2N3055 driver
The one of the most popular, easiest and non-efficient converters for high voltage transformer ;-) It cointains only bipolar NPN type transistor, two resistors and HV transformer. Also, a DC power supply will be needed. A several dozen of watts and 12V DC output will be enough (the most siimply and efficient solution is to use main transformer with iron plates or toroidal one and rectify bridge and smoothing capacitor. This circuit has few disadvantages. Transistor receives high voltage spikes (a very short moments, where voltage has value higer few times than working value). Those spikes are unfortunately needed - voltage across primary is boosted by voltage of this spike and thanks to that we do have arcs, as they should be. Because of this reason transistor heats a lot and a good cooling is very suggested. Big heatsink and optionally fan will be suitable for this task. Or use more powerful transistor. Electric arcs, which were achieved with 2N3055 transistor, 27Ω and 240Ω resistors and computer power supply unit, were 1cm long, but I've heard of people achieving few centimeters. High voltage transformer that was used, I bought on Warsaw's stock exchange, where are many interesting things for hobbyist and I've paid something around 3 euro. His photo is located below at right side. After confronting with little problems, I've managed to force converter to work and get arcs that samples are shown below in photo and short film.
How it does work?
We're powering up power supply. The input voltage appears, so current starts to flow. Current firstly goes through 220ohm resistor, then through transistor's base, making this semiconductor conductive. At this moment, two things occurs simultaneously - the current from power supply starts to flow through primary coil and it causes decreasing base's current, causing transistor being non-conductive. But this takes some time, so primary's current creates a electromagnetic field in ferrite core. The field's intensity is proportional to current value, so it's alternating. According to Faraday's law of induction, alternating electromagnetic field induces current in secondary and feedback coils. Again, there two things are happening simultaneously. At secondary side, a high voltage impulse appears, at feedback side current flows through base and end the one cycle. This cycle, depending on parts, repeats by several or tens of thousands times per second. Also, there's high possibility that the fequency will be in hearing range (up to 24kHz) and you will hear annoying squeals.
First arc! Yeah! Download it [1.2MB]
One of the most common failures is a incorrect plugging feedback winding (the one that powers base, a B point in scheme). Generally transistors works in way if we need to open them, a voltage across base and emitter must be applied (in MOSFET's they are called accordingly gate and source), then again bipolars like 2N3055 needs required current. Emitter is connected to ground, so, for current flowing from base to emitter, base has to have positive potential. Depending on that how we plug feedback winding, this potential can be positive or negative. If it's negative, current won't flow through base and transistor won't open. In this case swapping feedback windind leads is enough to reverse polarity. Incorrect plugging shouldn't affect negatively on transistor's lifetime.
Results can be improved and it can be done by increasing supply voltage. It's important for current to be DC. If it has sinusoidal wave or pulse characteristic, converter can open in moments, when supply voltage has low value. Bridge rectify and capacitor with dozen of thousands microfarads (i.e. 5000µF) is enough to have quite smoot current like as baby's ass. But keep in mind that increasing supply voltage, you increase high voltage spikes and it causes transistor heating more. 2N3055 isn't a good one for high flies and its popularity is caused by probably low price and good accessibility. On certain webpage (look first page in links) I've found interesting comparing of transistors in driver's functionality context. I think it may be very useful, so I'm going to quote it:
| 2N3055 | 60V | 15A | 60V 15A Doesn't work worth jack |
| 2N3773 | 140V | 16A | Works OK, typical output of 7,000V |
| 2N5038 | 150V | 20A | One worked, for a second or two |
| NTE2308 | 400V | 12A | Worked great for 20 seconds, then got fried |
| NTE152 | 60V | 7A | Fried instantly |
| NTE284 | 180V | 16A | Works great, typical output of 18,000V |
| TIP120 | 60V | 5A | Fried instantly |
| TIP31 | 40V | 3A | Fried instantly |
| MJ21194 | 250V | 16A | Fried instantly |
| MJ15022 | 200V | 16A | Fried instantly |
| MJ15024 | 250V | 16A | Fried instantly |
| MJL1302A | 200V | 15A | Fried instantly |
| KSE13009 | 300V | 12A | From Fairchild, works grrrrreat!!! |
| MJE13009 | 400V | 12A | NPN 400V 12A from ON Semiconductors, seems to work well so far (probably the same "made in mexico" transistor as KSE13009). |
Thereform, it's a good idea to look for NTE284, KSE13009 and MJE13009. Unfortunately, from what I've seen in web, they're hard to get. At least, in my country. And if you find something, then probably you will notice that magazine's state is equals to zero. It's best to look in internet auctions or electronic stock-exchanges or buy from friend old-date electronic guy :) If someone find good source, I would be pleasant if he gave links - I will put them there. Czech KD503 will be suitable - from what I've readed, it works better than 2N3055, but is harder to find. Even there are people that claims them as artifacts and sells for much higher price than they're worth.