The FOD817D3S is a 1-channel phototransistor optoisolator from onsemi's FOD817 family, rated for 5000Vrms isolation between input and output. That 5 kV figure is the dielectric withstand rating per UL 1577 — it means the part provides reinforced insulation for safety isolation in AC mains circuits, not just basic functional separation. With a DC input and transistor output, it fits the classic role of passing a logic-level or low-current DC signal across a high-voltage barrier — think PLC digital inputs, motor drive feedback, or SMPS secondary-side regulation loops where the controller sits on the primary side.
CTR range — the real selection parameter
Current transfer ratio is specified as 300% minimum and 600% maximum at If=5 mA. That wide band means the D3S bin guarantees at least 3× the output current relative to the input drive — so a 5 mA forward current yields at least 15 mA available at the collector, enough to saturate a TTL or CMOS logic input without an external buffer stage. The 200 mV Vce(sat) maximum at the output side confirms the transistor can pull a logic input low cleanly when driven into saturation — the voltage drop across the output is well below the 0.8 V VIL threshold of most 5 V logic families. Rise and fall times of 4 µs and 3 µs typical set the practical data rate ceiling at roughly 100 kbit/s in a saturated switching application — fast enough for 50/60 Hz zero-cross detection, relay coil feedback, or low-speed serial isolation, but not for high-speed SPI or CAN.
Temperature grade and board integration
Operating from -55°C to 110°C, the part covers the full industrial temperature envelope and extends into the lower end of automotive under-hood ambient — a motor drive sitting next to a hot heatsink or a power supply in an outdoor telecom cabinet stays within spec. The 4-SMD Gull Wing package (supplier device package 4-SMD) is a standard surface-mount footprint for optoisolators — the gull-wing leads provide visual solder-joint inspection access and relieve mechanical strain between the lead frame and the PCB pad during thermal cycling. Output rating is 70 V maximum collector-emitter voltage and 50 mA continuous current per channel — sufficient for driving a 24 V PLC input module or a 5 V logic gate, but not for directly switching a relay coil without a transistor buffer.
The base product number FOD817 covers multiple CTR bin grades (D3S is the 300-600% bin) and package options — the D3S suffix specifically identifies the surface-mount Gull Wing variant in tube packaging.