1. PS2561AL-1-A Substitution Conclusion The PS2561AL-1-A matches the original part in package, isolation voltage, and maximum Current Transfer Ratio (CTR). However, direct substitution is not recommended due to multiple degraded key parameters: its minimum CTR has been reduced from 80% to 50%, significantly lowering the worst-case optocoupler efficiency, which may lead to insufficient output signal amplitude or degraded drive capability; the output current rating has dropped from 50 mA to 30 mA, limiting load drive capacity; the output withstand voltage is reduced from 80 V to 70 V, decreasing high-side voltage margin; and the maximum LED forward current has been substantially lowered from 80 mA to 30 mA, restricting input-side drive conditions. These differences make the PS2561AL-1-A suitable only for applications with lower current and voltage margin requirements and wider acceptable CTR tolerance. If the original design relies on the higher performance margins of the original part, direct substitution may pose reliability risks.