I attempting to test if my PC111 opto-coupler is working properly (pulled from a PSU).
I wanted to use my multimeter first (so the device not connected to anything), setting the multimeter in the diode test mode.
Testing pin 1 and 2 (the LED) was fine. I saw the voltage on 1 end and OL on the other.
I assumed that the output transistor could also be testing this way as the base pin is also exposed,
I thought base -> emitter and base -> collector would show a voltage drop (like with a regular NPN transistor).
However this was not the case and always shows OL (I tried it with another opto-coupler also).
I then tried both opto-couplers in a simple circuit with an LED (hooking up 5V through 2K resistor) to turn on the LED using the transitor and that worked fine.
Someone also told me that I should check for continuity between emitter and collector when the optocoupler is activated but also that doesn't work.
So why can't I test the transistor using my multimeter that way (diode test and continuity) ?
I tried it with a second opto-coupler (pulled from a working PSU). A CNY17GF-2. Exact same thing. Multimeter diode test on transistor pins fails, simple circuit to turn on LED via opto-coupler does work.
I then bought a brand new CNY17 (same pin layout as the previous 2 opto-couplers), and there the multimeter diode test on transistor pins does work.