4170 Vrms isolation in a 6-DIP through-hole package
The 4N37VM: It provides 4170 Vrms galvanic isolation between a DC input LED and a phototransistor output with a base terminal, making it suitable for signal isolation in industrial control, PLC digital input modules, and power-supply feedback loops where the primary and secondary sides must be electrically separated.
CTR floor and switching speed — what they mean for the BOM
The minimum current transfer ratio of 100% at 10 mA forward current means the output transistor can sink at least as much collector current as the LED draws — a 10 mA LED drive yields at least 10 mA at the output, which is enough to pull a TTL/CMOS logic input low through a 2.2 kΩ pull-up resistor. The typical turn-on and turn-off times are both 2 µs, so the part can handle data rates up to roughly 250 kbit/s in a digital isolator application; for 10 MHz serial links you would need a logic-output optocoupler instead. The 300 mV maximum Vce saturation at the rated output current keeps the voltage drop across the output transistor low, so the logic-low level stays well below the 0.8 V threshold of a standard 5 V TTL input. The 30 V maximum output voltage gives headroom for 24 V PLC input circuits or for driving a small relay coil through an external transistor.