Why can't I test the transistor part of an optocoupler using my multi-meter?

[ddewaele]

I'm attempting to test if my PC111 opto-coupler is working properly (pulled from a broken PSU).

• pin 1 = anode
• pin 2 = cathode
• pin 3 = NC
• pin 4 = emitter
• pin 5 = collector
• pin 6 = base

I wanted to use my multimeter first (so the device is 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 (pin 6) -> emitter (pin 4)  and base (pin 6) -> collector (pin 5) 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 the optocoupler) to turn on the LED (with separate power source) 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 (this time 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.

Any idea why the multimeter diode test on transistor pins fails for the first 2 opto-couplers but turning on an LED using a basic opto-coupler circuit does work ?

[magic]

May have something to do with this:
https://www.eevblog.com/forum/projects/different-die-pictures/msg3173898/#msg3173898

The secondary of the dismantled optocoupler is clearly not a single transistor but an IC.

edit
Okay, H11L1 is specified to have a buffered output so duh
Still, there is a chance of some shenanigans in the supposedly "single transistor" optocouplers too.

[Andy Watson]

The manufacturers are messin' with you

https://www.datasheetq.com/ndown.jsp?url=/datasheet-download/Sharp/556034.pdf

Datasheet for PC110, PC111, PC112, and PC113 - says "PC111 and PC113 have no base terminals."

[ddewaele]

Datasheet for PC110, PC111, PC112, and PC113 - says "PC111 and PC113 have no base terminals."

No idea how I missed that And same thing with the CNY17GF apparently. Only the newer CNY17 have their base pin exposed.

Thx !