5300 Vrms isolation, 100% CTR — the fit for a legacy isolated feedback path
The 4N373SD is a 1-channel optoisolator from onsemi's 4N37 family, with a DC input and a transistor-with-base output. Its 5300 Vrms isolation rating covers the reinforced insulation requirement for medical and industrial power supplies where the primary-to-secondary barrier must withstand a 5 kV dielectric test. Minimum current transfer ratio of 100% at 10 mA forward current means the output transistor saturates with a 10 mA LED drive — a standard logic-level or MCU GPIO can drive it directly without a pre-driver stage. Switching is rated at 2 µs turn-on and 2 µs turn-off typical, placing it in the medium-speed band for optoisolators — adequate for 100 kHz SMPS feedback loops or 10 kbaud serial isolation, but not for high-speed data links.
Obsolete — last-time-buy and sourcing reality
onsemi has marked the 4N373SD as Obsolete. The 6-SMD gull wing package and Tape & Reel packaging match the original production format, so existing pick-and-place feeders and reflow profiles carry over without a line change.
Temperature grade and output drive
Rated for -55°C to +100°C operating temperature, the 4N373SD covers the full industrial temperature range with margin into military-grade cold — suitable for outdoor telecom cabinets, engine-bay electronics, or cold-chain monitoring. Maximum output voltage is 30 V, and Vce saturation is 300 mV max at the rated drive — the output can sink a 10 mA load with less than 0.3 V drop, keeping the logic-low threshold well within TTL or CMOS input specs.