Galvanic isolation with Darlington drive
The 4N29SR2M is a single-channel optoisolator from onsemi that couples a DC input LED to a Darlington phototransistor output with a base terminal. The 4170Vrms isolation rating makes it suitable for breaking ground loops and protecting low-voltage logic from high-side transients in industrial controls, motor drives, and power supplies. A minimum current transfer ratio of 100% at 10mA forward current means the output can sink at least as much current as the LED draws — no gain compression at the low end. The Darlington configuration delivers a continuous output current of 150mA and a Vce saturation of 1V max, so it can drive relays, solenoids, or PLC input modules directly without a buffer transistor.
Switching speed and load timing
Turn-on time is 5µs typical, but turn-off stretches to 40µs maximum. That asymmetry caps the practical switching frequency around 12 kHz — fine for 50/60 Hz line-cycle detection, relay coil switching, or status feedback from a contactor, but not for high-speed data isolation. If your application needs symmetric switching below 10µs, the 4N38SM (transistor output, 5µs turn-on/off) is a faster alternative, though its output current is limited to 100mA.
Active production, surface-mount package
It ships in Tape & Reel or Cut Tape, which matches automated pick-and-place assembly. The 6-SMD gull-wing footprint is the surface-mount variant of the classic 4N29 — the through-hole 4N29M is electrically identical but requires manual insertion.
Operating range and derating
The operating temperature spans -40°C to 100°C, covering industrial enclosures and outdoor telecom cabinets. The maximum forward LED current is 80mA, and the typical forward voltage is 1.2V — a 220-ohm series resistor from a 5V logic rail will bias the LED at about 17mA, well within the safe zone and above the 10mA test point for the CTR spec.