dc42:
Your revised schematics need amending. If you are going to connect the indicator LED in series with the opto isolator, then the 1N4148 diode should be in parallel with the series combination of the indicator and the opto isolator. Otherwise, if the 80V input is reversed, you will blow the LED.
Absolutely right, I forgot about that, Thanks for looking into it and telling me.
Attached the corrected schematic. (I left the SW2 switch to do some tests in the simulator)
dc42:
Another issue with both the original and the revised circuit is that if you reverse the 80V input, then almost the full 80V appears across the 2.2k resistor, and it will dissipate about 3W. This is a consequence of adding the zener diodes and reducing the series resistor.
Anything that can be done with this? I understand what you are saying but I cant see this can be fixed, right?
dc42:
You haven't told us what the forward voltage of your indicator LEDs is (or even what colour they are), so I can't calculate the exact current flowing through the circuit.
Sorry about that, its a regular 5mm green led. I dont know the forward voltage as I got them from the store without further information. According to LED - Basic Green 5mm - COM-09592 - SparkFun Electronics
Could we say "1.8-2.2VDC forward drop
Max current: 20mA
Suggested using current: 16-18mA
Luminous Intensity: 150-200mcd"
I really appreciate your help dc42, now and also in the past you have helped me.
btw. I dont need very bright leds on the PCB, just the minimum to see if the channel is ON or OFF. So the current through the LED+optocoupler can be lowered, 5mA is ok?
