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For the last several weekends, I’ve been working on building my first spark gap Tesla coil from the ground-up. After many long nights and blown capacitors, it finally worked!

This is the story of how I came to build one of these remarkable, expensive, loud, dangerous, and incomparably cool high voltage toys.

WARNING: If you find yourself bitten by the Tesla bug, you might be in a hurry to throw together some parts to see the magic lightning. You should know that there are at least a dozen ways in which you can hurt, burn, blind, deafen, or kill yourself, your friends, or your neighbors. Read all you can about Tesla coil safety before you decide whether to attempt to build one of these things for yourself. Don’t say I didn’t warn you, but feel free to tell ‘em I sent you.

Pix, movies, and more after the jump.

A what coil?

The Tesla coil is a legendary gadget found in physics labs and at geek gatherings and art events all over the world. It is named after its inventor, Nikola Tesla (1856-1943). Tesla is considered by some to be one of the most brilliant inventors of all time. He had a lifetime of incredible achievements: his experiments and inventions led to the AC motor, polyphase power, radio communications, long distance AC transmission, radar, remote control, and dozens of other fundamental technologies we take for granted every day.

He is also generally considered to be the archetypal mad scientist. Some of his experiences as a young boy seem to indicate that he had an eidetic (photographic) memory, and many of his writings have a grandiose, almost mystical tone to them. I have the impression that he was the sort of man who perpetually chased his own grand unification theory that would harness the forces of the entire Universe for the betterment of all mankind. Fortunately for us, he was the “benevolent” variety of mad scientist. Once a millionaire, he died destitute and alone in a hotel room in New York at the age of 86.

His coil was developed to investigate the properties of electricity at very high voltages, on the order of a couple of million volts. He ultimately wanted to develop a global system of enormous Tesla coils that would transmit information and power globally for free without wires, for the universal improvement of mankind. If he had succeeded, TeslaNet would have been the first global Internet– in 1902, with power included!

There are many great stories and legends floating around about what Tesla did in fact achieve during his lifetime. Unfortunately, there is also quite a bit of conflicting information about how to go about actually building one of his famous coils. At the end of this article I have listed a bunch of resources that I have found useful in designing and building my first coil project.

Isn’t it dangerous?

Oh my yes. Electricity behaves in some very unusual ways at a couple of hundred thousand volts. While the lightning itself is obviously a potential hazard, there are many other dangers that are often overlooked. Some of these hazards include RF burns, noxious gasses, induced currents, fire, static discharge, and of course everybody’s favorite, electrocution. In addition, coils have an amazing ability to destroy expensive electronics, such as cell phones, cameras, computers, and test equipment.

Read all you can about Tesla coil safety before you construct your own coil, and use caution and good sense when it is operating.

Basic theory

You can think of a Tesla coil as a resonant step-up transformer. You supply it with low voltage at high current, and it spits out very high voltage at low current. The basic spark gap Tesla coil consists of six major components:

A power supply. The coil takes a supply of a few thousand volts and steps it up to several times that. I used a donated neon sign transformer for my power supply. It provides 9000 VAC at 60 mA.


A capacitor bank. The power supply charges a capacitor bank, which should be sized to closely match the characteristics of the supply. I ended up with eight of these Russian 2000 nF 10 kV capacitors in parallel. The cap bank quickly builds up energy which is then released through…


A spark gap. The gap acts as a high voltage switch, allowing energy to transfer from the capacitor bank to the primary coil. It operates 120 times per second (twice per AC cycle). It gets quite hot, releasing very bright light (including UV), lots of noxious gasses, and a ton of noise. The magnet is in place to quickly push the hot ions of the spark away from the gap, breaking the connection in time for the next cycle. This is called “quenching the gap”. Energy flows through the gap to…


The primary coil. This coil is made of eleven turns of 1/4″ soft copper tubing, with 1/4″ spacing between the turns. The red material is an old plastic cutting board. The power on the primary coil tends to induce a current in…


The secondary coil. This was made from a piece of 4″ PVC tube, wrapped with about 950 turns of 24 AWG magnet wire. The wire was wound by hand and then sprayed with a polyurethane sealant to hold it in place. Finally, the secondary terminates at…


A silly tinfoil helmet (also known as the top load). This provides some capacitance, and is the discharge terminal where the streamers get their big start on a nanosecond journey to oblivion.


