STEP 3: POWER SOURCE
A Tesla Coil requires high voltage to create corona discharges into the air. A transformer steps up fairly low voltages into high voltages. There are many different types of transformers but the two that are most commonly used are Neon Sign Transformers and Flyback Transformers. A NST is rated at about 15kVolts and 30-60mA and would probably be the easiest to use. However, an older version is needed (approximately 20 years old) because the newer ones have safety features that would automatically be triggered by the tesla coil. However, a good replacement is the flyback transformer. It is a little more difficult to utilize because you need to wind it and build a driver for it. This page explains exactly how to do that.
Begin by winding the flyback with magnet wire. You need to wind two coils, one for the primary and one for the feedback. The primary should have 10-15 windings and the feedback should have about 5. The next challenge is finding the ground pin out of the 10 on the flyback. You can find this pin by following these instructions. Once you have these you can connect it as follows:
Picture of 2n3055 transistor taken from http://www.digikey.ca/product-detail/en/2N3055G/2N3055GOS-ND/918206
As you can see, one end of the primary windings is attached to the positive end of the power source, while the other is attached to the 2n3055 transistor. This is a fairly inexpensive transistor that can be found at digikey.com. I would highly recommend getting at least a few because I fried at least 6 before I figured out the best way to run the coil.
The negative end of the power supply is attached to the diode and transistor. Here the diode is very important because it prevents any noise coming back from the coil which can fry the 2n3055 chip. I originally forgot to add in a diode, so that is why I fried so many of my chips. Also, putting the transistor in a heat sink is highly recommended beacuse overheating will also fry it. Here is my final flyback and driver circuit:
The parts of the flyback that are connected to the actual coil are the ground pin that you found earlier and the high voltage output(the one that looks like a suction cup). These two ends are attached to either end of the spark gap.
So how does it work?
The flyback is a special type of transformer that steps up a low voltage of around 12V to as high as 15kV. This is important because we need thousands of Volts to get sparks from the Tesla Coil. It is also notable that the newer flybacks, like the one I used, have an internal diode which makes the current that is outputted DC (Direct Current), not AC(Alternating Current). A Tesla Coil however can be operated by either DC or AC.