Isolation and signal transfer for industrial DC buses
The FODM121B is a single-channel optoisolator from onsemi that passes a DC signal across a 3750Vrms galvanic barrier using a phototransistor receiver. The 4-SMD gull-wing package suits reflow assembly on a standard surface-mount line, and the –40°C to 110°C temperature grade covers industrial control cabinets, motor-drive feedback, and isolated sensor interfaces without a separate high-temp variant.
CTR margin and switching speed
Current transfer ratio is specified at 50% minimum and 150% maximum, both measured at If=5mA. That 3:1 spread means the LED drive current must be set so the transistor side delivers at least 2.5mA collector current at the low end of the CTR range. Rise and fall times are 3µs typical each, which limits the practical data rate to roughly 100 kbps for a clean digital waveform — adequate for 24V PLC inputs, relay drivers, and slow optocoupler feedback loops. Output transistor is rated for 80V Vce and 80mA continuous collector current. The 400mV Vce saturation at the rated current keeps conduction loss low enough that the part does not need a heatsink in normal use.