My motivation came from an industrial indicator I came across once (Stahl Ex-e indicator) that had this feature. I liked the idea of one device for all voltages!

Erik's suggestion of a big resistor and zener has the downfall of large dissipation at the higher voltages.

The DC range is going to be 12-32V.
The AC range is going to be 110-220V, 50/60Hz.

The DC solution is easy - LM317 as constant current source.

The AC solution is easy - use a capacitor (X2 rated!) to act as a reactance of the required value. This takes care of the power dissipation.

The trick is to combine these two solutions into one - with as few components as possible and be CE compliant!

The net result is to have an input that the customer has little chance of destroying by wiring it incorrectly. You might say I have little confidence in my customers, but the reality is that strange things do happen in the real world. The other point is that is makes design easier - universal input. Just like the advent of universal input switchmode supplies- you no longer need to worry about 100V,220V 50 or 60Hz - it all works!


So in answer to Kai's questions:

1/ reaction time - < 100mS. This is to be used as a switch input.

2/ Heat disipation. As little as possible (<1W)

3/ Opto coupler choice is open. I would hope to use a cheap opto like a 4N35 or suchlike. The circuit should be able to run a standard led: 1.2V @20mA.

4/ Can separate inputs be used? That's what I'm trying to avoid! If that was the case, the solution would be simple.

It could be that the solution I'm trying for is going to be larger and more problems than just supplying terminals for low voltage and high voltage!