to reach charge from static sources or electric fields presents, and also due to memory effect that appears in dielectric materials long time under electrical stress. For security, in case of storing capacitors for a long time it is convenient to store them shortcircuited.


Spark Gap

Sparks produced in the gap electrodes are the result of thousands of amperes passing through the air, which produces, additionaly to terrible noise, great amount of ultraviolet radiation, in fact, the light produced in a spark gap is very similar to that produced by an arc welder, but brighter than it. Human eye experiments no pain with UV light, but nocive effects appear later. So never look at the arcs produced in the gap, althrough its visual study with adequated protection (welder mask) gives many data of equipement operation.

Moreover, rotary spark gaps use a spinnig disk with electrodes rotating at high speed, which makes that both disk and electrodes to aquire a notable amount of kinetic energy, which can be translated into an evident risk of mechanical failure, disk breaking and parts projection at high speeds, wich can make considerable damages. The best way to avoid it, is shielding spinning parts using a plastic material strong enough (metacrilate, policarbonate...) with appropiated thickness thath ensures no fragments are projected in a posible spinning disk failure.


High Voltage Supply

High voltage transformer present in this part of the design is capable of giving a current of hundreds of miliamperes at a voltage about 10,000 V, risk of death by electrocution is evident, most having in mind that hv transformers usually have nude high voltage terminals. In this case, current frecuency is the domestic AC frecuency, 50Hz/60Hz, which make skin effect unapreciable, passing now most of current through internal organs of the human body, causing severe damages in tissues due to Joule effect or thermal disipation of electric energy in a resistive medium. In this case, risk of cardiac failure is specially high, cause at this frecuencies is very high the possibility of ventricular fibrilation.

High voltage transmision line between hv power source and rest of circuit is a high risk element and must be constructed using high voltage rated cable, as the type used in distribution systems in car motors or in neon signs. Usually this type of cable supports voltages up to 40 kV.


Low Voltage Line

Low voltage line (120-220V) which powers the system is another potential source of electrical risk, in the way that must be avoided at all that arcs generated in discharge terminal can impact into supply low voltage line. This, moreover of injecting a dangerous voltage peak in the line, produces a fault to ground from electric house line, through highly conductor ionized path which is the electric arc. If unfortunately, a person enters in contact with this arc could die, but not caused by HV effects as by the high low voltage current from house line. This risk is diminished installing diferential protections.

All the ways,  it is recomendable to build all design using appropiated materials. Even resulting more expensive safety is esential for continuing the experimentation.