Isolation barrier and output drive
The 4N29SM provides 4170Vrms galvanic isolation between a DC input LED and a Darlington with base output transistor, rated for continuous output current up to 150mA per channel. Minimum current transfer ratio of 100% at 10mA forward current ensures the output can saturate with modest LED drive, suiting logic-level interfacing in PLCs and motor drives. The Darlington configuration with base access allows external pull-up or speed-up networks, useful when switching inductive loads or driving a second transistor stage.
Switching speed and saturation voltage
Turn-on time is 5µs typical, while turn-off extends to 40µs maximum — the Darlington's high gain trades off switching speed, so this part is better suited to 10-50 kHz PWM or DC-level shifting than fast serial links. Vce saturation is capped at 1V, keeping the output voltage drop low enough to drive 5V logic families without an extra pull-up resistor.
Temperature grade and application environment
Rated for -40°C to 100°C operating ambient, the 4N29SM fits industrial control cabinets, outdoor telecom enclosures, and engine-bay electronics where commercial-grade parts would drift or fail. The 30V maximum output voltage and 80mA maximum LED forward current set the rail and drive limits — derate the output current above 25°C per the onsemi datasheet curve.