Re: How do Atom Computer Ignition modules work?

[legendre]

@All

I finally had some time to devote to understanding this system, and I believe I’ve figured out how the circuit is supposed to work.. correct me if I’m wrong on this, as I’m not much of a solid-state guy.

As a cycle begins, the module is "off" and presents a virtual open-circuit to the primary of the mag coil. Once the voltage across the coil rises to the level required to turn-on Q1 (which might be somewhere between 700mV and 1200mV being a Darlington device) current flows through R1 into the base of Q1 and Q1 turns ‘on’. Q1 has huge current gain, and it turns on very hard, which now presents a near short-circuit across the mag coil. The mag coil current now flows hard through Q1.

The voltage at the R2-R3 junction is too low to do anything with Q2, yet – it’s only 25% of the voltage across Q1, so Q2 remains ‘off’. As the cycle continues, current in the coil builds as does the voltage across the coil. Eventually, the voltage at the R2-R3 junction rises high enough (about 700mV) to turn-on Q2. When Q2 switches ‘on’ it shorts R1 to ground, starving the base of Q1 of current, causing Q1 to abruptly switch ‘off’. C1 has the dual effect of delaying the time at which Q2 turns ‘on’ and ensuring that Q2 remains on long enough to permit a full discharge of the coil.

When Q1 switches ‘off’, this has the same effect as opening the points – the coil current breaks, the magnetic field collapses, and a high-voltage is induced in the mag coil secondary, firing the plug. Just as with points, a second, lower voltage (but still several hundred volts) also appears across the mag coil primary – but the transorb diode takes care of that.

So.. changing the ratios of R2-R3 should alter the timing, as should changing the value of C1. Either should also effect spark quality (voltage and duration). As for R1, I’m not sure how critical its value is – it seems to be primarily there to limit the base current of Q1, lest it become damaged. it would seem that R1 should be the lowest possible value that keeps the base current within the design limits?