The FOD814SD is a single-channel optoisolator from onsemi with a transistor output and a 5000Vrms isolation rating — the kind of spec you reach for when you need reinforced insulation between a mains-voltage domain and a low-voltage control circuit. This part accepts both AC and DC input, which suits zero-cross detection on a mains line or monitoring a universal-input power supply where polarity is not fixed.
Key ratings that drive the BOM decision
The current transfer ratio (CTR) spans 20% to 300% at a 1 mA forward current — a wide bin that gives you headroom for degraded LED output over life, but also means you should design the pull-up resistor for the minimum CTR to guarantee the output saturates at end of life. Vce saturation is 200 mV max — low enough to keep the logic-level output clean when driving a 3.3V or 5V CMOS input, but check the collector current: the 50 mA output rating limits direct drive of a relay coil or LED string without a buffer transistor. Rise and fall times are 4 µs and 3 µs typical — fine for mains-frequency isolation, but not for high-speed data or gate-drive feedback where a digital isolator or high-speed optocoupler is needed.
It is ROHS3 compliant, so no exemption paperwork needed for EU or California markets. Packaging options include Tape & Reel and Cut Tape — the reel suits volume pick-and-place; the cut tape works for prototypes or small-batch builds.