MorganS:
Always consider wires entering your box as untrustworthy. Even if the upstream device has current limits, that does not prevent that wire from being dragged across the battery terminals during maintenance.Usually the voltage divider on 12V inputs is sufficient for most automotive wiring screwups. But static discharges are thousands of volts. ESD protection via TVS diodes is pretty easy these days.
Look up "load dump". That is the most violent electrical event that can occur in a vehicle.
Thanks for the input, would you recommend a TVS diode on all inputs then to be sure? I will read up but is there a particularly good PN you'd recommend for this application?
WattsThat:
IMO, the 10k series led resistors for the PC817’s are marginal. Given the current transfer ratio of those parts, I’d consider lower value series resistors for better noise immunity. They may work on the bench but add some noise and a higher ambient and you may have issues.I’d also be cautious of relying on the internal pull up resistors. They are relatively high values and not the best for noise immunity. In addition, there is the question of minimum load requirements when you’re replacing filament lamps with solid state inputs. Lots of unknowns there, depending upon the age of the vehicle.
Thanks for your input too! Having read the datasheet, I've now changed the values of the input resistor to 470 ohms - the PC817 can handle max forward current of 1A and min of 50mA so 470 should do the job I think...
When you mention not relying on the internal pullup resistors, what other method would you recommend?
With regard to the minimum load piece, the wiring loom is going to be completely replaced so nothing will be existing. The power distribution will come from the units mentioned above which will replace all relays and fuses, and the engine/alternator will be a 2019 Chevrolet LT4 motor so very new design and hopefully modern levels of electrical noise if such a thing exists...