1. 4N45 Substitution Conclusion The 4N45 cannot serve as a direct replacement for the 4N30 due to significant degradation in critical safety isolation and output drive capabilities. Forced substitution may introduce system safety hazards or functional failures. The key differences are as follows: First, the isolation voltage is reduced from 5300Vrms to 3750Vrms, representing a nearly 30% decrease in the withstand insulation voltage between the high‑voltage and low‑voltage sides. If the original design relies on the higher isolation rating, the replacement carries a risk of dielectric breakdown. Second, the output capability is substantially weakened. The maximum output voltage drops from 30V to only 7V, and the output current decreases from 150mA to 60mA. This directly limits its ability to drive relays, motors, or larger loads, potentially causing the original circuit to malfunction. Third, although the 4N45 exhibits a significantly higher current transfer ratio (CTR) – 200% to 1000% compared to 100% – which makes it more sensitive and requires less input current, this advantage cannot compensate for its fundamental shortcomings in isolation withstand voltage and output power. Substitution may only be considered in applications where the isolation voltage requirement is extremely low (e.g., <3750Vrms) and the load voltage and current are very small. Otherwise, a full reassessment of the circuit design is mandatory.