TESLA COIL: HISTORY, HOW IT WORKS, WHAT IT IS FOR - PHYSICAL - 2024

The Tesla coil is a winding that functions as a high-voltage, high-frequency generator. It was invented by the physicist Nikola Tesla (1856 - 1943), who patented it in 1891.

Magnetic induction made Tesla think about the possibility of transmitting electrical energy without the intervention of conductors. Therefore, the idea of ​​the scientist and inventor was to create a device that would serve to transfer electricity without using cables. However, the use of this machine is very inefficient, so it ended up being abandoned shortly for this purpose.

Figure 1. Demonstration with the Tesla coil. Source: Pixabay.

Even so, Tesla coils can still be found with some specific applications, such as in pylons or in physics experiments.

History

The coil was created by Tesla shortly after Hertz's experiments came to light. Tesla himself called it "apparatus for transmitting electrical energy." Tesla wanted to prove that electricity could be transmitted without wires.

In his Colorado Springs laboratory, Tesla had at his disposal a huge 16-meter coil attached to an antenna. The device was used to conduct energy transmission experiments.

Experiment with Tesla coils.

On one occasion there was an accident caused by this coil in which dynamos from a power plant located 10 kilometers away were burned. As a result of the failure, electric arcs were produced around the windings of the dynamos.

None of that discouraged Tesla, who continued to experiment with numerous coil designs, which are now known by his name.

How does it work?

The famous Tesla coil is one of the many designs that Nikola Tesla made in order to transmit electricity without wires. The original versions were large in size and used high voltage and high current sources.

Naturally today there are much smaller, compact and homemade designs that we will describe and explain in the next section.

Figure 2. Schematic of the basic Tesla coil. Source: self made.

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

Source (F)

The source consists of an alternating current generator and a high gain transformer. The source output is typically between 10,000 V and 30,000 V.

First LC 1 resonant circuit

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 circuit 1 also has a capacitor C1 and a coil L1 connected in series.

Second resonant circuit LC 2

The LC circuit 2 consists of a coil L2 having a turn ratio of approximately 100 to 1 relative to coil L1 and a capacitor C2. Capacitor C2 connects to coil L2 through ground.

The L2 coil is usually a wire wound with an insulating enamel on a tube of non-conductive material such as ceramic, glass or plastic. Coil L1, although not shown like this 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 shape of a spherical or toroidal dome and is connected in series with the L2 coil.

The other board of C2 is the nearby environment, for example a metallic pedestal finished in a sphere and connected to ground to close the circuit with the other end of L2, also connected to ground.

Mechanism of action

When a Tesla coil is turned on, the high voltage source charges capacitor C1. When this reaches a sufficiently high voltage, it makes a spark jump in switch S (spark gap or explosor), closing the resonant circuit I.

Then the capacitor C1 discharges through the coil L1 generating a variable magnetic field. This variable magnetic field also passes through coil L2 and induces an electromotive force on coil L2.

Because L2 is about 100 turns longer than L1, the electrical voltage across L2 is 100 times greater than that across L1. And since in L1 the voltage is of the order of 10,000 volts, then 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 the order of a million volts ionizes the air, produces a spark and is discharged abruptly through the ground. Discharges occur between 100 and 150 times per second.

The LC1 circuit is called resonant because the accumulated energy in the capacitor C1 passes to the coil L1 and vice versa; that is, an oscillation occurs.

The same happens in the resonant circuit LC2, in which the magnetic energy of the coil L2 is transferred as electrical energy to the capacitor C2 and vice versa. That is to say, that in the circuit a round trip current is produced alternately.

The natural oscillation frequency in an LC circuit is

Resonance and mutual induction

When the energy supplied to the LC circuits occurs at the same frequency as the natural frequency of oscillation of the circuit, then the energy transfer is optimal, producing a 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 the LC1 circuit to the LC2 and vice versa to be optimal, the natural oscillation frequencies of both circuits must match, and they should also match the frequency of the high voltage source.

