Before you are going to read the texts on my experiments sites, please have a look on this page, thanks.
At the beginning there has been a man: Nicola Tesla (if you want to know it: he lived from 1856 till 1943). I'm sure that you already have heard of his surname, as Tesla is the unit of measurement for the magnetic fluxdensity (this is the strength of a magnetic field). But one of the experiments of Nicola Tesla, which is much more important to us right now, has to do with a strange coil which should be called by his name later: the Tesla-coil.
First of all I will have to tell you something about the theoretical background of a tesla coil. If you are absolutely not interested in this, I am afraid that I cannot help you, it would be the best to skip this section. For the others: Let's go!
A tesla coil is in principle a big transformer with some special properties. At the beginning there is a "low" voltage of some ten thousand volt on the primary coil with just some few windings which induces a much higher voltage on the secondary side, the actual tesla coil with it's distinctive look and quite a lot of windings. This side produces the high voltage. This is just like a normal transformer but Nicola Tesla made the important difference.
The frequency of the current on the primary side is much higher than what you get out of your socket at home and it is chosen in such a way that it exactly fits to the proportions of the secondary coil. This big coil can be seen as in inductivity together with a tiny capacitor (which is the two ends of the coil). As such it forms a LC-oscillator with a typical frequency. If the frequency of the primary current is selected to match the resonance of the tesla coil, the efficiency of the whole apparatus improves a lot and through this resonance phenomenon, the tesla coil can produce those huge voltages.
Stays one problem: How to produce such a high frequency? It is created by a LC-oscillator itself. Therefore you have to build a capacitor that has together with the primary coil the right frequency to achieve resonance. (But keep in mind that this thing has to cope with some ten kV!) This capacitor is being charged through another transformer and a so called spark gap closes the circuit if a certain voltage is reached. During those tiny parts of a second where the arc closes the current through the spark gap, the tesla coil actually works. If everything it build correctly you can achieve several ignitions per second and enjoy the cool glow of your work and the lightnings created by the coil.
Together with a friend we started to build a tesla coil. The theory of a tesla coil is not easy but actually building one is even more complicated. We tried to calculate all the required parameters and wanted to build the parts to match those values but this turned out to be nearly impossible.
We started with a high voltage transformer out of an old TV. Those can create up to 24 kV which should be sufficient. Therefore we had to convert our 12 V DC input signal into AC with a little circuit we found on the Internet (it used some kind of especially expensive transistors - don't ask how many were roasted during our tests). Those twentyfourthousand volt had to be stored in a capacitor (ca. 20 nF) and those cannot be easily bought for little money. Well, be constructed our own capacitor. The principle was simple enough: You just put many layers of aluminum foil and clingwrap onto each other and isolate the whole thing with salad (no kidding) oil. The whole thing has then been put into a self-made box with Plexiglas windows (we wanted to see our baby). This was a oily work but finally it was done.
The spark gap was easy to make: Just put two nails or screws in such a way in a box that the size of the gap can be changed, that's all.
Well, one part to go: the tesla coil itself! The principle is again easy but it's a boring work. We had a PVC pipe 30 cm in length and with a diameter of 10 cm. On this we had to put over 1000 m of very fine coil wire, one winding exactly beside the other. The whole thing has then been fixed by a layer of paint.
And this was it; we didn't exactly meet or calculated requirements, but at least all parts were together.
We could finally start to put all parts together now. However even the place where the coil would stand had to be chosen wisely. At first, we haven't thought much about the place and chose a room with a TV standing near by, well, after a first test, we noticed some strange colored lines on the glass screen but the TV wasn't turned on! We were lucky, it wasn't broken, however we learned not to try this near a computer. ;-)
After some adjustments and tuning everything worked. That was the moment we worked for. By the way: The sparks itself aren't life threatening, as they are AC with a very high frequency. At such frequency a effect called "skin effect" prevents the current from going through your body, everything stays (as the name says) on the surface. So it won't kill you, but the point where the little lightnings hit your finger can still get quite hot, which could hurt a little.
And now, just for the fun of it, imagine the atmosphere: You are in a dark room and there is a deafening "boom" several times a second made by the spark gap. Before you there is the tesla coil with a round metallic plate on its top around which a plasma is glowing in a pale blue color. In your nose you can feel that acid smell of ozone created by all those discharges. You bring your hand near the plane and lightnings are coming towards you with a length of about 30 cm. The nearer you get the stronger the bolts are and slowly the pain in your fingers increases... Can there be something more beautiful?
One last remark: I'm sure that you know "Star Wars". Did you ever wanted to have such a lightsaber, too? No problem: Just bring a fluorescent tube within some meters of the tesla coil and it will begin to glow, ready to fight the final fight with Darth Vader.
Looking back, I tend to say that it has been worth the trouble. I will try to get some pictures the next time we fire our coil up.
I know that this report is weak on the technical side of a tesla coil. It hasn't been intended to serve as a construction manual, for this there are better sources to be found in the Internet. One good starting point could be the page of the friend I already mentioned above. Oh, and there is, of course, always the possibility to write a mail to one of us.