I am trying to build a spark generator using an ignition coil. I am generating a square wave(ish) signal using a 555 timer and switching (@
500Hz) a transistor (2N3055) with the ignition coils primary connected to the 12V and collector of the transistor. My problem is that I am only getting sparks of a mm or less. I need my sparks to be larger to jump the gap of a spark plug. I have tried the ignition coil and transistor interchanged so the collector is connected to +12V, emitter is connected to the primary one end and the other end of the primary is connected to the GND. I have tried a 2.2U capacitor across the primary but still no effect. Can any one recommend anything?
You need to allow the primary current to build up to its maximum (dwell time). The spark is produced by breaking the primary currnet and letting the primary voltage rise to a high value so you need a transistor that will withstand it (not a 2n3055) probably easier to use a high voltage (1200v) igbt or an avalance rated mosfet.
Get yourself a transistor that's rated to stand 350 or 400 volts. You'll never get anywhere with a 2n3055.
Mosfets and IGBT's are pretty convenient because you don't need that heavy base current a bipolar power transistor takes in an application like yours that has high collector curent. See, bipolar transistors with high voltage ratings (which you need for your circuit) have low gain. It might take an amp of base current. Running that through a power resistor from a twelve volt supply is going to waste a lot of your battery power and get really hot. Plus, a bare 555 won't sink that much current; you would need another stage between the timer and the power transistor to shunt the current to ground and turn your transistor off. You can avoid using a power resistor by using a high voltage PNP power transistor with the emitter connected to the negative coil terminal, the collector grounded, and the base connected to a mechanical switch or a timer circuit that can sink a lot of current. You could use the same method with a NPN connected at the positive coil terminal. But a
555 won't sink enough for any of these methods.
Probably best for your application is a coil driver IGBT. You can use Google or go to the Digikey web site and search for coil driver IGBT's. With a mosfet or IGBT, you can drive it directly from the 555. I suggested Digikey because of their "parts search" feature, which you can find on their site map. Scroll down to the "discretes" heading, find IGBT's, then search for coil driver. You can use a 1 or 2 k pullup resistor on the gate, and use the 555 to ground it out.
Now that I think about it you don't need a pullup resistor. You can drive an IGBT or mosfet directly from the 555 (probably through a small resistor, like 5 or 10 ohms).
Do a google search for "ignition coil driver" and you'll find lots of examples. Evidently a lot of people have had success using a 555 driving a
2n3055. Although a power mosfet would be a better choice. If it's not already, make the 555 frequency and duty cycle adjustable. Some of links from the google search will show you how to do that. Optimum pulse duration and repition rate will depend on the particular coil you are using. Also be sure to use a good sized heatsink on the power transistor, especially if you stick with the 2n3055. Don't try to drive the 2n3055 directly from a cmos 555, it can't provide enough current to drive it. CMOS version -10/+50mA, bipolar version +-200mA. If all you have is the cmos version then definately switch to power mosfet or insert some kind of higher current driver between the 555 and the 2n3055.
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