onsemi has marked the 4N373S as Obsolete. Available through independent surplus channels; quantities are lot-specific and confirmed at RFQ. No stock-holding claim — sourced per BOM request.
Isolation and switching parameters
Rated for 5300Vrms isolation — this is the reinforced-isolation class for mains-referenced circuits in industrial power supplies, motor drives, and medical equipment where a single fault must not create a shock hazard. The 30V output max limits the collector-side swing to low-voltage logic or control rails. Minimum current transfer ratio of 100% at 10mA forward current guarantees the output transistor saturates with a 10mA LED drive — useful for 5V logic interfaces where the input side is driven from a GPIO or comparator. Typical turn-on/turn-off time of 2µs is fast enough for 100kHz PWM or serial data at 115kbaud but marginal for higher-speed isolation. Vce saturation is 300mV max — the voltage drop across the output transistor when fully on, which matters for power dissipation in low-voltage loops. Forward voltage typ 1.18V and max forward current 100mA define the LED drive budget.
Housed in a 6-SMD gull wing package with a 6-SMD supplier device package. Surface-mount — the gull wing leads provide visual solder-joint inspection and some stress relief for the optocoupler body during board flex. Tube packaging means it ships in anti-static tubes, not tape-and-reel, so pick-and-place feeders need a tube handler.
Temperature grade and environment
Operating temperature range -55°C to 100°C — this covers industrial and outdoor telecom enclosures but not under-hood automotive (125°C is the usual ceiling for under-hood). Single-channel DC input, output is a transistor with base — the base pin allows external bias or Miller-clamp circuitry for switching-speed optimisation.