5300Vrms isolation in a 6-DIP through-hole package
The onsemi 4N25300 is a single-channel optoisolator in the 4N25 family, providing 5300Vrms galvanic isolation between a DC input LED and a phototransistor output with the base brought out. The 6-DIP (0.300", 7.62mm) through-hole package suits legacy board layouts where surface-mount reflow is not an option — common in industrial control cards, PLC I/O modules, and power-supply feedback paths that need a clean isolation barrier.
20% minimum CTR at 10 mA — the coupling margin that matters
A minimum current transfer ratio of 20% at 10 mA forward current means the output transistor sinks at least 2 mA collector current when the LED is driven at 10 mA. That is the floor for worst-case coupling — if your downstream logic threshold needs 1 mA, you have 100% headroom before the link drops out. The typical forward voltage is 1.18 V at that drive current, so the LED drop is predictable across temperature. Switching is rated at 2 µs turn-on and 2 µs turn-off typical — fast enough for 100 kHz data rates in isolated serial links or gate-drive feedback, but not for high-speed digital isolation above 1 Mbps. The 30 V maximum collector-emitter voltage on the output side covers 24 V logic rails and 15 V analog supplies with margin.
onsemi has marked the 4N25300 as obsolete. No pin-compatible direct replacement from onsemi is documented, so a BOM substitution would require validating the CTR, isolation voltage, and switching speed against the application.