[EDIT] I see Sunglasses already covered some of my reply, but here it is anyway, just for context. Also keep in-mind I do not claim to be an expert or know everything, but I've learned a lot working with many types of signal conditioner circuit designs for quite a long time now, so I hope this is helpful:
zylinx wrote: ↑Thu Aug 06, 2020 5:38 pm… First I noticed the 5.1V Zener diode come into question from a MS circuit, I noticed this in one of my open tabs:
The John Zener(5.1V) is used for some specific ignition systems which do not pull the load all the way to ground potential - the application of this diode depends on installation (it is recommended to install a jumper in place of the John Zener for first ignition tests - if noise or lack of triggering is present then the diode should be installed).
source: http://www.bgsoflex.com/v22/msv22.html , Hope that helps clear that up.
It does for the original intent of certain problematic installations (force opto to ground and shut off). It does not for additional considerations not taken into account in the pieced-together MS circuit, which is what romP44 is addressing. See below.
zylinx wrote: ↑Thu Aug 06, 2020 5:38 pmI was wondering If the 4N35 and 4N25 interchangeable between NickZ's nad dazq circuits ? or is there something im missing in the datasheets.
They are very similar, but have a few different characteristics, especially in voltage isolation, maximum LED current, and substantially different transfer ratio. While they can be interchangeable in some cases, the differences may need some attention for proper operation. For examples, in romP44's circuit, they should work equally well, but in NickZ's the 35 could have some advantage. One difference would be in the tighter minimum saturation limits against the lack of the Zener in Nick's.
zylinx wrote: ↑Thu Aug 06, 2020 5:38 pmAlso would it be ok to use a 100V Zener instead of 110V ?
Maybe. That depends on the coil's output waveform and the ringing voltage after spark burn. The point in improving the circuit would be to "cut-out" secondary triggers, only using the initial HV spike while ignoring all others.
zylinx wrote: ↑Thu Aug 06, 2020 5:38 pmI tried to make NickZ's circuit but LED2 keeps blowing when I test by putting 19V DC across coil input. I thought it would be able to withstand 19V easily but the 2.2K resistor gets warm and LED is very bright then dies, not sure if I also damaged the opto-isolator aswell. Am I being stupid ? Should this circuit be able to handle 19V on its input ? I would think so. I must have made an error somewhere.....
Yes, we are back to using only the HV spike, by using the HV zener, to do a few things. One is to greatly limit the on-time of the opto LED (or any indicator LED) to only that short discharge, which limits the average power through the LED and therefore limits the possibility of burn-out. No big deal, as that can be compensated with resistor values. However, the HV Zener moves the timing of the trigger signal to the actual coil firing point.
With lower-volt Zeners (including the MS 5.1V), the trigger signal is always high ("12V"), until the dwell pulls the voltage low. This makes the trigger point the falling edge at the beginning of dwell, not coil firing. Rising edge would be even more sloppy as a coil voltage slope. Add to that the change in coil dwell duration (on-top of timing change) that will randomly move the trigger point even further. While your engine may or may not be substantially bothered by the large change in trigger point (and therefore injection point), some certainly will. The HV Zener (if properly selected) solves all of these issues.
Just to get picky, a Zener higher than system voltage (generally up to 18V), yet lower than spark burn (generally 40-60 volts, but widely varied) would also work to get a longer signal that rises at discharge and ends at spark burnout, but would still have multiple triggers at the end of spark burn with the ringing voltage. This can be avoided with a de-bouncing circuit or software de-bouncer, and is why some systems can use these mid-range-voltage Zeners successfully without confusing the processor. I hope that helps with some of the various considerations with these seemingly simple signal conditioners.