Problem In Optoisolator IC

By on June 30, 2009


The most common fault for an Optoisolator IC is the short circuit between the collector and the emitter pin and you can test it by using an analog meter set to X 10 K ohm and it should not show two readings. If there are two readings then it is either consider shorted or leaky. The LED diode inside the Optoisolator IC rarely have problem and you can test it using analog meter set to X 1 Ohm and it should have one reading.

In power supply, if the Optoisolator have problem it will cause many different types of symptom like no power, low power or even power shutdown. Since this IC is not difficult to test, i suggest that if you come across problem with the power supply with the symptoms that i have mention above, you may check on the Optoisolator IC first.  What about Optoisolator IC that breakdown when under load?  Yes in my repairing career i do came across Optoisolator IC that breaks down when under full operating voltage but it was rare too. If you have test all the components in the power supply section and still could not find out the fault, i guess there is no harm in replacing the IC and retest it again. Who knows the power supply may come back to life again.



  1. Waleed

    July 1, 2009 at 8:00 am

    thanks for the tip. i have this LG DVD/vcr compo machine that goes to stand by mode few seconds after I turn on the mahine. I checked all the power supply and bud not check this one, I will give it a try and check it out. I will keep you posted. thanks again for the tip.

  2. Waleed

    July 1, 2009 at 2:18 pm

    can the testing be done with a digital fluke meter? just by using the diode setting and the transitor setting. please let me know

  3. admin

    July 1, 2009 at 7:03 pm

    Hi Waleed,
    Yes you can but not accurate as compare to use an analog meter.


  4. admin

    July 1, 2009 at 7:04 pm

    HI Waleed,
    Hope you will find the fault.


  5. beh

    July 1, 2009 at 11:34 pm

    hello jestine

    i have seen optisolator with six leges if you tell me what are those two extra
    leg going to do? with optoisolators i will be more thankful

  6. admin

    July 2, 2009 at 11:26 pm

    Hi beh,

    The extra two legs, one is no connection which located at the LED side and the other one is Base pin.Please click on the link below:


  7. ravichandran

    July 9, 2009 at 7:51 am

    thanks for the tip. i have this smps that goes to stand by mode few seconds after I turn on the mahine. I checked all the power supply and bud not check this one, I will give it a try and check it out. I will keep you posted. thanks again for the tip.

  8. lokesh mohapatra

    November 17, 2009 at 4:42 am

    Thank you very much sir.We have done a project on remote fan regulator where opto isolator is used.your tips helped us and now our project is ready.

  9. admin

    November 17, 2009 at 7:18 am

    Hi Lokesh,

    You are welcome!


  10. bill

    March 3, 2010 at 12:03 am


  11. Altaf

    March 5, 2010 at 7:40 am

    i have an airplus Xtreme G wireless Router. its some astonish problem when i on this device it works properly but some of time it hangs and some of time not hangs couple of dose. so please tell me Is it power supplu problem or the problem or ruoghter board. and please tell me its solution
    thanks jestine
    ur student Altaf Hussain from Pakistan

  12. admin

    March 5, 2010 at 8:59 am

    Hi Altaf,

    The best way to confirm is to make sure the power supply produce good output. Check for dry joints and all e-caps with esr meter.


  13. Daniel F

    September 27, 2010 at 1:04 pm

    I recently came accross a Toshiba LCD Television with a shutdown problem. The complicated circuit used 4 NEC 2561A optoisolators.
    As you suggested I found the datasheet first to make sure it never used a diac internally, these use LEDs.

    I tested all 4 optoisolators using your method above with an anologue meter and none of them gave a reading when I tested on the x10 kohm scale between the emitter and collector. In other words, no reading either way.

    I then tested the same pins using an expensive fluke Digital Meter and it read 0L meaning 'open circuit' or reading off scale.

    Here's the datasheet link for anyone interested.

  14. admin

    September 28, 2010 at 9:58 pm

    HI Daniel,

    Not necessary the optoisolator have problem because in certain part number, the internal resistance is very high you will not get any reading. You can connect two analog meter in series like a battery and set to x 10 k ohm and retest the ic again. If it still does not show any reading, most probably the internal transistor already got problem.


  15. Gerald Giasson

    December 9, 2010 at 7:40 pm

    Hi Jestin,

    I am trying to purchase this part number (NEC2561DE351) from Digikey . However, the above part number does not quit fit the Digikey part list and in addition there are various specs related to this part. Which Digikey part would best fit the above part number? The above part came from a power supply. The opto is marginal and causing the power supply to go into an over voltage shut down. FYI. Thanks.


