Ignition Coil Driver

From the previous stage, it is now time to send alternating current to the ignition coil. An automobile ignition coil is a transformer that will step up 12 VAC to 10,000-50,000 VAC. They are reasonably cheap and available at Checker Auto Parts or your local automobile store. The 555 timer can only output about 200 mA of current, and we will need up to 10 amps. Thus, a circuit with a transistor must read the 555's output and switch on and off a larger amount of current accordingly.

The right side of the schematic shows the ignition coil driver. The ground must be connected with the ground from the oscillator, so then when the 555's output turns on, the MOSFET transistor will switch and there is a path from the source to the drain (S to D). The path from source to ground has a resistor in between for current limiting, so the ignition coil cannot exceed 12 amps of current. The value for R3 is a 1-ohm, 10W power resistor. The output from the drain connects to the coil inputs, and then to the positive lead of the power supply. A metal-oxide varistor is across the source and drain transistor connections to prevent against induced surges. The parts for this are:

• 276-149 Circuit Board
• 276-2072 MOSFET IRF510 transistor
• 271-132 1.0 ohm 10W power resistor
• 276-1368 Heat sink for transistor
• 276-568 Heavy Duty MOV Surge Protector

The transistor will heat significantly when switching heavy currents, so the heat sink is highly recommended. If you do not have a screw to fit it, you will need to buy the accompanying mounting hardware from RadioShack as well.

Upon completion of the next step, you will be able to read about how the ignition coil connects to this. Right now I am waiting for my cheap ignition coil source to come through.