Tesla Coil

!!!Information provided here are NOT sufficient for safe construction and operation of a TC!!!
Touching devices does not harm me due to high frequency of the power source
Do not try anything similar at home, unless you fully understand what you are doing

The beginning

One fine day, I managed to come by induction coil (for car ignition). I found out that it kind of sparks when (dis)connected to a battery. So i bought a relay to reconnect it instead of me. I huddled the complex into small box from candies and grounded on it. I placed a switch onto the bottom of the box (so that it is disconnected when the box is put down). I placed pot onto the top and mounted small surfaces (metal) on it, which I connected onto the induction coil. Afterwards few individuals of type "wheee, that is shiny" tried to observe it from all sides. Also this way I managed to get into state, that when I have something strange somewhere placed, it stays there.

Apart from pedagogical function, it made sparks when it was raised and no one was touching the electrodes. I take for sparks and discharges. I browsed for a while. I haven't felt like building Van de Graaff generator so I decided for Tesla Coil.

Czech pages http://rayer.ic.cz helped me a lot, however it is good to go through more sources on this, including foreign pages.

Little bit of theory

SGTC Schemas

TC (Tesla Coil / Tesla's Transformer) is actually device to make high voltage on high frequency.

Device consists of two circuits, primary and secondary. Primary circuit consists of coil with low number of turns (usually up to 12) and "something" which oscillates on given frequency (the resonant frequency of the secondary circuit). I describe "something" later.

The secondary circuit coil has 800 to 2000 turns (it is advised to apply lacquer on it, so the turns won't run around on temperature changes.) and is by no mean conductively connected with the primary circuit (or at least it should not be, occasionally one founds schemas with common grounding). This coil has its inductivity and inter-winding capacity. The capacity can be raised by adding an additional capacity (spherical or toroidal top). The secondary circuit also has the discharge electrode, that is the part (usually on top) from which discharges are expected, unless the role of the electrode is played by the additional capacity itself. Sometimes both are missing, so discharges occur just from the ending of the secondary coil. So the basic parameters are circuit capacity (which is approximately 20% smaller than simple sum of the inter-winding capacity and the additional capacity) and circuit inductivity. With use of Thomson's formula (or measured) we can get resonant frequency of the secondary circuit, onto which we need to tune the primary circuit.

Resonance knowledge comes handy here, simply during loose-coupling the energy is transferred from oscillator (the primary coil) into resonator (the secondary coil). This energy transfer rapidly falls as the frequency gets away from the resonation frequency of the resonator.

And now, hot to achieve oscillations of the primary circuit corresponding with the resonation frequency of the secondary one. The frequency can be roughly from 300 kHz to 4 MHz (at these frequencies skin effect occurs as well as the frequency is far away from frequencies at which human body operates). Actual resonant frequency depends on the actual secondary circuit. We differentiate TC by how is the resonant frequency achieved on the primary circuit.

SSTC (Solid State Tesla Coil) - semiconductors are used to achieve desired frequency. These are quiet, oscillations can be practically un-attenuated and uninterrupted (well not modulated by low frequency). Disadvantages are, that today semiconductors are not robust enough or $$$ unavailable. And even when you get some, they produce lot of heat and needs cooling and burns out easily. On the contrary, this is the way how to achieve nice corona, however it is suited for smaller power output. Circuit itself is usually made by full-bridge or half-bridge, however it is not my case (at least not yet), so I won't write about this much more. Occasionally simple self-oscillating exciters are used, mostly for demonstrational purposes. Browse some other pages for more information on SSTC.

VTTC (Vacuum Tube Tesla Coil) - vacuum tubes are used to achieve desired frequency. I really like this idea, however except one kenotron (usable more like heavy duty diode) I don't possess any vacuum tube, so nothing. BTW: VTTC is AFAIK the only way how to achieve long straight pseudo-steady stream pointing to the top.

SGTC (Spark Gap Tesla Coil) - Tesla Coil with spark gap. Really simple schema, just LC circuit with desired frequency connected onto power source. This one may be the closest one to the original Nicola Tesla's plans. Schemas are used the ones you saw above. I prefer the second one, as primary winding is under power only when needed (during oscillations). How does it work? The capacitor is charged from the low frequency source, until the voltage is strong enough to ionize the spark gap into level that it discharges. This completes the circuit and oscillations begin. Once the power is too low to keep the discharge in the spark gap alive, the circuit disconnects thus allowing the capacitor to charge from the power source again. Oscillations are attenuated, and practically modulated by the spark gap frequency. Spark gap can be static or rotary.

