Galvanic isolation for signal transfer in industrial control
The onsemi FODM121 is a single-channel transistor-output optocoupler that provides 3750Vrms galvanic isolation between a DC input LED and an output phototransistor. This isolation rating is sufficient for basic insulation in mains-connected industrial control circuits, PLC digital inputs, and motor drive feedback paths where ground loops or voltage transients must be blocked. At the 50% floor, a 5mA input still guarantees 2.5mA of output drive, enough to saturate a downstream logic gate or relay coil interface.
Output ratings and switching speed for BOM planning
The output transistor is rated for 80V collector-emitter voltage and 80mA continuous output current, with a Vce saturation of 400mV max. The 80mA ceiling means it is not suited for driving solenoids or incandescent lamps without a buffer transistor. Rise and fall times are 3µs typical each, setting a practical switching ceiling around 100 kHz for digital signals. For 50/60 Hz mains zero-cross detection or low-speed serial data, this is more than adequate; for high-speed fieldbus isolation it falls short — the propagation delay asymmetry would skew the bit timing. The forward LED requires 1.3V max at up to 50mA continuous. The 50mA absolute maximum forward current means the series resistor must be sized to keep the LED within its safe operating area across the full input voltage range, especially if the input rail exceeds 5V.
Active lifecycle and compliance documentation
The surface-mount 4-SMD Gull Wing package (supplier device package 4-SMD) is compatible with standard reflow soldering profiles. The tube packaging is typical for prototype and low-volume assembly; for high-volume pick-and-place, verify with your distributor whether tape-and-reel options are available on alternate order codes within the same family.