What the isolation and CTR tell you on the bench
The H11A817AS is a 1-channel optoisolator from onsemi with a DC-input LED driving a phototransistor output. Its 5300Vrms isolation rating means it can withstand that potential between input and output for one minute per the hi-pot test — enough for reinforced insulation in mains-powered equipment where you need to keep the low-voltage control side electrically separate from the line side. The current transfer ratio (CTR) is specified at 80% minimum and 160% maximum, both measured at 5 mA forward current. That spread tells you the LED efficiency varies part-to-part — in a design where the output must pull a logic input high at a known current, you size the input resistor for the minimum CTR, not the typical, or the circuit may not latch at the low end of the bin.
Package, temperature, and output limits
Housed in a 4-SMD gull-wing package (supplier device package 4-SMD), this part is surface-mount and suited for reflow assembly. The phototransistor output is rated for 70 V maximum across the collector-emitter, with a continuous output current of 50 mA per channel. Saturation voltage is 200 mV max at the rated current — that low Vce(sat) keeps power dissipation in the output transistor manageable when switching a downstream gate or relay coil. Rise and fall times are both 2.4 µs typical, which limits the switching speed to low-frequency signals — think 50/60 Hz line sync, relay drive, or status feedback, not high-speed data. The forward voltage drop on the LED side is 1.2 V typical at 5 mA, with an absolute maximum forward current of 50 mA.
Lifecycle reality — this part is obsolete
onsemi lists the H11A817AS as obsolete. Sourcing now runs through independent distribution and surplus channels — quantities are confirmed at RFQ, and the available stock is what it is. If you need a pin-compatible replacement, the H11A family shares the same 4-SMD footprint and pinout — a cross-reference search against the base part number H11A will surface functionally equivalent parts from onsemi and other manufacturers, but each must be qualified for CTR bin, isolation, and temperature grade against your BOM.