Static spark gap can be simple, two spheres apart, however it is not too effective because it heats a lot and is hard to cool. Static spark gap can be also composite, multiple conductive pipes apart, so the discharge occurs between pipes 1 and 2 first and then between 2 and 3 etc. so the spark power is spread through larger surface, which can be cooled better. For even higher power, various rotary spark gaps are used. Roughly two electrodes far apart opposed by disk with conductive electrodes on sides reaching almost the opposing electrodes. Discharges are then controlled by rotating of the disc (as discharge can occur only when electrodes are aligned). Engine operating the spark disc should be well isolated, as discharge through the engine may not have desired effect. One of the problems with the rotary spark gaps and power source with alternating current is, that discharges does not occur during the peak charges of the capacitor (synchronous engines solves this, but they are expensive).

The heart of SGTC is the resonant capacitor, which have to withstand high voltage at high current. It charges from maximal positive charge to maximal negative charge rapidly and repeatedly. So it can't be bought. Well, it can be ordered from some pro companies, but it is expensive. Therefore exists few alternatives for its homemade construction.

Leyden jar, jars with salty solution covered in aluminum foil or again placed in another salty solution. Very disadvantageous as it has too high resistance and the whole idea of liquid with high voltage is not a good one.

MMC Multi Mini Capacitor, parallel-series composition of smaller capacitors. Very often used, I just need to say, beware of electrolytic capacitors (they dislike alternating current and happily explodes). Disadvantage is higher price when buying hundreds capacitors, or longer salvaging time till desired amount of capacitors are salvaged.

Make of your own, from conductive foil (usually aluminum foil) and some dielectric. The capacitor can be either: rolled one (2 plates of dielectric foil, 2 plates of conductive foil, which is fun to determine its final capacity) or parallel plate model (alternating dielectric pieces and conductive pieces). Parallel plate model, as well as the other one, needs some reserve area on dielectric (i.e. conductive foil does not cover the whole surface) to prevent unwanted discharges. Good example can be multiple CDs or LPs covered by aluminum foil, where each odd aluminum surface is connected onto one electrode and each even one onto the second electrode. With the appropriate dielectric, these can be constructed to really high voltage with desired capacity. Disadvantages are literally huge dimension of final product, and again, big inner loses.

Skin effect and others

From certain frequencies the current flows only through a surface, even on cupper cube. It could be simplified like this: the higher the frequency is, the shallower is the part through which the current flows. Therefore a hit from a TC should not hit any inner organs, however it can still burn (the discharger itself has 5000 - 50 000K). Due to skin effect, solid cable is as good as pipe for the primary winding. Relatively safe skin effect can be achieved from around 50 kHz.

Even with skin effect, a discharge can cause unpleasant burns and it can resonate with cardiac valves, which can cause problems to some people. Moreover any spark gap as well as discharge itself emits amounts of ultraviolet lights and ozone.

One needs to be aware of the fact that except from correctly working secondary winding, other parts of such device contains deadly voltage without skin effect, because common power supply operates on 50Hz / 60Hz. In case of undesired short circuit of secondary winding with other parts, the whole device becomes extremely dangerous (HF discharge from discharge electrode creates conductive connection, which can be traversed also by these unwanted lower frequencies).

In case that anyone is decided to construct the device, it is very advised, to consider your knowledge from school you have, what more knowledge you need to study and definitely read through more materials then just this page. Most importantly study work safety and respect principles of safety.

First success

Schema for TV transformer Scheme for high school technical competition
transformer experiments Schema for TV transformer - real

At first, I created just smaller (SSTC), schema for which I found on the internet. Spark around 3 mm. Nothing extra, well it was just start. The secondary coil have (I still have it somewhere) 433 turns.

With the same secondary coil, I decided to make SGTC. Capacitor consisted of few CDs with aluminum foil. I utilized transformer from old TV Merkur as HV transformer (left schema). The schema was so clumsy, the transistor lasted for only 5 min. Transistor in TO3 case can be seen on the photo, which can be salvaged from old things. For the actual high school technical competition I bought irfp150n transistor in TO247 case and so I had to make cooler for it (I drilled hole for it into used CPU cooler. I achieved 2-5 cm sparks with this schema. Once I even had corona (from TC discharge electrode, not from the TV transformer). I participated with this schema in high school technical competition (the right schema). The vacuum tube was utilized as HV diode, so I could actually charge the capacitor on some reasonable value which could operate the spark gap (the transformer was too weak for its operation frequency, around 60 kHz).

