How Does a Tesla Coil Work

What is a Tesla Coil

Tesla coil is an amazing device that can create very high voltages. Its purpose is to create bolts of artificial lightning. You may have seen them in real life, in a museum or art display. Or perhaps you saw one on TV, or in the movies.  We wrote this document for those who wonder how a Tesla coil works. While there are many variations, they are all a type of transformer.

A Tesla coil producing a large artificial lightning bolt.
A Tesla coil makes lightning bolts.
A diagram of a standard transformer.
A basic transformer


A transformer is a device that can either increase or reduce voltage. They work by magnetically coupling 2 coils together. These coils are referred to as the primary coil and the secondary coil. Simply put, when 2 coils are close together, they can transfer energy between them. In a typical transformer, the ratio of turns determines the output voltage. 

Not so for the Tesla coil. The secondary coil has resonant properties. Because of this, it achieves much higher voltages. The main property of a coil is inductance. The large surface area of the secondary coil has capacitance as well. When a circuit has both inductance and capacitance, it is said to be resonant.


A resonant circuit stores energy. You can think of it as pushing someone on a swing set. By matching their rate, you are putting energy into their forward momentum. With each push, they swing higher and higher. The same can be said about how a Tesla coil works. With each push at the coil’s frequency, the voltage becomes higher.

An animation of a resonant circuit.
A diagram of a Tesla coil

Tesla Coil Parts

Now that we’ve gone over the basics, let’s look at a typical Tesla coil diagram. While there are several variations, most have 3 main pieces in common. We already discussed the 2 coils, named the primary and secondary. Next, let’s take a moment to discuss the topload. The topload is the large torus shape at the top of the Tesla coil. It has two main purposes.

First, it provides a nicely shaped electric field. This protects the machine from arcs and flash-overs. Sparks like to form where there are sharp points or edges. To combat this, the large smooth surface of the topload helps prevents sparks from forming. More basic than that, it just moves the spark further from the secondary coil. 

Second, the topload is a capacitor. The size of the topload determines its capacitance. As a result, we adjust this value for several electronic reasons. For one, it can help us lower the frequency of the coil. This can come in handy if we want to use lower frequency transistors. In general, all Tesla coil parameters are chosen to enhance performance.

By adding a breakout point, we can control where the spark forms. While this piece isn’t necessary, it helps direct the sparks. This is an important aspect of Tesla coil safety. If you omit the breakout point, the spark will form in random spots on the topload.

Types of Coils

Now that we’ve learned what a Tesla coil is, let’s go over the two main types. The original Tesla coil, like those made by Nikola Tesla, used a spark gap. Spark gap coils were the main type of homemade Tesla coil for decades. Solid state coils are the modern version. Both types are popular with hobbyists today.

A Tesla coil being built.
A diagram of a spark gap Tesla coil
Spark Gap Tesla Coil Diagram

Spark Gap Tesla Coils

A spark gap in a Tesla coil is two electrodes positioned close to each other. You can think of it as a switch. A transformer is used to charge a capacitor to a high voltage. Eventually, the voltage becomes so high that an arc forms in the gap. This allows current to flow into the primary circuit. As the energy transfer is complete, the arc extinguishes and the process repeats.

Solid State Tesla Coils

A solid state Tesla coil replaces the spark gap with modern technology. Transistors, MOSFETs, IGBTs, are all examples of modern switches. These machines are typically much more efficient and reliable than their predecessors. Not to mention, their capabilities are vast.

The solid state machines are capable of many things. Because of their ability to use microcontrollers, a wide variety of spark effects are possible. Even so, Tesla coil modes fall into two main categories.

A CAD rendering of an H Bridge
H Bridge for DRSSTC
A simulation of continuous wave mode.

Continuous Wave Mode

CW mode, as the name suggests, operates at 100% duty cycle. If you look at the waveform on a scope, it just looks like an infinite sin wave. This mode makes a mostly silent and bushy spark. This is also the mode used to make high-quality audio sounds. They accomplish this by using various forms of modulation.

Interrupted Mode

In interrupted mode, the high-frequency signal operates in short bursts. By shortening these bursts, higher primary currents are achieved. Because of this, we can produce very long sparks with less input power.

A DRSSTC waveform depicting 2 bursts.
A schematic of a solid state Tesla coil.
SSTC schematic

Single Resonant Coils

This is the most basic type of coil.  It is often just called an SSTC. The term “single resonant”, refers to the resonance of the secondary. A solid state driver is usually coupled to the primary with a capacitor. The chosen capacitor value is meant to avoid primary resonance. 

This is how a Tesla coil works using the most basic techniques.

Class E Tesla Coils

In this type of coil, the primary capacitor and coil are resonant. Class E circuits are common in commercial RF amplifiers. Many early solid state coils were based on these circuits. They are a nice choice because they only use a single transistor. The primary and secondary tuning is slightly mismatched to achieve ideal switching attributes. This sets it apart from other coupled tuned circuits. Their main pitfall is that the changing spark load makes them hard to calibrate.

A schematic of a class E Tesla coil
Class E Tesla coil circuit
A schematic of a DRSSTC
DRSSTC schematic

Dual Resonant Solid State Tesla Coils

A dual resonant coil is most often called a DRSSTC. These machines use a half or full bridge to drive a series tuned primary circuit. These are the most powerful variety. The tuned primary circuit makes it possible to put much more energy in. DRSSTCs are the foundation for more complex varieties, such as QCW Tesla coils. We will explore that subject more in-depth in a later article.

Come Back Soon!

These machines are always improving. We intend to update this document as the tech and our understanding of them evolve. We hope it has shed a little light on the subject of how a Tesla coil works. If you would like to see something covered in more detail, please contact us!