Ignition coil high voltage generator

Abstract

This high-voltage generator uses a car ignition coil along with a pulse generator circuit to generate high voltage pulses.

The Circuit

IC1B in the circuit (figure 1.1) is a square wave generator with an output frequency of approximately 300 Hz. R3 and D3 control the duty cycle of the square wave signal. The output signal of IC1B is buffered by IC1A and the transistor T1. The following transistor (T2) is a driver for the power transistor T3. T3 is a BUV48A with a maximum collector-emitter voltage of 1000 V and a maximum current of 15 A.
The zener-diodes D2, D4, D5 and D6 protect T3 against high voltage spikes from the primary winding of the ignition coil. If the voltage on the collector of T3 reaches 720 V, the zener diodes start to conduct. The resulting current into the base of T3 leads to a current flow through T3 and limit the voltage on it's collector.
The operating voltage of 6 to 15 volts is applied to terminals X1 and X2, the ignition coil is connected to X3 and X4.

A few thoughts on ignition coils

Ignition coils are used to power the spark plug in spark ignition engines. A four-stroke engine needs a spark every other revolution per cylinder. That means, a four-stroke engine at 5500 rpm will need an ignition frequency of 183 Hz for four cylinders and 367 Hz for eight cylinders. So the 300 Hz that has been chosen for this circuit is good for powering such ignition coils.
One could even have two or more coils in parallel to increase the output current. This is done in some sports cars.
Attached to the engine, the ignition coil has a more or less defined load that consists of the spark gap in the manifold and the spark gap of the spark plug. Therefore, the output voltage is limited by the spark. Without load, the ignition coil will produce a higher output voltage and the stress on the insulation is increased which can lead to internal breakdown and damages the coil. It is advisable not to run the coil without load for longer periods of time to enlarge its lifetime.

Operation

Discharge pictures

Some pictures showing the generated discharges.

Figure 1.2: Discharges	Figure 1.3: Discharges
Figure 1.4: Discharges