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Problems In Isolator IC

By on March 3, 2010

 opto ic

There are few problems that can be found in Optoisolator IC (4N35, PC817 and etc). The most common problem is the internal transistor shorted or turn leaky. The diode LED could have open circuit but rarely shorted. Apart from that, optoisolator IC could also breakdown when under load (remember, it has transistor and diode LED inside). It could also cause intermittent problem. If all the components in the power supply tested good and the power supply still don’t work, then i suggest that you directly replace the Optoisolator IC and retest the power supply.



  1. Jimmy Lane

    March 3, 2010 at 3:36 pm

    I had an unusual problem years ago in a brand new Panasonic 25" tv/vcr combo. It had a terrible sqealling sound and I found a shorted zener in the secodary side of the power supply. This diode monitors one of the source voltages and when it hits the zener voltage shorts and kills the voltage so it won't get to high. I noticed that one leg on the optoisolator was not through the pc board. After I unsoldered it I found the leg bent over and not through. Without the reference voltage from the secodary feeding info to the primary side the switching transitor turned on harder putting out to high a voltage. After remounting everything worked good. That taught me those zener diodes from from a voltage source to ground are very important and do there job when needed. Hopes this helps someone.
    Jimmy Lane

  2. H-O

    March 3, 2010 at 5:05 pm

    Hi Jestine.

    What about replacement of these ICs are they all different or can they be replaced with another one, not exactly the same for testing of say a switchmode PSU?

    thank you.

  3. admin

    March 3, 2010 at 9:24 pm

    Hi H-O,

    If the internal components are the same then you can replace it. Some optoisolators have triac or even diac inside so you must know the internal components before you replace it.


  4. Waleed Rishmawi

    March 4, 2010 at 6:47 am

    can it be alos tested with a fluke digital meter or it has to be with analog meter? thanks

  5. admin

    March 4, 2010 at 8:37 am

    HI Waleed,

    Testing with analog meter is much accurate then using digital meter.


  6. admin

    March 4, 2010 at 8:38 am

    HI Jimmy,

    Thanks for sharing this tip!


  7. Noel

    March 6, 2010 at 10:05 am

    Hi Jestine,

    I have experience in repairing SMPS with output of +12V,-12V and +5V keep blowing the filter cap at the -12V side.And I found that the optoisolator is open.Its a pc2501 optoisolator.
    Hope can help to some readers.


  8. admin

    March 8, 2010 at 4:14 am

    HI Noel,

    Thanks for sharing!



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