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

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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.

  • \$\begingroup\$ Good question. You would assume a photo transistor to behave just like a normal transistor when not exposed to light. And when you use your multi-meter on any old BC546/7/8 or whatever standard NPN transistor base to emmitter you find it works? You should get collector to emitter current when the LED is on even without the base connected, don't you get that? \$\endgroup\$ Commented Aug 26, 2020 at 15:31
  • \$\begingroup\$ When testing an NPN BC139 transistor here the test-cycle as described in the url works. The base pin was only used during multimeter testing (out-of-circuit, not powered). With the LED test there is no point in using the base as the LED will drive the base pin of the transistor. \$\endgroup\$
    – ddewaele
    Commented Aug 26, 2020 at 15:36
  • \$\begingroup\$ Transistors in the 4N25 and 4N35 are normally measurable NPN transistors. There is also data on the base current in the documents. I didn't find one at Sharp. \$\endgroup\$
    – csabahu
    Commented Aug 26, 2020 at 16:29
  • \$\begingroup\$ Just bought a brand new opto-coupler CNY17 and there the diode test with multimeter on the transistor pins does work. On another "old" optocoupler pullen from a working power supply, a CNY17GF-2, I am again unable to perform the diode test with multimeter on the transistor pins. (and again the LED circuit test works). \$\endgroup\$
    – ddewaele
    Commented Aug 31, 2020 at 6:36

1 Answer 1


In the older CNYF and CNYGF series, the base of the transistor is not routed out, so it cannot be measured.


PC111 have no base terminal too:


  • \$\begingroup\$ Thx I could not find a datasheet of the old CNYGF .. .so that explains why the old CNYGF chip fails the test. But the Sharp PC111 has the base routed .... so does that mean the opto-coupler is bad ? (and if so, why is it able to turn on the LED) \$\endgroup\$
    – ddewaele
    Commented Aug 31, 2020 at 8:10
  • \$\begingroup\$ PC111L have no base terminal too... \$\endgroup\$
    – csabahu
    Commented Aug 31, 2020 at 8:23
  • \$\begingroup\$ totally missed that PC111 has no base terminal .... \$\endgroup\$
    – ddewaele
    Commented Aug 31, 2020 at 8:43
  • \$\begingroup\$ PC111 has, but PC111L not. :) \$\endgroup\$
    – csabahu
    Commented Aug 31, 2020 at 8:46
  • \$\begingroup\$ Sure ? After your comments I read in the datasheet that "PC111,PC113 have no base terminals" \$\endgroup\$
    – ddewaele
    Commented Aug 31, 2020 at 11:46

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