  16. admin

    December 12, 2010 at 10:35 am

    HI Gerald,

    I could not find any info in Digikey too. Try this link:


  17. Gerald

    December 18, 2010 at 6:48 pm


    Thank you :).


  18. wycliff

    December 23, 2010 at 4:30 am

    thanks waleed but i've got this dvd power supply and the fuse blew up and another IC(VIper22A)also blew could it be this optoisolator causing the problem. thanx again

  19. Amir Mukhtar

    February 21, 2012 at 8:25 am

    Thanks you
    Nice guidance about opto isolator device.

    Amir Mukhtar

  20. leonard williams

    May 22, 2012 at 7:35 pm

    i would like a 42 inch lcd screen  to buy any idea what the price is like

    • Jestine Yong

      May 27, 2012 at 4:04 am

      HI leonard,

      Sorry I do not sell spare parts-try


  21. Arcie Valeza

    July 7, 2014 at 9:08 pm

    Thanks for sharing your knowledge on repairing technique it's a great help...

  22. Arcie Valeza

    July 7, 2014 at 9:22 pm

    Jestine Yong,
    Thanks for a technical guide on OptoIsolator IC,how to test it and to check if it is functioning not all technician know about this,really thanks for sharing ur technical expertise..

    • Jestine Yong

      July 7, 2014 at 9:38 pm

      HI Archie,
      The best is to direct replace Opto ic as some will breakdown when under load.


  23. Jared

    February 1, 2017 at 9:30 pm

    hi Mr. jestine
    I just like your articles they really boost my spirit of repair
    but is there way to modify this opt isolator ics incase there is no
    close alternative to bring hope ?

    • Jestine Yong

      February 2, 2017 at 11:24 am

      Hi Jared,
      It is all sealed thus it would be tough to modify one.
      Optoisolator ICs are very common in the market and you should be able to find one online.


  24. mohammed

    May 14, 2017 at 6:24 pm

    I have switchable power supply it's funny normally 3_10
    many of and sometimes don't run but restart cyrcle when I fixed it it run normally for 48 hours and then return to restarw cycle what's problem please help me

    • Jestine Yong

      May 16, 2017 at 7:27 am

      Hi Mohammed,

      What have you checked and replaced?


  25. Mohammed

    May 17, 2017 at 7:04 pm

    Hi Mr Justine thanks you for replayed
    In first time replaced small mosfet in the second circuit after that its run normally for 5 days and return to restart cycle while that happened short electricity caused by main cable and after test components find bigger mosfet shorted and replaced it with a irfp460 mosfet the original one is 24n60c3 after that the power is not run its restart after sound heard (tek) and after replaced the another mosfet (24n60c3 with irfp460 mosfet) and replaced the 3 optioisolator (sharp pc718) finally the power supply is run very easy and pc become fast and better but after a few days it's returned to restart after hearing voice (tek) from the power supply
    Note : the voice(tek) hearing from power supply in the two cases (normally run and in restart cycle)

    • Jestine Yong

      May 20, 2017 at 11:55 am

      Hi Mohammed,

      What kind of power supply you are repairing? How many output voltages it has? When you test the power supply did you connect to the mainboard?


  26. mohammed

    May 21, 2017 at 2:01 am

    I have pc power supply called cooler Master gx 750w when I tested all voltage is very good testing out of pc and when it's run out of pc it's run normally without any restaret the problem begin with start all loading of pc however some times run many hours without any problem but finally it must arrive to restart recycl behavior when I'm test another power supply on the same pc it's run without any problem

    • Jestine Yong

      May 23, 2017 at 10:27 pm

      Hi Mohammed,

      Sorry I don't quite understand your meaning. Do you mean the power supply have problem when it is connected to the motherboard? When used another power supply connected to the motherboard have no problem?


  27. mohammed

    May 24, 2017 at 4:14 pm

    yes but the problem appear after many hours of use

    • Jestine Yong

      May 25, 2017 at 9:40 pm

      Hi Mohammed,

      You may have to check for dry joints and use a coolant/freezer to find out the faults.


  28. Eric Rosano

    November 16, 2017 at 3:21 pm

    What if i short the emitter and collector, what will happen? because on my circuit when i shorted it the machine works. if it is not shorted it doesnt work.

    • Jestine Yong

      November 16, 2017 at 9:12 pm

      HI Eric,

      It depends on circuit deign. Some power supply can start when you short the emitter and collector pins while other will cause no power, power blink and power shutdown problem.



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