I have been reading the schematic of ATX power supply and try to find out how that power supply can be converted into building an inverter. So I was searching for components which I can reuse from the ATX power supply circuit to build a diy inverter. I came across the PC817 optocoupler which is used in the ATX power supply circuit. However I had also the 4N35 optocoupler IC which is a good alternative to te PC817.
The Key difference is that 4N35 has an extra base pin for the transistor, which lets you control response time or pull-down the transistor for faster switching. The PC817 is simpler (no base pin) and comes in a smaller 4-pin package. They can be interchangeable only in simple on/off isolation circuits if you ignore the 4N35’s base pin, but pin layouts are different, so you can’t just swap them directly without rewiring. The PC817 is a 4 pin optocoupler IC where the 4N35 is a 6 pin optocoupler IC.
* PC817 = 4-pin DIP
* 4N35 = 6-pin DIP (you can leave the extra pins floating if unused).
So functionally, a 4N35 can work as an alternative to a PC817 in many simple isolation circuits, but it’s not a drop-in replacement because:
In the Performance differences aspect, CTR ranges and speed are similar enough for most low-speed microcontroller or power-supply isolation tasks. If your original PC817 circuit is not sensitive to CTR variation, 4N35 will likely work fine.
If you wire it correctly and leave the 4N35 base pin unconnected (or tied as needed), it can be used in place of a PC817 in basic on/off optocoupler applications. But you can’t just plug it in without checking the pinout.
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