How does this work

[recoil]

After been told of an electronic replacement for points in lawnmowers motors that were great for getting a higher performance, i was eventually able to track down and buy this nos unit. My question to anyone with an electronic knowledge is how does it work? The only wire, besides the unit being earthed is attached to the wire that once run to the points. What i understand is the points through opening and closing make and brake the power to the coil, at the right time according to the cam they run on. There for without the points how can the timing be right? and how could it be adjustable?

[Wally]

I monitors the magneto primary current by measuring volt drop across an inline switching transistor. A zener diode triggers a second transistor when the volt drop reaches peak and a blocking diode is switched in circuit, which causes an interuption in current flow and the ensuing spark.

[recoil]

Thanks Wally, i have a basic idea of what you are saying. So what makes the exact point of voltage drop which triggers the spark, the exact point the points would have been opening. Also then i assume the timing could be adjusted by moving a slotted coil clockwise or anticlockwise.

[Wally]

At some point the current flow will reverse as the pole face moves past the armature (primary). This momentarily increases the potential difference to breakdown voltage of the zener.

The magneto is normally installed so that the change in flux lines and subsequent current direction are in synch with the breaker points opening +- the capactive effect of the condenser. This means you shouldn't need to make any adjustments