Modern Physics

Tesla Coil: History, How It Works, What Its For

A Tesla coil is a coil that acts as a high voltage and high frequency generator, invented by physicist Nikola Tesla (1856 – 1943), which he patented in 1891. Tesla magnetic induction did consider the possibility of transmitting electric energy without driver intervention.

The scientist and inventor’s idea was to create a device that could transfer electricity without the use of cables. However, the use of this machine is very inefficient, so it was soon abandoned for that purpose.

Even so, Tesla coils can still be found in some specific applications, like high voltage towers or physics experiments.


The coil was created by Tesla shortly after the emergence of the Hertz experiments. Tesla himself called it “an apparatus for transmitting electrical energy”. Tesla wanted to prove that electricity could be transmitted wirelessly.

In his Colorado Springs lab, Tesla had at his disposal a massive 16-meter coil connected to an antenna. The device was used to perform energy transmission experiments.

On one occasion, there was an accident caused by this coil in which we burned dynamos from a plant located 10 kilometers away. After the failure, electrical arcs were produced around the dynamo windings.

None of this discouraged Tesla, who continued to experiment with various coil designs, now known by the name.

How it works?

Tesla’s famous coil is one of many designs made by Nikola Tesla to transmit electricity wirelessly. The original versions were large and used high voltage and high current sources.

Today, of course, there are much smaller, compact, homemade designs that we’ll describe and explain in the next section.

A design based on the original versions of the Tesla coil is shown in the figure above. The wiring diagram in the previous figure can be divided into three sections.

Source (F)

The source consists of an AC generator and a high-gain transformer. The power supply output is usually between 10000V and 30000V.

First resonant circuit LC 1

It consists of a switch S known as “Spark Gap” or “Explosor”, which closes the circuit when a spark jumps between its ends. The LC 1 circuit also has a capacitor C1 and a coil L1 connected in series.

Second resonant circuit LC 2

The LC2 circuit consists of a coil L2 that has a rotation rate of approximately 100 to 1 relative to coil L1 and a capacitor C2. Capacitor C2 is connected to coil L2 through ground.

The L2 coil is typically an insulated enamel wire winding on a tube of non-conductive material such as ceramic, glass or plastic. Coil L1, although not shown in the diagram, is wound on coil L2.

Capacitor C2, like all capacitors, consists of two metal plates. In Tesla coils, one of the C2 plates is usually in the form of a spherical or toroidal dome and is connected in series to coil L2.

The other board C2 is the nearby environment, for example a metal plinth terminated in a sphere and grounded to close the circuit with the other end of L2, also grounded.

Mechanism of action

When a Tesla coil is put into operation, the high voltage source charges capacitor C1. When it reaches a high enough voltage, it blows a spark on suiche S (spark or explosion), closing the resonant circuit I.

Then capacitor C1 is discharged through coil L1, generating a variable magnetic field. This variable magnetic field also passes through coil L2 and induces an electromotive force in coil L2.

As L2 has about 100 turns more than L1, the electrical voltage at L2 is 100 times greater than at L1. And as in L1 the voltage is around 10 thousand volts, in L2 it will be 1 million volts.

The magnetic energy accumulated in L2 is transferred as electrical energy to the capacitor C2, which, when it reaches maximum voltage values ​​of around a million volts, ionizes the air, produces a spark and is abruptly discharged by the ground. Downloads occur between 100 and 150 times per second.

Circuit LC1 is called resonant because the energy accumulated in capacitor C1 passes to coil L1 and vice versa; that is, a swing occurs.

The same happens in the resonant circuit LC2, in which the magnetic energy from coil L2 is transferred as electrical energy to capacitor C2 and vice versa. That is, in the circuit there is an alternate current going and going.

The natural oscillation frequency in an LC circuit is

Tesla Coil: History, How It Works, What Its For 3

Resonance and mutual induction

When the power supplied to the LC circuits occurs at the same frequency as the circuit’s natural oscillation frequency, the energy transfer is optimal, producing maximum amplification in the circuit current. This phenomenon common to all oscillating systems is known as resonance .

The LC1 and LC2 circuits are magnetically coupled, another phenomenon called mutual induction .

In order for the energy transfer from circuit LC1 to LC2 and vice versa to be optimal, the natural oscillation frequencies of both circuits must match and also the frequency of the high voltage source.

This is achieved by adjusting the capacitance and inductance values ​​on both circuits so that the oscillation frequencies match the source frequency:

Tesla Coil: History, How It Works, What Its For 3

When this occurs, power from the source is efficiently transferred to circuit LC1 and from LC1 to LC2. In each oscillation cycle, the electrical and magnetic energy accumulated in each circuit increases.

When the electrical voltage at C2 is high enough, energy is released in the form of lightning by discharging C2 into the ground.

