Darlington output with base access — what it means for the BOM
The H11B1SR2VM is a 1-channel optoisolator from onsemi with a Darlington phototransistor output that brings the base terminal out to a separate pin. This base-access pin lets you add a speed-up resistor or a pull-down to tailor the switching threshold and turn-off time — useful when driving a logic input that needs a clean edge rather than a slow Darlington saturation ramp. Rated for 4170 Vrms isolation, the part provides galvanic separation between a low-voltage DC control side and a higher-voltage load side, with the output capable of sinking 150 mA continuous at up to 30 V. The minimum current transfer ratio of 500% at 1 mA forward current means the LED drive current can be kept low — a 1 mA input yields at least 5 mA of collector current at the output, which is enough to saturate the Darlington pair and drive a relay coil or a PLC input module directly.
Switching speed and the base-pin trade-off
Turn-on time is 25 µs typical, turn-off 18 µs typical — these are slower than a standard phototransistor opto because the Darlington pair's internal gain stores more base charge. The base pin lets you add a resistor between base and emitter to speed up turn-off at the cost of reduced effective CTR; without it the part is best suited for DC-level or low-frequency switching (below ~10 kHz) where the 25 µs delay is negligible. Operating temperature spans -40°C to 100°C, covering industrial cabinet and appliance environments. The 6-SMD Gull Wing package is reflow-compatible and occupies the same footprint as the industry-standard 6-pin DIP Gull Wing — no board spin needed if migrating from a through-hole H11B1 variant.