Isolation barrier and output drive
The 4N29M is a single-channel optoisolator from onsemi with a 4170Vrms isolation voltage rating. That rating is the peak repetitive voltage the barrier can withstand — for a 250Vrms working voltage (common in mains-referenced control circuits) you have a comfortable margin, but always confirm the working voltage and creepage distance against your end-equipment safety standard. The output is a Darlington pair with the base brought out to a separate pin, giving you the option to add a speed-up resistor or a pull-down to improve turn-off time. The typical turn-on time is 5 µs, turn-off 40 µs max — fast enough for 10 kHz PLC input modules or relay replacement, but not for high-speed serial isolation. Minimum current transfer ratio is 100% at an LED forward current of 10 mA. That means with 10 mA into the LED, the output can sink at least 10 mA collector current.
Rated for operation from -40°C to +100°C, the 4N29M covers industrial control cabinets, outdoor telecom shelters, and HVAC panels without needing a wider-temperature commercial part. The input is DC only — confirm your drive signal is a steady DC level, not an AC line or a pulsed waveform that could dip below the LED forward voltage (Vf typ 1.2 V,) and drop out. Maximum continuous LED forward current is 80 mA, so a current-limiting resistor is always required.
It is ROHS3 compliant — no lead in the solder pins, no red-list substances.