5000Vrms isolation for industrial noise rejection
The 6N138SM is a single-channel optoisolator from onsemi that delivers 5000Vrms of galvanic isolation between a DC input and a Darlington output with a separate base pin. That isolation rating is the spec that decides whether this part can sit on a 24V PLC input card or a motor-drive interface without arcing across the creepage gap. The Darlington configuration gives a minimum current transfer ratio of 300% at only 1.6 mA forward current, which means a 3.3V or 5V logic output can drive the LED directly and still saturate the output transistor — no extra driver stage needed.
Switching speed and timing margin
Turn-on time is 1 µs typical, turn-off is 7.3 µs typical. That 7.3 µs off-time is the bottleneck — it limits the practical data rate to roughly 70 kHz, so this part is a fit for slow digital signals (status flags, fault latches, relay commands) rather than high-speed serial links. If you need faster switching, the 6N139SVM sibling offers 240 ns turn-on and 1.3 µs turn-off, but at the same 5000Vrms isolation and Darlington output topology — a straight speed upgrade without changing the footprint.
That means you can commit this part to a BOM without planning a qualification cycle for a replacement. If your pick-and-place prefers tape-and-reel, the 6N138SDM variant is the same die and package but supplied on tape and reel.
Base pin access — external resistor option
The Darlington output includes a separate base pin (pin 7 on the 8-SMD package). Leaving the base open gives the standard Darlington response. Tying a resistor from base to ground (typically 10–100 kΩ) pulls leakage current away and speeds up turn-off — useful if the 7.3 µs off-time is marginal for your timing budget.