Super Simple Powerful Flyback Driver

First things first, a disclaimer: 
I am not responsible for any bodily damage or property damage that is a result of one building or using this circuit. High voltage can be dangerous or lethal when used incorrectly, and it is up to the user of the circuit to use high voltage safety rules and maintain their own safety at all times.

Flyback transformers are some of the most commonly used high voltage supplies by hobbyists- mainly due to their relative simplicity and how easy they are to find. Every single CRT (big, heavy kind) TV that people are throwing out now has one, the bigger the TV the bigger the transformer.
But unlike many transformers that you find in electronics, which are designed to handle standard 60 hertz AC power, and are step down transformers, flyback transformers operate at much higher frequencies- around 16 khz or sometimes higher. Many modern flybacks also put out DC voltage. Older flybacks had AC power output, making them the ones you want. The AC flybacks can operate at higher power, since there are no rectifier/multiplier circuits built into them like the DC flybacks.
That being said, DC flybacks are easier to get. So for this project they are recommended. Make sure that the flyback transformer you are using has an air gap. This means that the core is not a loop all the way through, but is rather shaped like a C with the gap being about a mm thick. Almost all modern flybacks have an air gap, so if a new on is being used, checking is not necessary.

Now, onto the circuit. I am going to say right now this is not my circuit. I found it on the 4hv.org forums here. This Instructable is intended to take much of the theory presented in the thread and condense it into a quick how-to for beginners. 

This circuit uses the venerable 2n3055 transistor, which is loved/hated by flyback driver builders. Loved because it is so easy to get (even Radioshack sells them) and hated because they generally stink. They are prone to burning out, sometimes spectacularly. That being said, this circuit works incredibly well with it.

The bad reputation for the 2n3055 comes from it being used in the usual single-transistor flyback driver, which puts a lot of stress on the transistor. In this circuit, a few components have been added which dramatically increase the power that the circuit can put out. The theory of the circuit is discussed later in the Instructable. 

This circuit uses very little in the way of parts. The parts needed are listed on the schematic on this page. I will say this though; Many of the parts can be substituted, and the circuit can be tuned by doing so:

The 470 ohm resistor can be changed for a different value. In my circuit, 450 ohms was used, made by wiring three 150 ohm resistors in series. This value is not really critical to the operation of the circuit, but to minimize heat the highest value that the circuit runs on should be used. 
The bottom resistor can be changed for more power. The smaller the value, the more power produced. In my circuit, 20 ohms was used by wiring two 10 ohm resistors in series. Making the value smaller means more heat though, which in turn means shorter amounts of time the circuit can be operated. 
The capacitor placed near the transistor (.47 uf) can be changed for different power outputs. Larger values will lead to more output amperage (and hotter arcs) but less voltage. I stuck with a .47 uf cap.
The number of feedback turns (the coil with three turns) can change power output as well. More turns gives more output  amperage, not voltage. 

What makes this circuit differ from the single transistor drivers that are more common is the addition of the diode and capacitor that is wired in parallel with the diode. The diode protects the transistor from surges of reverse polarity charge that tend to kill the transistor without it. The diode on the schematic does not have to be the same type however. I used a GI824 that I harvested from a TV. When choosing the diode, you want one that has a high voltage rating and fast switching. To figure out if your diode works, google BY500 and find a datasheet, and then find the data sheet for your diode and compare them. If it has a comparable or higher voltage and power rating and a comparable or faster switching time, you're all set to use it.
The capacitor in this circuit is what is key to the high output power. The transistor oscillates at a frequency set mainly by the primary and feedback coils working in tandem. the capacitor (even though it might not appear so) is wired across the primary coil, forming a capacitor-inductor (LC) circuit. LC circuits resonate at a certain frequency, and by tuning the circuit so the transitor osciallates at the frequency of the LC circuit, the amount of power output is greatly increased. LC circuit theory is the same theory as used by Tesla coils to produce their extreme power output. This circuit can be tuned by adjusting the capacitor value, and the number of primary/secondary turns.

