Triac-output optoisolator for AC load switching
The onsemi FOD4218SD is a 1-channel optoisolator with a triac output, designed to drive AC loads like solenoids, valves, and small motors from a low-voltage control side. Rated for 5000Vrms isolation between input and output, it provides galvanic separation for safety in equipment that must meet UL, cUL, and FIMKO standards. The 800 V off-state voltage rating means it can directly switch 240 VAC or 277 VAC lines with margin, and the 10 kV/µs minimum static dV/dt ensures it won't falsely trigger on fast mains transients or relay contact bounce.
With no zero-crossing circuit (random-phase triac driver), the FOD4218SD fires the triac at any point in the AC cycle — useful for phase-angle dimming or soft-start, but it generates more EMI than a zero-crossing part. The 60 µs turn-on time is fast enough for 50/60 Hz line switching; the 500 µA hold current (Ih) means the output triac stays latched once triggered until the load current drops below that threshold near the zero crossing. Operating from -55°C to 100°C, this optoisolator suits industrial enclosures, outdoor telecom cabinets, and motor-drive environments where the ambient temperature near the power stage runs hot.
The 1.28 V typical forward voltage (Vf) at 30 mA max DC forward current (If) means the LED side drops about 1.3 V; a series resistor from the logic supply sets the drive current.
Fully ROHS3 compliant, with no restricted substances above the threshold; the part also holds cUL, FIMKO, and UL safety agency approvals, covering North American and European regulatory acceptance.