That’s about it for the major components. Of course, the devil is in the details: you’ll also want a suitable table to hold all of the stuff, structural supports, a really good grounding wire, a grounding ring to attempt to catch errant strikes before they hit the primary, and other miscellaneous bits. I would estimate that over the course of the last month, about $500 and 40 hours was spent on Tesla coil R&D.

Designing the coil

One invaluable resource I found for designing my first coil is the incredible Deep Fried Neon site. Rather than providing step-by-step instructions, DFN attempts to put you into the proper mindset to come up with your own coil design. Their online calculating tools are indispensable, and help let you play “what-if” games based on the materials you have to work with and what you can find on eBay. Using their guide, I came up with the following values:

• The neon sign transformer provides 9 kV at 60 mA, for a total power of 540 Watts. The theoretical maximum spark length would be about 40″ (about 1 meter), assuming dry air at sea level and everything in perfect tune.
• The matching capacitor should be 0.0176 uF. I tried a bunch of unusual capacitor designs, but in the end I used eight of the above 2000 nF 10 kV capacitors in parallel. I screwed them together between two pieces of aluminum bar to make a nice, compact capacitor bar.
• The secondary coil was made from 4″ PVC wrapped with 942 feet of 24 AWG magnet wire. The inductance should be around 15 mH, with an internal capacitance of 7.7 pF.
• The top load was made from a piece of 4″ diameter vent hose, two pie pans, and some aluminum tape. Its capacitance should be roughly 16 pF.
• The primary coil needs to be somewhere from 18 to 25 uH, depending on how accurately I built the rest of the coil. That translates to roughly 8 turns of 1/4″ copper tube, with 1/4″ between each turn. You can set the number of turns using this handy clamp.
• Depending on the placement of the tuning clamp, the coil should resonate somewhere from 240 to 284 kHz.

Construction

As with all DIY engineering, implementation is the beautiful tension between what would be ideal, what you can afford, and whatever is actually available.

One lesson I’ve learned on this project is that “good enough” is fine, but “almost good enough” can be disastrous. In trying different capacitor designs, I destroyed five different capacitor banks before I realized the importance of quenching the gap. You can easily make (very bad) loud sparks by setting the gap size too wide, or by accidentally providing a good conductive path anywhere near the cap bank.

You should spend quite a bit of time considering how to make a good RF ground independent of your mains power. I use a large battery clamp on very heavy wire fixed to a large pole in the ground, such as a chain link fence post. Also consider the routing of your cables and pipe. Keep high voltage conductors (from the NST, primary, and secondary) far away from other connections (like the ground and mains lines).

If you are in any doubt about the quality of a connection or component, either upgrade the component itself or find a new way to use it.

In action

Stupid things you shouldn’t do

Of course, fluorescent and neon bulbs will glow in the presence of a high voltage field, even without being directly struck by a streamer. One completely unsafe and idiotic thing you shouldn’t do is to ground one end of a fluorescent tube and hold the other end close to the top load. It will light up twice as bright, and you will have several hundred thousand volts flowing through your hands.

You can see in the next pic that the glass isn’t really much of an insulator here.

What if the ground wire comes loose? Or if your shoes aren’t really all that insulating? Or if a streamer strikes a wire carrying mains voltage, creating a low resistance path between the grid power and ground through you? That could be incredibly bad for you and your next of kin, which is why you should never, ever do this. No matter how cool it looks.

What’s next?

For my first ever excursion into the land of coiling, I think I’ve had a very successful run. This coil isn’t terribly different from other coils made in the last hundred years, but I have learned a lot along the way. Now that solid state modulated coils are all the rage, I want to try something different for Tesla 2.0…

Links

• Tesla Coil Safety from Pupman.com
• Deep Fried Neon for calculations and wiring advice
• Wikipedia on Nikola Tesla and the Tesla coil: tons of links
• Quenching Spark Gaps
• Tesla’s Colorado Springs Notes from 1899 to 1900. This is an excellent book, which really gets you inside the mind of the man who invented these crazy things.
• Tesla Downunder. Even more stupid things you really shouldn’t do with electricity.

Special thanks

Thanks to all of the folks at the PNA who made this long-term project possible. Happy coiling!