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About Optoisolator IC

By on October 14, 2009

The function an Optoisolator IC in power supply is to send a signal back to the Power IC so that it can control the “On” time of the switching frequency. Because of this the output voltages at the secondary side always stable. Now, in certain power supplies you will notice that ther are more than one Optoisolator IC. The other Optoisolator IC could be part of the standby circuit or even for controlling the power ON/OFF in CRT Monitor. That means if the VGA signal cable did not plug into the computer CPU, the Monitor power supply will shutdown.

Assuming if one of the Optoisolator have problem, it could cause no power, power shutdown or even power blink. You must solder out the Optoisolator IC and test with your analogue meter. If the Optoisolator IC that control the  power supply on/off have problem (usually open circuit in the phototransistor) you can simulate a test by soldering a jumper across the transistor pin. Of course this will defeat the purpose of the circuit. That means, the Monitor power supply will be always On even without plugging in the VGA cable to the CPU!

If the Optoisolator IC control the regulation have problem, then solder a jumper across the phototansistor pin will not have any affect to the power supply. In this case i suggest that you get a new Optosisolator IC and retest the power supply again.



  1. jgmanza

    October 15, 2009 at 1:35 am

    Hello Jestine, I have a CRT TV on my bench for two weeks now, and there is good power supply over the Main Filter Capacitor. If I put the probe to hot ground and red probe to center pin of power fet it still have same voltage. If I put the black probe to cold ground and red probe to both side of all secondary diodes there is no voltage.

    I was told that it is the optoisolator by a technician who fix CRT monitors here. Could that be so. I ask because I test the opto according to ebook with Analog meter and it test good out of circuit.

  2. admin

    October 15, 2009 at 10:23 pm

    Hi Jorge,
    Have voltage at the center pin of FET does not mean there will be power output at secondary side. The best is to use the scope to test on the signal at the gate and the drain pin of FET. There is a possibility because the optoisolator IC send a signal to the power IC to control the "On" time. Bad components in the primary side could have affect the signal from reaching the power IC. I suggest that you check all components in the power side and if possible direct replace the power IC and retest the set again.


  3. Abdoulie Bah

    November 2, 2009 at 9:32 pm

    Thanks very much it great to know wish you well.

  4. admin

    November 3, 2009 at 7:30 am

    Hi Abdoulie,

    You are welcome!


  5. samson

    July 7, 2010 at 6:03 am

    nice to know functions of opto isolator ic.

    how can know that its faulty on switching mode power supply.

  6. admin

    July 7, 2010 at 8:11 am

    Hi Samson,

    You need to solder it out from the power supply and test it.



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