On the completion I failed among others on answering the question: "What is it good for? Anyway the schema is not very good, as the secondary coil is conductively connected to the primary one."

For this construction, i also had few really small secondary coils (top of the toothpaste tube) with capacitor made from really small piece of dielectric with two pieces of aluminum foil. These small coils made small yet nice corona.

Naturally I played with the transformer as well, HF HV source can entertain for a while... For example: little Jacob's ladder.

V Discharge from the transformer
V Small Jacob's ladder
V Light bulb near transformer discharge point
V Light bulbs and glow bulb near transformer discharge point
V Small SGTC

Long term project

Once I salvaged one dispersion transformer from a scrapyard and my friend gave me another one on 230V/50Hz. This solved problems with connection and conversion of DC into AC, as it can be directly plug into main.

V Jacob's ladder

Prototyping was made on wooden base, mostly without computations. This try was not much successful and quickly advanced into later base. The photo shows remains of the old base and preparations for the new one.

Base reconstruction

Transformer from the scrapyard is dispersion one, so I use it for first tries as it handles well short circuits. Another reason for this choice is the fact that I miss any choke in the schema, I just didn't bothered with it yet and it can be omitted with dispersion transformer on this power levels. However it would be nice to come by more powerful transformer.

The mess
Transformer Transformer

As skeleton of the project I used PMMA, I found some home and a classmate donated me some more. I draw few days on the PC how it should look, then I traced it on the PMMA (considering effective utilization) and then i played few days with saw and drill.

Primary winding support

Time to glue parts together. I tried various glues, the best seemed specific superglue, however it didn't bond as I needed. So I browsed a little, asked few people and concluded that the best choice would be to dissolve some PMMA in chloroform and use it instead of glue. It was funny to obtain some chloroform, but I managed to. If anyone tries it, let air to ventilate a lot, as chloroform can put you to sleep and sun light impacting its vapors makes toxic phosgene.

Glue test

When I finished with using saw, drill and glue I started winding primary winding, thick solid cupper cable. The construction is such a cube, with top and bottom face and four edges for holding. The capacitor occupies the bottom face and the top one contains trapezoids with primary winding. The secondary coil belongs in between those trapezoids on the grounding port. Transformer uses to be placed outside of the construction and spark gap is made as a gap between cables (I haven't made anything better yet).

Winding of the primary coil Details of primary coil Construction from side Construction from front

The primary coil also contains one more turn, which serves as safety for when I achieve bigger sparks than the secondary coil itself, they get grounded by this safety ring instead of short-circuiting other parts. It is kind of safety for device and user. This turn is connected onto the bottom of the secondary coil and are both grounded (ground is not shared between primary and secondary circuit, nor is it room heating). I use lightning rod as grounding, metal pole dig deep into ground would be suitable too.

Port for secondary coil

Secondary coil consists of PVC tube with 2000 manually winded turns (long night). height of the winding is 717 mm and diameter 112 mm. After winding I applied few layers of acrylate lacquer, for dielectric isolation as well as mechanical strength of turns (I didn't want to wind it again). I use my program for computations, however the calculator from Rayer's page can be used too.

V Applying of lacquer onto secondary winding
Secondary coil + capacitor Applying of lacquer onto secondary winding

Secondary circuit consists of already described coil and on the top of it is toroid made of tube from fume hood (with no connection yet, it just lays there). Toroid also serves as discharge electrode.

I made connector port on the bottom of the secondary coil for easier connection with the base. i made the port from piece of metal. I tightened it in the place with sticks and superglue and then poured it with dissolved PMMA, so it has its durability.

Port on the secondary coil

Used schema is the one from the right in the theory section. As a capacitor I chased many LP, from which I made parallel plate one. It has large loses, but handles the dispersion transformer (14 kV) well.

Making of the capacitor Completed capactior

I tried also making an MMC, but for 14 kV it needs quite a lot of pieces, and with only a part (correct capacity, insufficient voltage strength), I have to adjust spark gap accordingly, as to prevent MMC from detonating, so the final sparks are smaller than from the parallel plated one.


I manage to get quite nice sparks from it, approximately 15-20cm. More or less event the corona, however not standalone one, only when some grounding is near discharge electrode (e.g. my finger).

V Lighting neon tube - dark
V Discharges into my finger - dark
V Light bulb near tesla coil - dark
The whole complex