I've been given a circuit to test and pointed out that the potential reverse voltage across the led in the opto will exceed the spec. The cct is a dual input with an opto on each channel, which feed into a bus buffer device for then reading form the motherboard that this plugs into.
So there are TVS diodes on the 2 inputs (La+ and Lb+) that protect against transients and clamp at 15V. But the problem is that at 15V we are exceeding the 6V reverse rating of the opto LED, which is fed from 3V3 through a resistor. To be honest the design is supposed to work up to 24V so it is worse.
I could use a voltage shunt across the LED to clamp the reverse voltage seen by the LED, but this will mean that in the inputs inactive state (a voltage of either 5, 12 or 24V on La/b+) we will draw current form the input/consume power. As we have up to 32 of these channels in the design that's not really ideal. I also cannot affect the level of input voltage signals as this device is only supposed to 'listen' in to the signals which are part of another system.
I thought about using a schottky on the input signal to block the voltage as we're only interested/active when the input is pulled to GND. But without the LED conducting there is no circuit the other side of the schottky so it doesn't do anything.
Any ideas gratefully received - thx