1. Overview

Tesla Coil is a resonant transformer invented by Nikola Tesla in 1891 , used to produce ultra-high voltage, low current, and high frequency electricity. In particular, Tesla coils can produce brilliant arc effects, so thousands of electronics enthusiasts are still making them.

Normally, air is considered to be insulating. When the voltage across the electrodes is high enough, the molecules in the air are ionized to form various ions. At this time, the air is broken down and the air becomes a conductor. When current passes through, an arc is formed.

So how much voltage is needed to break down the air? 3KV/mm (this value changes with pressure, temperature, and humidity). In other words, if you want to generate an arc at a distance of 1cm, it will require 30KV. It can be seen how high the output voltage on the Tesla coil is. Of course you can say that this is nothing in front of the natural thunder and lightning that cuts through the sky.

2. Circuit composition

I found a relatively intuitive physical connection diagram of the Tesla coil on the Internet:

Figure 2-Tesla coil physical connection diagram

Corresponding The schematic diagram is as follows (the positions of the electrodes and capacitors have been adjusted from those in the actual picture):

Figure 3 - Tesla coil circuit schematic

The main components are:

• Power input: usually mains power, such as AC 110V 60Hz/220V 50Hz, etc.;

• Step-up transformer (transformer 1): By increasing the voltage through the ratio of turns, the voltage can be increased to thousands of volts. The working frequency is low (mains frequency 50~60Hz), usually with a magnetic core;

• Spark gap: consists of two electrodes with a short gap between them, described as "Spark Gap" in the figure ”;

• Resonant transformer (Transformer 2): It is composed of the primary coil (Primary Coil) and the secondary coil (Second Coil) in the picture. The so-called Tesla coil refers to this;

• Capacitor (Capacitor 1): plays the role of energy storage, and forms an LC resonant circuit with the primary coil (Primary Coil) in the resonant transformer;

You may think that in the resonant transformer The secondary coil (Second Coil) is open circuit and no current flows. But in fact, the parasitic capacitance of the coil, as well as the capacitance between the annular body electrode (Torus) at the top of the coil and the earth, combine to form the "equivalent capacitance" (capacitance 2), and when the voltage is high enough, it will break down the air , the current flows through the capacitor:

Figure 4-Special The "invisible" equivalent capacitance on the Tesla coil

So, the complete Tesla coil circuit diagram is as follows:

Figure 5 - Tesla circuit schematic with equivalent capacitance added

3. Working principle

(1) Spark Gap

To understand the Tesla coil, you must first understand the Spark Gap.

The spark gap is equivalent to a switch here:

• In the initial state, the transformer 1 pair of capacitors forms a charging circuit, as shown in the figure (a) below;

• When the capacitor voltage reaches a certain value, the cremation gap is turned on, which is equivalent to a short circuit. The capacitor and the primary coil of transformer 2 (Primary Coil) form an LC resonant circuit, as shown in the figure (b) below;



Figure 6 - Spark gap non-conduction (a) and conduction (b)

(2) LC resonant circuit

In (b) above, the capacitor and inductor form a classic LC resonant circuit. In short, the energy stored in the capacitor is gradually released and transferred to the inductor; then the energy in the inductor is gradually released and transferred to the capacitor, and the cycle repeats. The period of energy interaction is the resonant frequency (related to capacitance and inductance). Taking into account the inevitable resistance consumption on the circuit, the energy will gradually decrease.



Figure 7-LC resonant circuit

• Regarding the LC resonant circuit, you can check the previous article for a more detailed introduction.

When the energy decreases, the high voltage on the two electrodes of the spark gap is difficult to maintain, and the spark gap is disconnected and returns to the state in (a) above.

Note that through the values of L and C, the resonant frequency of the LC circuit is very high (hundreds of KHz). Combining the opening and closing of the spark gap and the oscillation of the LC resonant circuit, the schematic diagram of the voltage waveform on the capacitor is as follows:



Figure 8 - Schematic diagram of the waveform change of the capacitor

Compared with the resonance process of the LC circuit, the charging process of the capacitor is slower, which is why Figure 6 (a), (b) Slow Circuit and Fast Circuit are marked respectively.

Of course, compared to the 50/60Hz input power supply, the slow circuit is still very fast! So in one cycle of the input power, the slow circuit and the fast circuit occur many times as a large whole.

(3) Resonant transformer

Finally, the resonant transformer consists of a primary coil (Primary Coil) and a secondary coil (Second Coil). The primary coil also belongs to the above-mentioned LC resonant circuit.



Figure 9-Resonant Transformer

The working principles of resonant transformers and ordinary transformers (transformer 1 in Figure 3) are different. Ordinary transformers are based on the principle of electromagnetic induction, that is, the primary coil and the secondary coil are wound around the same magnetic core, and the two coils share For the same magnetic flux, the ratio of the voltages of the two coils is related to the ratio of the turns of the two coils:



Figure 10 - Ordinary Transformer

Resonant transformer is based on the principle of resonance (resonance). There is resonance in the field of sound waves. For example, a soprano singer can shatter a glass with her voice. This is because the resonant frequencies of the two are the same, and energy is maximized through sound waves to be transferred to the cup, causing it to vibrate and break. There is also resonance in the field of electromagnetic waves. If we look carefully at the resonant transformer in Figure 9, it actually contains two LC resonant circuits. If the two resonant frequencies are the same, the energy will be maximized for interaction.

Resonant transformers operate at high frequencies and usually do not have a magnetic core; the magnetic flux in the two coils is not the same, so the two coils do not need to be close together or aligned.

The formula for the voltage ratio between the primary coil and the secondary coil of the resonant transformer is as follows. It is related to the inductance of the two coils (affected by the number of turns, diameter, and length) and the quality factor Q of the resonant circuit (representing the selected frequency capability) and coupling coefficient K (affected by distance):



Figure 11 - The ratio of the voltage of the primary coil and the secondary coil of the resonant transformer

When we look at the physical picture of the Tesla coil in Figure 2, we can indeed see the number of turns of the two coils of the resonant transformer. A large ratio means a high boost.

The energy interaction of the resonant transformer is relatively complex. Energy can be transferred from the primary coil to the secondary coil, and also from the secondary coil to the primary coil. I have not studied it in depth, so I will not expand on it here, but you You can imagine that common RFID applications are based on this principle:



Figure 12-Energy transmission at the RFID resonant frequency

IV. Summary

Today we introduced the Tesla coil, which has two cores The output voltage of a resonant transformer composed of an LC circuit is so high that it can electrically breakdown the air.

The difficulty in making a Tesla coil is that the resonant frequencies of the two LC circuits must be consistent, which requires a lot of effort to debug the annular body electrode (Torus) on the resonant transformer.