400 V, 400 mA — the headroom decision
The 1N647-1 is a general-purpose standard recovery rectifier rated for 400 V reverse voltage and 400 mA average rectified current. The 400 V Vr covers a 240 VAC line (340 V peak) with margin, but not a 400 VAC line (565 V peak). The 400 mA Io is continuous — peak repetitive current is higher but not specified here; for a capacitive-input filter, the surge current at power-on may exceed the rating unless a series resistor limits it. Standard recovery (>500 ns, >200 mA Io) means this diode is not intended for fast switching. At line-frequency rectification (50/60 Hz) the recovery time is irrelevant; in a flyback snubber above a few kHz the stored charge will cause excessive dissipation and junction heating.
Junction temperature and package limits
The junction temperature range is -65°C to 175°C — military temperature grade. The DO-35 glass package is hermetic, so moisture ingress and thermal cycling in avionics or downhole environments are not a concern. At 400 mA forward current and 1 V forward drop, dissipation is 400 mW; in still air the DO-35's thermal resistance (approx 200°C/W junction-to-ambient) means the junction rises 80°C above ambient, leaving 95°C headroom to the 175°C limit. Reverse leakage is 50 nA at 400 V — low enough for high-impedance bias circuits or voltage-dropper applications where leakage current would otherwise shift the operating point. The leakage doubles roughly every 10°C, so at 125°C junction it is about 1.6 µA, still negligible for most circuits.