Isolation and CTR — the two numbers that decide the fit
The FOD817DS is a 1-channel DC-input transistor-output optoisolator from onsemi's FOD817 family, delivering 5000Vrms of galvanic isolation between the input LED and the output phototransistor. That isolation rating covers reinforced insulation per IEC 60950-1 for mains-referenced circuits — think power-supply feedback loops, motor-drive interface signals, or any BOM where the control side must stay electrically separate from the high-voltage bus. The Current Transfer Ratio (CTR) is the headline parametric: 300% minimum to 600% maximum at a forward current of 5 mA. A 300% floor means the output transistor can sink at least three times the LED drive current — so a 5 mA input yields at least 15 mA of output drive capability. This wide CTR band gives the designer margin across temperature and LED aging, but it also means the saturation voltage (Vce(sat)) at 200 mV max stays low enough to keep the output logic-level clean even at the low end of the CTR range.
Speed and temperature envelope
Rise and fall times are 4 µs and 3 µs typical respectively. That places the usable switching frequency around 100 kHz or lower — fine for 50/60 Hz mains zero-crossing detection, relay drive, or slow digital isolation, but not for high-speed communication like I²C or SPI. The output can handle up to 70 V across the collector-emitter and 50 mA continuous current per channel, so it drives small relays, logic inputs, or low-power loads directly.
The FOD817DS comes in a 4-SMD Gull Wing package (supplier device package 4-SMD), which is a surface-mount version of the classic DIP-4 footprint. The Gull Wing leads give a visible solder fillet for AOI inspection and better thermal cycling reliability than a flat lead. The package is supplied in Tube — not tape-and-reel — so factor that into your pick-and-place feeder setup or plan for manual placement on prototype runs.