Opto Isolator

[Iano]

Can anyone tell me if an opto isolator still protects circuits from each other when both ground pins of the chip are connected to a common ground?
Surely current doesn't travel back across the internal LEDs??   

[kaz911]

Try to remove the ground from your circuit pins and see if any current will flow. If there is no "ground" there is no current flow. The ground can be "floating" (think virtual ground) or the ground can be earth connected (like a normal bench oscilloscope). Ground is just a "definition"

And one last pointer.. can you measure current on the negative cable from your power supply with a clamp meter? That is usually connected to your board's ground. And yes - you can measure current on your negative cable.

If you accidentally attach 110/220v A/C to the input - what happens to your circuit?

So yes - you can force current through your ground plane from "the other side" of an opto isolated circuit if you connect grounds together.

[Iano]

Thanks for the pointers.  I will see what I can do. 
This is only a 40V DC (max) circuit. 

[kaz911]

40 v DC is actually quite a lot.. Dependent on the Amps of  course.

Anyway - post a bit more of what you are trying to do - and how you are powering it.

I'm working with Isolated CAN at the moment - with DC power input either from CAN side or from both - and they need to be fully isolated as well.

But why do you need to connect the two grounds together? can't you just keep them separate? or does your isolated side need to get DC power from other "other side"

[Paul Price]

The answer is no! If the grounds are connected together you have no isolation. Connected together means no isolation.

It is  possible to us an optocoupler for something other than isolation of grounds, for instance the output portion of the device can float within a circuit to function as a switch of a way to change the circuit it is meant to control and in this way isolating the controlled device circuitry from the ground-referenced control input. But if the two grounds are tied together, this idea is to control something with the optocoupler's output and to not achieve ground isolation.