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  • Writer's pictureKristjan Salev


Updated: Sep 16, 2021

Tesla coil  makes lightning

Tesla coil is an electrical device that generates extremely high voltages. Since the voltage is very high, you can see how electrical sparks are emitted from the coil, just like miniature lightning arrows. It was first introduced by the famous inventor Nicola Tesla in 1891 when he was trying to develop the transmission of electrical power without wires. In 2005 Joe DiPrima taught Tesla coil to sing and now these music Tesla coils are already quite popular gadgets. From an earlier post "Plasma Sphere" we could read how a small Tesla coil inside this gas-filled sphere ionizes the gas and creates a wonderful light show, or even powers a light bulb near the sphere. In this post we will try to get an answer to the question, what are the main components of this device, and how does it work.

Tesla coil contains two circuits: a primary circuit and a secondary circuit. The main idea is to achieve resonance between the primary and secondary circuits, which generates a very high voltage in the secondary circuit. There are many articles on the internet, that use a good analogy with the swing. If you push the swing at exactly the right moment when the swing starts to move away from you, it is quite easy to add momentum to the swing and the oscillation amplitude of the swing quickly grows very large.

primary and secondary circuit resonance

Using the swing analogy, if the momentum is added at exactly the right moment and resonance is achieved between the two oscillating circuits, then the energy in the secondary circuit is constantly increasing. The potential energy ( E = mgh ) of a person on a swing is constantly increasing as the amplitude and therefore the height above the ground increases.

So, the Tesla coil has two electrical circuits, both consisting of an inductor and a capacitor. Capacitors are devices that store energy in an electric field and inductors are devices that store energy in a magnetic field. The capacitor in the primary circuit is charged by the alternating current transformer.

Tesla coil schematic

A capacitor consists of two parallel plates with a non - conductive material between them. When the primary circuit is connected to the high voltage transformer, the capacitor begins to charge. When the electric field and thus the voltage between the capacitor plates becomes very strong, the electrical neutrality of the air between the spark gap breaks down and an electric spark occurs between the electrodes. In essence, the spark gap is like a switch, that connects the primary and secondary circuits. In solid-state Tesla coils, the spark gap is replaced with more modern switches, like transistors or MOSFET.

If a spark occurs between the electrodes, the capacitor instantly discharges, causing a huge current to flow in the primary circuit. The electric current then passes through the primary inductor, which is an insulated wire with a small number of turns. When current passes through the inductor, a magnetic field is generated around the inductor (2).

Tesla coil primary circuit

The current flow in the circuit decreases, as the capacitor discharges. At the same time, the magnetic field around the inductor starts to decrease (the magnetic field changes). According to Faraday's law, a changing magnetic field generates a voltage across the inductor. This induced voltage allows the capacitor to recharge, but with a voltage of opposite polarity (3). Then the cycle starts again, but now the direction of the current is reversed (4). Such a circuit in which energy oscillates between the electric field of a capacitor and the magnetic field of an inductor is called an LC or resonant circuit. The oscillation occurs as long, as there is a spark between the electrodes, that closes the circuit and allows the current to flow back and forth.

The alternating current frequency is 50 Hz in Europe and 60 Hz in the United States, which means, that the current and voltage change 50 or 60 times per second (50 or 60 cycles per second). Depending on the size of the spark gap, the spark may occur several times during the cycle. Each time there is a spark between the electrodes and the primary circuit is closed, the energy between the capacitor and the inductor can oscillate tens or hundreds of thousands of times per second.

The secondary circuit also consists of an inductor and a capacitor. The toroid at the end of the inductor acts like a capacitor with respect to the ground. Thus, we can view the toroid as one plate of the capacitor and the ground as the other plate of the capacitor. Primary and secondary inductors are loosely coupled, they are not wound on an iron core as in conventional transformers, but there is a large air gap between them.

Tesla coil primary and secondary circuit resonance

As we learned before, when an electric current passes through the primary inductor, a magnetic field is generated around it. When this oscillating magnetic field passes through the secondary inductor, the voltage is also induced across the secondary inductor. The secondary inductor is much larger, tightly wound with wire. The more turns the secondary winding has in relation to the primary inductor, the larger the induced voltage in the secondary inductor. The energy in the secondary circuit oscillates between the capacitor and the inductor in the same way as in the primary circuit, they have the same oscillation frequency. At exactly the right time in the cycle, the magnetic field from the primary circuit transfers energy to the secondary circuit. But with each cycle primary circuit loses its energy, while the secondary circuit gains energy. When there is not enough energy in the primary circuit to keep the spark gap conducting, the spark goes out. The whole process starts again when the capacitor in the primary circuit is fully charged.

Miniature tesla coils generate less output voltage, usually 10 000 volts or more. Larger ones however can generate hundreds of thousands of volts. When the voltage is very high, the air gets ionized near the toroid and becomes electrically conductive. Miniature lightning arrows exit the surface of the toroid and try to find their way to the ground.

By switching the Tesla coil on and off a certain number of times per second, we can even make sound waves. This principle is used in musical Tesla coils and plasma speakers.

When the air gets ionized, it heats up and expands, generating pressure waves. Sound waves are nothing but the pressure waves in the air. More sound waves per unit of time produce a higher pitch and fewer sound waves produce a lower pitch.

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