At the end of the last century (1999) when I was finishing my studies in electrical engineering I was fortunate to know the works of the most incredible engineering genius Nikola Tesla, but he was never sufficiently recognized. There are innumerable contributions to the progress of humanity, especially in electrical engineering, among others nothing less than alternating current and radio.

Nikola Tesla, a genius out of his time

I have always felt great interest in electricity and, naturally, phenomena related to high voltages such as lightning strikes in storms have always seemed fascinating to me and the possibility of experimenting with them in a more or less controlled manner appeared before me thanks to the Tesla coil, a device capable of generating voltages of the order of one million volts without the need for equipment of industrial proportions. So I decided to build my own coil, but how does this device work and what are its construction parts? What risks entails its construction and what precautions should be applied for a reasonably safe operation? The answers to these questions were obtained based on research and experimentation during the construction of the three or four models that I operated.

One of the few photos I took of my 1999 Tesla coil (analog camera with manual exposure)

A classic Tesla coil (today there are more evolved versions that incorporate power semiconductors and digital control circuits) is made up of relatively few elements. Basically these are two electromagnetically coupled LC oscillators that play the role of the primary and secondary circuits of a transformer. However, unlike a conventional transformer, it has no ferromagnetic core. If it had it, the enormous tensions with which one works would make the task of obtaining adequate electrical insulation between windings very complicated, in addition to requiring a considerable size to avoid the magnetic saturation of the core.

Electrical schematics of a clasic Tesla coil

A conventional 220V / 12kV medium voltage transformer powers the system by charging the capacitor C1. When the voltage in it reaches the sufficient value, the GAP conducts, which is no more than two electrodes separated by a certain distance by air. The air has a dielectric strength, in theory, of about 30kV per centimeter, but in practice due to humidity among other factors it is reduced to about 10kV / cm. This means that if the electrodes are 1cm apart when the voltage between them exceeds 10kV an electric arc will form, then the GAP behaves as a closed circuit and causing the accumulated charge in the capacitor C1 to circulate through the primary coil L1, generating an alternating current with a specific frequency imposed by the values ​​of L1 and C1 called natural frequency.

The electromagnetic field generated by the primary induces current in the secondary, formed by the coil L2 and the capacitor C2 which also has its own oscillation frequency. When the natural frequency of the primary is equal to that of the secondary, the phenomenon of resonance occurs that makes the transmission of energy between the two windings optimal even without the presence of a core that confines the magnetic flux.

The huge electric field ionizes the air forming streamers.

As the number of turns of the secondary winding is much greater than that of the primary winding (although it really is more a matter of difference in inductances) a voltage is generated much higher than that applied to the primary, which is already important (10kV – 20kV) , so that the toroidal terminal that culminates the winding (it is the element C2 of the scheme) accumulates tensions of hundreds of kilovolts that ionize the surrounding air and make their way forming what is known as streamers or channels (we would say that they are lightning…) . When a streamer reaches land a very bright arc is formed leading an important current, this is already a lightning bolt!

Ground arc (left terminal), where a halo of ionized air is observed. Approx 500kV. The primary winding in the form of an inverted cone with 4-5 turns and the secondary one, vertical cylindrical with about 1000 turns, as well as the toroidal discharge terminal are also appreciated.