> But if you tap into the loop at all, you're connected to that source
> whether you like it or not. I don't understand what the issue is.

You are connected, but with two wires you only get ground and current,
no voltage.

My idea was to feed the current through the LED of the first
optocoupler, and at the secondary side feed the output of an opamp
through the LED of the second optocoupler, regulating for equal current
through both receivers.

> About the only approach I can think of that uses *zero* power on the
> current-loop side is some sort of Hall-effect sensor. See

It is difficult to use zero power at the current side, and not needed
anyway. That's one of the things the 4 mA is for.

Wouter van Ooijen

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Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products

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