Equivalent Part Number For PC817 Optoisolator IC
Optoisolator IC with the part number of PC817 can be easily found in many switch mode power supplies. If the Optoisolator IC have problem it would cause no power, power blinks, low power, power shutdown, intermittent shutdown and all sort of weird power problems. There is no doubt that the Optoisolator IC can be tested with multimeter but I found that the most accurate way is to direct replace the IC and retest. The reason for it this type of IC can breakdown when under load.
This means it will checked ok but failed when under full operating voltage. You can use PC123 part number as replacement but I found another part number that can also be used which is NEC2581A or PS2581A. Here is the link to the datasheet:
Note: The best is to use back the same original part number and if you could not locate the original part number then the only choice you have is to use the replacement number. This applies to all semiconductor devices.