This is achieved by adjusting the capacitance and inductance values ​​in both circuits, so that the oscillation frequencies coincide with the source frequency:

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

When the electrical voltage across C2 is high enough, then energy is released in the form of lightning by discharging C2 to ground.

Tesla coil uses

Tesla's original idea in his experiments with these coils was always to find a way to transmit electrical energy over long distances without wiring.

However, the low efficiency of this method due to energy losses by dispersion through the environment made it necessary to look for other means to transmit electrical power energy. Today wiring is still used.

Plasma lamp, which helped develop Tesla's experiment.

However, many of Nikola Tesla's original ideas are still present in today's wired transmission systems. For example, step-up transformers in electrical substations for transmission over cables with fewer losses, and step-down transformers for distribution in homes, were devised 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 function safely. They are also used in various shows to generate lightning and sparks, as well as in some physics experiments.

It is important to take safety measures in high voltage experiments with large Tesla coils. An example is the use of Faraday cages for the protection of observers and metallic mesh suits for performers who participate in shows with these reels.

How to make a homemade Tesla coil?

Components

No high voltage AC source will be used in this miniature version of the Tesla coil. On the contrary, the power source will be a 9 V battery, as shown in the diagram in figure 3.

Figure 3. Schematic to build a mini Tesla coil. Source: self made.

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

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

The primary coil can be built with a common wire with plastic insulation, but the secondary coil requires a thin wire covered with insulating varnish, which is the one usually used in windings. The winding can be done on a cardboard or plastic tube that is between 3 and 4 cm in diameter.

Using the transistor

It should be remembered that in the time of Nikola Tesla there were no transistors. In this case the transistor replaces the "spark gap" or "explosor" of the original version. The transistor will be used as a gate that allows or not the passage of current. For this, the transistor is polarized as follows: the collector c to the positive terminal and the emitter e to the negative terminal of the battery.

When the base b has positive polarization, then it allows the passage of current from the collector to the emitter, and otherwise it prevents it.

In our scheme, the base is connected to the positive of the battery, but a 22 kilo ohm resistor is inserted, to limit the excess current that can burn the transistor.

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

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

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

How the Mini Tesla Coil Works

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. So a current abruptly appears that passes through the primary coil, continues through the collector, leaves the emitter, and returns to the battery.

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 goes from the bottom of the L2 coil to the base of the transistor. This current abruptly ceases the positive polarization of the base so that current flow through the primary stops.

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

What happens when the current circulates?

During the cycle of rapid current growth in the primary circuit an electromotive force was induced in the secondary coil. Since the ratio of turns 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 capacitor C capable of ionizing the gas at low pressure in a neon tube or fluorescent lamp that approaches the sphere C and accelerating the free electrons inside the tube. as to excite the atoms that produce the light emission.

As the current abruptly ceased through coil L1 and coil L2 discharged through the air surrounding C toward ground, the cycle is restarted.

The important point in this type of circuit is that everything happens in a very short time, so that you have a high frequency oscillator. In this type of circuit, the flutter or rapid 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 of the original versions will not be produced.

However, with the help of a fluorescent light bulb or a neon tube, we can observe how the combined effect of the intense electric field generated in the capacitor at the end of the coil and the high frequency of oscillation of that field, make the lamp light up just approaching the condenser sphere.

The strong electric field ionizes the low pressure gas within the tube, leaving free electrons within the gas. Thus, the high frequency of the circuit causes the free electrons within the fluorescent tube to accelerate and excite the fluorescent powder adhering to the inner wall of the tube, causing it to emit light.

You can also bring a luminous LED closer to sphere C, observing how it lights up even when the LED pins are not connected.

References

1. Blake, T. Tesla coil theory. Recovered from: tb3.com.
2. Burnett, R. Operation of the Tesla coil. Recovered from: richieburnett.co.uk.
3. Tippens, P. 2011. Physics: Concepts and Applications. 7th Edition. MacGraw Hill. 626-628.
4. University of Wisconsin-Madison. Tesla coil. Recovered from: wonders.physics.wisc.edu.
5. Wikiwand. Tesla coil. Recovered from: wikiwand.com.