Tesla Coil

This is a high-voltage resonant transformer I built in 2007, in the summer following high school. It takes ordinary 120V AC power, and kicks it up to nearly 1 million volts, yielding some very interesting effects! This project is dedicated to my uncle, Tony Bowers (1948-2007), who helped inspire me to pursue a career in electrical engineering.

November 9, 2007

Sometime during the summer, I was bored and got this crazy idea to build a Tesla Coil!

My Uncle Tony was the guy who got me interested in electronics at a young age. He gave me my first electronics kit back when I was in second grade. I remember him telling me about Tesla one time when he came over for dinner on the weekend. He told me about how Tesla was the real genius behind the electric motor and the radio, but how, despite this, he never really got the credit or recognition he deserved, even though his AC system won out against Edison's DC system, and is what we use to get power to out homes today!

And then recently, I saw the movie The Prestige, which sort of "sparked" my interest in Tesla again. In it, a fictional form of the Tesla coil is the central plot device. But there were several other scenes that were kind of intriguing. In one particular one, Tesla lights up a lightbulb by just holding it. And in another, a field full of lightbulbs are lit wirelessly from miles away. So after watching the movie, I went to see if this had any truth to it. It turns out that this part was quite true! Those two scenes were based directly on actual inventions.

Upon further investigation, I came across the Tesla Coil design and decided to try and build a replica of Dr. Nikola Tesla's "Apparatus for Transmitting Electrical Energy" (Patent #1,119,732). Put simply, it is a resonant transformer, that converts the low frequency, low voltage AC power in your house, to high-voltage, high-frequency energy. The result of this is that it disperses into the air an immense, high-frequency electromagnetic field. This is shown by the presence of brilliant purple sparks emanating from the toroidal top of the contraption, as well as the fact that it causes unconnected fluorescent tubes to glow spontaneously from several feet away.

The schematic is actually pretty simple. 120C 60Hz AC power comes in and goes into a high-voltage transformer. In my case, I used a 15kV neon sign transformer. Then, this feeds into an LC-circuit that acts as a resonator. A bank of capacitors is constantly charging up and discharging through a spark gap. It flows through the primary coil at the bottom, and this electromagnetic wave travels up through the secondary coil at the top. The secondary coil is made from about 1000 turns of fine copper wire, and is attached to a toroidal top-load, from which the power is transmitted.

I began construction in August. My grandpa, who is a carpenter, helped me a great deal throughout the construction of the project. Here is the plywood base we built to house the components.

Then, I began work on wrapping the 1000 feet of 24 gauge copper wire around a 5 inch diameter PVC pipe. This took about three or four hours to do by hand!

When finished, the wrapped length was 19 inches and the total resistance was 26 ohms. I coated it with polyurethane varnish to insulate the windings. While that was drying, I got to work on the capacitor bank...

The capacitors used were fifteen 2000v, 0.1 mFd polypropylene film capacitors. They're set up in series to increase the voltage rating. I later added terminals so that I could change the capacitance level to tweak the performance. On the back are 10M ohm bleeder resistors so that these guys don't retain any charge after running the coil.

Next we put together the torus, which was made from a PVC endcap attached to a segment of aluminum dryer duct. There is a terminal made from a bit of copper tubing where the top of the secondary circuit hooks into it.

Next up was the spark gap. I used Richard Quick's design, which consists of a PVC cylinder with several copper tubes lined up inside it. This allows for a larger surface for the spark to discharge, which makes for better heat dissipation, and better spark "quenching" with the use of a fan that blows through the tube.

Next was the winding of the ~10 turns of 8 gauge copper wire that makes up the primary coil. It is in a conical form at an angle of 30 degrees.

Then of course, I needed a high-voltage transformer. I originally bought a 10kV 30ma NST from ebay, but that turned out to have some sort of internal protection circuit to prevent shorting, and as a result, it refused to operate when there was any strange load on it. Later, I bought an older, heavier duty 15kV 30ma NST from ebay, and this turned out to work great. (I eventually fried this one though, more on that later.)

And here is the whole thing put together:

After tweaking it a great deal, I finally got it to work pretty well. Here is a video of it in operation:

Here are some still images. This one shows the coil discharging to a grounded terminal:

These next two stills are low quality, but they show how the coil lights up an unconnected fluorescent bulb!

The coil was actually running pretty well, until an arc in the bottom hit an unshielded part of the NST power supply casing! This caused the power transformer to work intermittently. Note that for these initial tests, I didn't have a proper filter on the coil, which is probably why this arcing occurred. I found the same NST on eBay for pretty cheap, so not a huge loss.

At some point, I'd like to understand the actual theory behind the coil, rather than just randomly changing the capacitance and inductance and seeing what works. I suspect that once I find the mathematical model behind the coil's operation, that I could tune it and get the sparks to be 2-3 times longer what they are now. But for my first exploration into high voltage electricity, it isn't half bad.