Drive opto with Wet and Dry inputs circuit?

Thread Starter

Visinet

Joined Feb 1, 2016
3
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.
 

Thread Starter

Visinet

Joined Feb 1, 2016
3
I would assume that at some threshold voltage, say 3V and below, the LED/opto would be ON, same as if it was connected to gnd. Everything above a threshold, say 5V, it would be off. I can deal with the actual polarity of the point in SW. I have to monitor a number of external status points in my system and I want to be able to support both DRY CC and WET voltage levels without having to change any jumpers or such. Hope that makes sense. My current design requires changing a jumper to select between WET and DRY input signals which simply selects if the opto's led voltage is supplied internally by my system or by and external signal.

Thanks again
 

crutschow

Joined Mar 14, 2008
34,284
You can use diodes to protect the input and provide the switching point as shown in the LTspice simulation below:
The opto is fully on at ≤1.5V input and fully off at ≥5V.

upload_2017-9-6_16-43-3.png
 

Thread Starter

Visinet

Joined Feb 1, 2016
3
Thanks a bunch. Just a follow-up question as I am more of a SW guy. Why the 2 diodes D1 and D2 and if the external voltage is a basic powers supply then how does it sink current from the Vd?

Thanks again.
 
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