What is a typical diode voltage that saturates the optocoupler?

[zape]

I suspect a dead optocoupler which is supposed to switch on the power supply and diode voltage is 1v. however the Vce is ~10v

[David Hess]

The diode is normally driven by a current and the optocoupler has a CTR (current transfer ratio) where the output current is some percentage of the input current.  20% to 80% is typical for transistor optocouplers.

If you want a safe value, the diode's forward voltage drop is about 1.5 volts so 5 volts applied through a 150 ohm resistor will yield 10 milliamps at the input and maybe half that at the output.

[Zero999]

I suspect a dead optocoupler which is supposed to switch on the power supply and diode voltage is 1v. however the Vce is ~10v

The diode is an infrared LED, which has a far higher forward voltage, than an ordinary silicon diode, normally around double. If you're only putting 1V across the diode, then hardly any current will flow, so the transistor will be off.

As mentioned above, pass about 10mA through the diode and the transistor should measure closed circuit.

[Wolfgang]

The GaAs LED inside an optocoupler does have a turn-on voltage, but this (as Silicon diodes) varies with temperature quite a lot. Drive OCs by current not by voltage !