This circuit does require a high wattage power supply, which is described next.

This part is up to the builder. For this circuit, a high power DC supply is needed that can deliver between 12 and 30 volts at 1 to however many amps you want. It's a good idea to overbuild the power supply so the circuit can get all the power it wants. That being said, if the circuit is built incorrectly and a power supply like this is used, stuff will go bang. But a regulated supply is not necessary for normal operation.

My circuit used a 300 watt transformer from an audio amplifier. The transformer has outputs for 2, 4, 15, 30, and 60 volts. The circuit requires about 12 to 18 volts for the 2n3055. I frequently operate mine using the 30 volt supply, but this is for short periods of time and with my transistor mounted on an overkill of a heatsink. At 15 volts, the circuit can be operated indefinitely, for after 30 minutes nothing is above room temperature. 

The Ac output from the transformer feeds into a 400 watt bridge rectifier mounted to a heatsink, and then through a 7800 uf capacitor rated to 70 volts in order to smooth out the voltage. Using similarly specked components, you can build your own power supply and have all the power you need.

Other power supplies that are viable are switch-mode power supplies, or SMPS's. These include laptop and car battery chargers, and the power supplies found in computer towers. These supplies often supply outputs of 12 volts at up to 10 amps. This is plenty for this circuit.

This is a very simple circuit to build. There are not any special assembly instructions, though it is a good idea to build it similarly to the way I did if you plan on playing around with it an tuning it. Everything is mounted on a piece of MDF, and the parts spread out to minimize crosstalk from wires close to each other and to allow for cooling. Everything is connected using heavy duty multi strand wire.  There are many pictures detailing different elements of the circuit, and if you are anything like me, often times pictures are more helpful than words.

One of the most important things about building this circuit is to heatsink the transistor. 2n3055's are called TO-3 packages, referring to the size and shape of the transistor package. You can order TO-3 heatsinks, but they are a little difficult to find. For mine, I used the heatsink from a computer processor with holes drilled in the flat side for the wires coming off the transistor to go through and out between the fins. The transistor is held in place with self tapping screws. It is important to remember to put thermal compound between the transistor and heatsink for proper heat conduction. The wires going to the flyback transformer also had alligator clips to clip onto the transformer in order to allow different transformers to be used with the circuit. This again is a good idea if you plan on having the circuit be as flexible as possible once constructed.

Another important aspect of this build is the windings on the flyback transformer. Enamel coated copper wire is best, but it is a good idea to add extra insulation between the core of the transformer and the windings. The core of the transformer can have sharp edges, and if they strip off some enamel the wires can short circuit. When winding my coils, I removed the metal clip holding the transformer halves together and wound one coil on each half, then put the halves back into the potted windings and reattached the clip. On some transformers, this wont be possible, and the wire has to be threaded between the coils and core. The coils have to be wound out of phase, which means that they run in opposite directions around the core.
Again, refer to the pictures for more detail.  

This is the best part of building anything. 
When using this circuit is is a good idea to not minipulate any of the wires unless they are attatched to a nonconductive rod. Also, it is a good idea to check the temperature of the transitors or resistors periodicly during operation, though the circuit should be shut off and unplugged while doing this. If any of the components feel warm with your finger near it, it is a good idea to leave the circuit unplugged and let the components cool completely. The capacitors can hold a dangerous charge, so always be careful when handling the circuit after use.
Also, wear rubber-soled shoes while using any high voltage circuit and only touch stuff with one hand if the circuit is in operation at the time. Make sure that before touching any components after the circuit had been in use, they are discharged to a grounded wire so you do not receive a shock. Also, do not attempt to tune the circuit while it is in operation. 

Many things can be done with this circuit, including using it has a high voltage DC power supply, using it to drive a Tesla Coil, a TEA leaser, melting salt, or just having fun with arcs. 

Go ahead and have fun with high voltage! Remember though to stay safe.