1. PS2501A-1-A Substitution Conclusion Direct substitution is not recommended. While the basic structure and withstand voltage are identical, its current transfer ratio (CTR) range (50% @ 5mA ~ 400% @ 5mA) differs fundamentally from the original part (100% @ 5mA ~ 200% @ 5mA). Its lower CTR limit (50%) is only half that of the original. Under identical input drive conditions, the maximum drive capability on the output side could be severely insufficient, potentially leading to unreliable circuit turn-on or signal transmission failure. Concurrently, its upper CTR limit (400%) is excessively high, which may cause the transistor to oversaturate, degrading switching speed or increasing power dissipation. This broad and shifted CTR range indicates significant part-to-part performance variation. Direct replacement could force the original design to operate at its margins or become unstable, representing a high-risk substitution.