Tesla Coil

Probably the most dangerous project we ever built in eLab, but still, also one of the funniest! The Tesla Coil, an air core electrical resonant transformer, invented by Nikola Tesla at the end of the XIX century, able to generate high voltages at high frequencies, resulting in electrical discharges just like small lightning.


First things first, there are many different types of Tesla Coils, first you have the Spark Gap Tesla Coils or SGTC, as the name says, these are based on a spark gap switching mechanism, they are the easiest to build but require a high voltage power supply; then there are the Solid State Tesla Coils or SSTC which rely on transistors such as MOSFETs or IGBTs to handle the switching part, these are the ones that can be turned into musical Tesla Coils, however they might be quite complex to design; and then there are others like the Vacuum Tube Tesla Coils or VTTC which use the good old vacuum tubes as a mean of generating the oscillation needed for the circuit.

The one we are building is an NST Spark Gap Tesla Coil, NST stands for Neon Sign Transformer, which will be our high voltage power source.

There are also a lot of different schematics, but we’ll be following this simple one:


The goal is basically to make sure that the resonant frequency is the same on both the primary and the secondary side. It’s just an LC circuit with L1 and C1 as well as L2 and C2.

Next we need our High Voltage power source, it should have a voltage between 9kV and 15kV. Easiest thing to find for this is an Neon Sign Transformer.

We used a 10kV 30mA NST:


Next thing we need is a capacitor that would withstand high voltages. These are usually expensive, but I can build one completely from scratch. Using the principle of the Leyden Jar, the original form of the capacitor, which can be easily reproduced with glass bottles.

A little bit of water, salt, oil and bottles inside a bucket, and we’re done! The glass bottles would serve as the dielectric, the salt water inside the bottles would be one electrode and the salt water outside the other electrode. The oil is just to prevent arcs. The more bottles in parallel, the more capacity we’ll obtain:


Next we need the switching mechanism, for this we’ll be using a simple static Spark Gap. This can be basically two electrodes apart form each other at the right distance. Some people even use rotational spark gaps, to control the switching frequency with the rotational speed of the spark gap itself. In this case we’re going for something simple, so two bolts should be enough:


Moving on to the secondary part of the circuit, we have the secondary coil. These are usually a single layer and long coil, so we used a PVC pipe and winded it with some enamelled wire to obtain the desired inductance:


Still on the secondary side, we’re gonna need a discharge terminal which serves as a secondary capacitor as well. Usually Tesla Coils have spheres or toroids, so we’re going for the toroid design. With some aluminium ducting, metal discs and tape, and we have our toroid:


Returning on the primary side, now we need a primary coil. For this the best material to use is copper tubing:


Now it’s time to put everything together and connect the primary coil to the homemade capacitor:


After that the Tesla Coil is ready for a test run:


And it’s working! It’s always mesmerizing to watch it work, it allows for stunning photos:


Here’s a small video by pplware:

And that’s it! This was just a small summary of the project. If you want to know more details about the design, calculations and construction, you can read the full article on my blog.

Be very careful while working with High Voltage, it can kill you! If you don’t know how to work with it safely, then you shouldn’t even try!

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