3750Vrms isolation in a 4-SMD Gull Wing package
The FODM124 is a single-channel optoisolator from onsemi that passes 3750Vrms of galvanic isolation between a DC-input LED and a phototransistor output. That isolation rating suits it for breaking ground loops in industrial sensor interfaces, PLC digital inputs, and low-power signal isolation where you need to keep the field side separate from the controller side. The 4-SMD Gull Wing footprint is a standard surface-mount package — it reflows on a normal lead-free profile without special handling.
CTR spread and output drive capability
The current transfer ratio is specified from 100% minimum to 1200% maximum, both measured at 1 mA forward current. That wide spread means the designer must bias the LED for the worst-case 100% CTR to guarantee the output transistor saturates — a 1 mA forward current yields at least 1 mA of collector current at the output. Compared to the FODM121B sibling, the FODM124 shares the same 3750Vrms isolation, same 3 µs rise/fall times, same 400 mV Vce saturation, and same temperature range. The difference is in the CTR bin: the FODM124's 100%-1200% range is wider than the FODM121B's typical bin, which gives more margin in designs where the LED drive current varies but also means the output saturation point is less predictable across temperature.