The onsemi 4N37SD is a single-channel optoisolator with a DC-input LED and a phototransistor output that includes a base terminal. It comes in a 6-SMD gull-wing package on Tape & Reel, suited for surface-mount assembly. The minimum current transfer ratio is 100% at 10 mA forward current, so the output transistor saturates reliably with modest LED drive. Operating temperature spans -55°C to 100°C, covering industrial and some automotive under-hood environments. Maximum Vce saturation is 300 mV, keeping the logic-level drop low when the output is on.
onsemi lists the 4N37SD as Obsolete. For existing BOM lines that require this exact part number, sourcing runs through independent distribution channels.