Key ratings and what they mean for the BOM
The onsemi CNY173TM is a single-channel optoisolator in a 6-DIP through-hole package, with a DC input LED and a phototransistor output that includes a base connection — the base pin allows the designer to tailor switching threshold or add feedback compensation, a feature missing from simpler three-pin optocouplers. Rated for 4170 Vrms isolation, this part meets the reinforced insulation requirements for mains-referenced circuits in industrial power supplies, motor drives, and PLC digital inputs — the 4170 Vrms figure is the one-minute hipot test voltage between input and output sides. Current transfer ratio (CTR) is specified at 100% minimum and 200% maximum at a forward current of 10 mA — this 2:1 spread means the designer must guarantee the logic threshold at the low end of CTR and still have margin at the high end to avoid saturating the output transistor in the on state. Switching speed is 4 µs rise and 3.5 µs fall typical, with turn-on and turn-off times of 2 µs and 3 µs respectively — adequate for 10–100 kHz isolated data transmission or for debouncing slow field signals, but not for high-speed digital interfaces like I²C or SPI without a Schmitt-trigger buffer on the output.
No official successor part number is recorded from onsemi.