I need a Opto isolated input circuit that allows the input to the opto to be either connected to gnd or a WET voltage source that ranges from say 0-30 VDC.This is how most commercial digital input modules, such as ones from say Advantech or ICP, work and I cant figure out how. For DRY mode it is basic and I figure the device simply internally routes a voltage, say 5V, through a resistor then the opto and out the pin of the device so when that pin is connected to gnd the opto is energized. What I cant figure out is how does that same pin also accept a WET voltage that ranges from 0-30Vdc. I would think that at some voltage, say around 10V, the reverse voltage across the opto would exceed it's Max rating.
Thanks for any insight.
Thanks for any insight.