What the 5000Vrms isolation and Darlington output mean for your PLC or VFD interface
The 6N138SDM is a single-channel optoisolator from onsemi that provides a 5000Vrms galvanic isolation barrier between a DC input and a Darlington output with base access. The 300% minimum current transfer ratio at a low 1.6mA forward current means you can drive the input directly from a 3.3V or 5V logic pin without an external transistor buffer, saving board space and BOM cost. The Darlington output with base connection gives you the option to add a pull-down resistor or a speed-up capacitor to tailor the turn-off time (7.3µs typical) for your application — useful in PLC digital input modules where the input filter time constant must match the scan cycle.
Parametric fit: CTR, forward voltage, and timing for logic-level interfaces
With a typical forward voltage of 1.3V and a maximum DC forward current of 20mA, the input side is compatible with standard 3.3V and 5V logic families. The 300% minimum CTR at 1.6mA ensures the output can sink the full 60mA per channel even when the input current is kept low to minimise power dissipation. The 1µs typical turn-on and 7.3µs turn-off times are asymmetric — the slower turn-off is dominated by the Darlington storage time. In a motor drive VFD command path, this asymmetry means the on-pulse width is preserved but the off-pulse may stretch; account for it in the dead-time calculation.