I do not want to hijack the thread, but it is specifically about the opto input topic, so… Indeed it can be used as a filter, but at 5.1V is not blocking much and certainly not any coil voltage ringing, permitting multiple pulses after coil discharge. Hardly worth having at that level. To prevent multiple triggers past the Zener per coil firing, the voltage should be much higher. The 24V in MS3 is the right direction, but still low enough to require cap damping. The capacitor comes-in at that point to dampen the additional pulses as the 2nd part of the filter. But, that filter would not be necessary (though I would still have it for more reasons) to dampen multiple ringing triggers if the Zener were of higher voltage, no? Check my thinking, please.
To the point - if the Zener voltage were much higher, all of the ringing could be entirely avoided for a single clean pulse. That would make the Zener worthwhile. Either high enough to clip the ringing (green line below), or high enough to miss the coil burn and spike (red line). Then the cap is only tuned to provide a smooth signal from the primary flyback pulse and blocking noise without dealing with larger ringing pulses. Alternatively, filter or clip with a different method. Borrowed 'scope trace due to laziness:
ign_pri_sec_voltage.JPG (222.31 KiB) Viewed 1361 times
I'd like to hear more comments about this from others, just out of curiosity, and possibly circuit improvement.
- By kettlekev
- By gpineau