Testing Optoisolator IC PC123

Some optoisolator ICs do not have any reading when you test on the collector and the emitter pins with analogue meter set to X 10k ohm range. You may have thought that it can be a bad opto since it does not show any reading. One good example is the PC123 optoisolator IC which is found in many SMPS will not show any reading when tested using the analog meter. In order to solve this problem just connect two analog multimeter in series (just like batteries in series) and set to x 10 k ohm range and test the opto. A good PC123 opto should show one reading on both meters.  If you still did not get any reading then the optoisolator IC is said to be open circuit. Hope this trick helps!

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18 Comments

1. 

   beh

   December 23, 2011 at 6:12 pm

   HI JESTIN

   can i use insulation tester for this test?
   beh

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2. 

   Amir Mukhtar

   December 26, 2011 at 6:52 am

   Sir,

   thanks you for your valuable trick in order to check Optoisolator device.
   Also, please help us in replacement devices. for example, similar types or equivalent components replacement especially which using in SMPS and LCDs. we are very great full to you.

   thanks you
   A student of Electronics
   Amir Mukhtar

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3. 

   admin

   December 26, 2011 at 12:44 pm

   HI Beh,

   You mean to test if the primary and secondary side have any short circuit?

   Jestine

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4. 

   beh

   December 27, 2011 at 1:48 pm

   i mean instead of using two series multimeter for testing this opto pc123 that mentioned in this short article

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5. 

   admin

   December 27, 2011 at 8:56 pm

   Hi Beh,

   I'm afraid it can't because the voltage to inject into the collector and emitter may be too high that can destroy the phototransistor.

   Jestine

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6. 

   admin

   December 27, 2011 at 8:58 pm

   HI Amir,

   In fact this part number is very common and if possible try use back the original number. If you really can't find one you can try 4N35 as replacement buy you may need to adjust the 6 pins to fit into the 4 holes because PC123 has 4 pins.

   Jestine

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7. 

   jgmanza

   December 29, 2011 at 2:12 am

   Hello Jestine, I checked an opto 817 and the photo transistor have no reading both ways. I got one from a used board and still the same. So i am here reading and I see this article. Thanks for the advice.

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8. 

   Amir Mukhtar

   December 29, 2011 at 5:45 pm

   Thanks you Sir for you guidance.

   Amir Mukhtar

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9. 

   Azhar

   October 12, 2012 at 11:44 pm

   Hi Jestine,
   I have problem to repair my old machine PC board which is the function to control machine sensor, i try so many places but no body can repair it , they claim that the part is old. Can you recommend me any body can repair it.
    

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   • 

     Jestine Yong

     October 13, 2012 at 1:47 am

     Hi Azhar,

     Sorry I do not know of anyone that repair PC board. Most of my friends are in TV/Monitor repair.

     Jestine

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10. 

    Dennis Mitchell

    December 18, 2012 at 4:09 am

    Possibly a simple test to test optocouplers ,PC123 in particular,apply approx 1 V Dc to diode side in conducting direction and read resistance at output  (PC 123) exhibits  3.76 Kohms.Reverse the voltage to diode side and should see no conducting.
    Refer to some FETS whereby you see infinite resistance at output ,however put a finger across the G and you will switch it and may stay in this state until a reverse voltage is applied. 

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    • 

      Jestine Yong

      December 18, 2012 at 5:56 am

      HI Dennis,

      Thanks for the tip!

      Jestine

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11. 

    Mohd Usman

    February 28, 2015 at 1:37 pm

    Can't we use digital multimeter for testing.

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    • 

      Jestine Yong

      March 2, 2015 at 9:31 am

      Hi Mohd,

      The output voltage from digital meter is not high enough to check on this.

      Jestine

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12. 

    RYAN Atong

    September 9, 2018 at 11:42 pm

    Hi Jestine, just want to make sure..regarding this opto PC123..I have the same IC..when I test it using x10k..Pin 1&2 - it shows 1 reading but it goes all the way to 0, and Pin 3&4 - both ways goes to 0. So this is a bad opto? Thanks

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    • 

      Jestine Yong

      September 12, 2018 at 7:31 am

      Hi Ryan,

      Not exactly because some opto's pin 3 and 4 does not have reading. The best is to direct replace and retest since it is easy to get from the market.

      Jestine

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13. 

    RYAN Atong

    September 9, 2018 at 11:46 pm

    Follow Up Question

    A good OPTO should show 1 reading when measuring collector emitter pins? 2 readings considered bad?

    Thanks Again

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    • 

      Jestine Yong

      September 12, 2018 at 7:29 am

      Hi Ryan,

      Yes

      Jestine

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