The PS2561BL-1-F3-Q-A can serve as a replacement for the original FOD817A3S, but several key differences must be considered. Its higher current transfer ratio (CTR of 100–200% vs. 80–160%) offers improved signal transmission efficiency, potentially reducing input current requirements, though care should be taken to avoid overdriving the output. The higher maximum output voltage (80V vs. 70V) provides greater voltage margin in application, enhancing reliability. However, the lower maximum forward input current (40mA vs. 50mA) requires that the input circuit design remain within this new limit to prevent damage. A slightly higher Vce(sat) (300mV vs. 200mV) will lead to a modest increase in output-side power dissipation, affecting efficiency. Differences in switching times—faster rise time (3µs) but slower fall time (5µs)—may influence high‑frequency response performance. Overall, substitution is feasible provided the input current meets the specified limit and the switching characteristics satisfy the application requirements. It is recommended to verify parameter compatibility in the actual circuit.