Simplified Solid-State Tesla Coil: The Beauty of Resonance

Polish version is here

Warning: I do not recommend building the described device for individuals who lack experience with high-voltage electronic devices. A properly constructed solid-state Tesla coil poses minimal danger during operation. However, keep in mind that the temperature of the electric arc is extremely high and can cause burns. Any construction errors may result in electric shock (e.g., due to incorrect power supply connections) or thermal burns (due to overheating components). The author assumes no responsibility for any injuries, damages, or losses resulting from the construction or use of this device; proceed at your own risk.

What Is It?

In short, a Tesla coil is an air-core transformer capable of generating high voltage on the order of millions of volts. Both the primary and secondary windings are tuned to resonance, enabling highly efficient energy transfer between them. This transformer allows you to explore high-voltage phenomena in relatively safe conditions. For example, it can ionize and illuminate rarefied gases:

The creator of this high-voltage generator is Nikola Tesla (pictured below).

Source: http://upload.wikimedia.org/wikipedia/commons/7/79/Tesla_circa_1890.jpeg, accessed: 01/23/2015

Nikola Tesla was an engineer and inventor from Serbia, holding over 112 patents for various electrical devices. His most well-known inventions include the electric motor, AC generator, autotransformer, bicycle dynamo, radio, hydroelectric power plant, solar battery, disc turbine, fluorescent lamp, and the Tesla coil (a resonant high-voltage coil).

The Serbian scientist also pioneered the first remotely controlled devices using radio waves. Initially, Guglielmo Marconi was credited as the inventor of radio, but in 1943, the U.S. Supreme Court awarded the patent rights to Nikola Tesla. Unfortunately, this ruling came after Tesla’s death, leading to the widespread misconception that Marconi invented radio. Marconi himself admitted to using Tesla’s earlier work, which earned him the Nobel Prize in Physics in 1909—an award he did not rightfully deserve.

After initially collaborating with Thomas Edison, Tesla became embroiled in a dispute with him. The conflict arose from Edison’s theft of Tesla’s invention. Edison, a proponent of direct current (DC), initially dismissed alternating current (AC), which Tesla recognized for its potential to be transformed into different voltage and current levels—a feat impossible with DC alone. Edison launched a media campaign to discredit Tesla and exaggerate the dangers of AC. Ironically, Edison later adopted Tesla’s system without paying him the royalties he was owed.

Tesla extensively experimented with AC and its transformative properties. Among his achievements were various transformer designs, with the resonant transformer—later named the Tesla coil—standing out. Historical photographs depict his laboratory:

Source: http://upload.wikimedia.org/wikipedia/commons/e/e5/Tesla_colorado_adjusted.jpg, accessed: 01/23/2015

How to Build It?

There are many types of Tesla coils. This article describes the simplest and safest one: a highly simplified solid-state Tesla coil (SSTC).

This device has low efficiency and should be regarded as a scientific toy with educational value. Building more advanced and efficient versions is possible but requires greater knowledge and skills. The design presented here can be very useful for teaching physics and electronics.

While browsing international websites on high-voltage electronics, I came across the following schematic:

As you can see, the schematic is surprisingly simple and should be easy to build. The upper left corner shows the coil connections and parameters: my secondary coil has about 1,000 turns of enameled copper wire (0.3mm diameter ~0.012 inches) wound on a PVC tube. The two primary windings are made using standard wires wound on another, slightly wider PVC tube. All coils must be wound in the same direction. The secondary coil should fit inside the primary coil assembly.

Now, let’s discuss the key component: the transistor. The schematic specifies an SU169 transistor, which might be difficult to find. Don’t worry—you can use a BU208 transistor, commonly found in vintage black-and-white televisions from Eastern Europe. It looks like this:

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If the BU208 looks rugged, that’s because it is. Soviet-era electronics might lack refinement, but they were built to last, often under harsh conditions. This powerful transistor can handle driving our Tesla coil without issues.

During operation, the transistor may generate significant heat, so it’s best to mount it on an aluminum heatsink of appropriate size. I used an aluminum bracket combined with an additional heatsink equipped with a cooling fan.

You can build the circuit on a printed circuit board, but given the small number of components, I opted for a perforated phenolic board with drilled holes. The connections were made underneath the board:

Front view

On the right, you can see part of the heatsink with the cooling fan. The aluminum plate shows the transistor leads, which are screwed onto the plate from the other side:

Rear view

The entire assembly was mounted on a small wooden board. I also added a switching power supply from an old scanner, delivering 30V/1.5A (approximately 1.5A at 30V) to power the Tesla coil. Additionally, I used a voltage regulator to power the cooling fan.

Full assembly

Next, connect the coil wound according to the instructions. Here’s what mine looks like:

Coil

After assembling everything, power up the device. To test its operation, bring a fluorescent tube close to the coil—it should glow brightly even at a certain distance. The free terminal at the top of the coil will produce a faint purple corona discharge. Holding a metal object near this terminal will produce purple sparks (generally harmless to humans). Bringing various types of light bulbs near the coil will reveal beautiful discharges caused by gas ionization under strong electromagnetic fields. The discharge color depends on the gas inside the bulb. Below are photos of the effects I achieved (with various bulbs, except the last one, which is a neon lamp):

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A compact fluorescent lamp held in hand lights up at a distance of over 0.5m (~1.6 feet) from the coil. Below is a video demonstrating this fascinating device. Unfortunately, the image quality is somewhat low because the shots had to be taken in darkness to capture the gas glow inside the bulbs (the camera didn’t fully capture the actual glow color, which appeared more vivid in real life). The red light is from a darkroom lamp I used to see the setup during filming.

Enjoy experimenting with this fascinating project! :)

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