Uses of Tesla Coil

Tesla’s original idea in his experiments with these coils was to always find a way to transmit electrical energy over a long distance without wiring. However, the low efficiency of this method due to energy losses due to dispersion in the environment made it necessary to look for other means to transmit electrical energy. Today we continue to use wiring.

However, many of Nikola Tesla’s original ideas are still present in today’s wired transmission systems. For example, voltage booster transformers in electrical substations to transmit using cables with less losses and voltage reduction transformers for distribution in homes, were designed by Tesla.

Despite not having large-scale use, Tesla coils continue to be useful in the high-voltage electrical industry for testing insulating systems, towers and other electrical devices that must operate safely. They are also used in different programs to generate lightning and sparks, as well as in some physics experiments.

In high voltage experiments with large Tesla coils, it is important to take safety precautions. An example is the use of Faraday cages for the protection of bystanders and wire mesh clothing for artists who participate in concerts with these coils.

How to make a homemade Tesla coil?


In this miniature version of the Tesla coil, the high voltage alternating current source will not be used. Instead, the power source will be a 9V battery, as shown in the schematic in Figure 3.

The other difference with the original Tesla version is the use of a transistor. In our case, it will be the 2222A, which is a low-signal, but fast-response or high-frequency NPN transistor.

The circuit also has a switch S, a primary coil L1 of 3 turns and a secondary coil L2 of at least 275 turns, but it can also be between 300 and 400 turns.

The primary coil can be constructed with a common cable with plastic insulation, but the secondary one requires a thin cable covered with insulating varnish, typically used in windings. The winding can be done in a cardboard or plastic tube with a diameter between 3 and 4 cm.

Transistor use

It should be remembered that in Nikola Tesla’s time there were no transistors. In this case, the transistor replaces the “gap gap” or “explosor” of the original version. The transistor will be used as a gate that allows current to pass or not. For this, the transistor is polarized like this: the collector c on the positive terminal and the transmitter and on the negative terminal of the battery.

When base b is positively polarized, it allows the passage of current from the collector to the emitter and prevents it.

In our schematic, the base is connected to the battery positive, but a 22k ohm resistor is interleaved to limit excess current that can burn the transistor.

The circuit also shows an LED that may be red. Its function will be explained later.

At the free end of the secondary coil, L2 is placed a spherical metal, which can be constructed by coating a polystyrene ball or a pong pin ball with aluminum foil.

This spherical is the plate of a capacitor C, the other plate being the environment. This is what is known as parasitic capacity.

Tesla Mini Coil Operation

When switch S is closed, the base of the transistor is positively biased and the upper end of the primary coil is also positively biased. For a current passing through the primary coil to appear abruptly, it continues through the collector, exits through the emitter, and returns to the battery.

Related:   Thermal Expansion: Coefficient, Types and Exercises

This current grows from zero to a maximum value in a very short time, which is why it induces an electromotive force in the secondary coil. This produces a current that runs from the bottom of coil L2 to the base of the transistor. This current abruptly ceases the positive bias of the base so that current flow through the primary ceases.

In some versions, the LED is removed and the circuit works. However, putting it on improves the transistor base bias cutting efficiency.

What happens when current flows?

During the rapid current build-up cycle in the primary circuit, an electromotive force was induced in the secondary coil. As the winding ratio between primary and secondary is 3 to 275, the free end of coil L2 has a voltage of 825 V with respect to ground.

Due to the above, an intense electric field is produced in the sphere of the capacitor C capable of ionizing the low pressure gas of a neon tube or a fluorescent lamp that approaches the sphere C and accelerating the free electrons inside the tube to excite the atoms that produce the emission of light.

As current abruptly ceases through coil L1 and coil L2 is discharged through the air around C to the ground, the cycle is restarted.

The important thing about this type of circuit is that everything happens in a very short time, so you have a high frequency oscillator. In this type of circuit, the rapid switching or oscillation produced by the transistor is more important than the resonance phenomenon described in the previous section and referring to the original version of the Tesla coil.

Proposed experiments with mini Tesla coils

Once the Tesla mini coil is built, it is possible to experiment with it. Obviously, the lightning and sparks from the original versions will not occur.

However, with the help of a fluorescent lamp or a neon tube, we can see how the combined effect of the intense electric field generated in the capacitor at the end of the coil and the high oscillation frequency of this field make the lamp light. condenser ball.

The intense electric field ionizes the gas at low pressure inside the tube, leaving free electrons inside the gas. Thus, the high frequency of the circuit causes the free electrons inside the fluorescent tube to accelerate and excite the fluorescent powder adhered to the tube’s inner wall, causing it to emit light.

A luminous LED can also be approximated to sphere C, noting how it lights up even when the LED’s pins are not connected.

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