The 4N25SD: No stock-holding claim is made here.
5300Vrms isolation — what it means for the BOM
The 5300Vrms isolation rating is the one-minute withstand voltage between input LED and output phototransistor. This is a reinforced-insulation figure per IEC 60950-1 and UL 1577 — it separates the low-voltage control side from a 250VAC mains circuit with margin for transient overvoltages. The 20% minimum current transfer ratio at 10mA forward current means the output transistor saturates with 2mA collector current when driven at 10mA input — adequate for driving a logic gate input or a small relay coil. The 2µs typical turn-on and turn-off time limits switching to below 250kHz, fine for 50/60Hz line sensing or slow digital isolation.
SMD gull-wing — board integration
Surface-mount 6-SMD gull-wing package, 0.100-inch (2.54mm) pin pitch. The gull-wing leads provide a visible solder joint for inspection and some compliance for thermal cycling between the PCB and the epoxy body. The 6-SMD supplier device package is the same footprint as the standard DIP-6 but with formed leads for reflow soldering. Operating temperature range -55°C to 100°C covers industrial and some automotive under-hood environments — the 100°C ceiling is the junction temperature limit, not the ambient; derate the forward current above 70°C per the LED derating curve.