Re: How do Atom Computer Ignition modules work?

[legendre]

Yay, success..

Absent a supply of +real+ D1071, I used a pair of ST 13007A transistors in a Darlington configuration. This is a common part used in switching supplies, motor controllers and even horizontal deflection circuits of small CRTs. It has a Vceb of 850V, Vceo of 400V, Ic of 8A (16A surge) and so far, no snubber diode has been needed. Darlington gain (hfe) is measuring at 1500 for small-signals – this is directly in line with calculated hfe (38 x 40). I also lack 1K5 resistors, so 1K3 are used instead. All resistors are 1/4W.

Anyway, just as expected, Q1 turns on hard, as soon as the coil voltage hits ~1.2V. When the coil voltage rises to ~2.4V (raising the base of Q2 to ~0.6V) Q2 switches on firmly, abruptly switching off Q1 and causing the coil to fire. For this exercise, I’m using a Bosch 6V ignition coil from a moped and a 12V / 1.5A DC bench supply. Coil is in the +ground+ circuit of the module / PSU loop, and the PSU (-) terminal is tied to earth ground.

There is also no cap in the circuit, yet, as it makes little sense at this point, using a manually-controlled DC supply. Once it’s being run on AC, the cap will become much more important.. it would seem to set a time constant allowing the coil voltage to rise +above+ the ~2.4V threshold before Q2’s base current starts to flow. This increases spark energy by permitting coil current to hit higher peaks before the spark is triggered.