The 4N25W is marked Obsolete by onsemi. No stock-holding claim is made here.
5300Vrms isolation and 20% CTR — what they mean for the circuit
The 5300Vrms isolation rating qualifies this part for reinforced insulation in mains-referenced circuits — think AC-line monitoring, motor-drive feedback, or medical equipment where the safety barrier must withstand a 5.3 kV transient without breakdown. The 20% minimum current transfer ratio at 10 mA forward current sets the drive margin in the receiver. If the LED is driven at 10 mA, the output transistor can sink at least 2 mA — enough to pull a 5V logic input low through a 2.2kΩ pull-up. Below that CTR, the output may not saturate cleanly. Turn-on and turn-off times are 2 µs typical each — fast enough for 100 kHz switching or serial data isolation, but not for high-speed protocols like CAN or Ethernet.
Through-hole 6-DIP — rework-friendly, clear pin 1
The 6-DIP (0.400-inch, 10.16 mm) through-hole package is about as rework-friendly as it gets. The 2.54 mm pitch matches standard prototyping grids and perfboard. Operating temperature spans -55°C to 100°C — the industrial/military band. The part survives cold-soak and hot environments common in outdoor or engine-bay electronics. Maximum forward current is 100 mA DC; the LED is robust enough for direct drive from a 5V logic output through a 330Ω resistor without exceeding the 1.18V typical Vf.