Isolation and CTR — the two numbers that decide the fit
The onsemi CNY17F3S is a 1-channel phototransistor optocoupler in a 6-SMD Gull Wing package. Its 5300 Vrms isolation rating means it can safely separate a 400 VAC motor-drive control signal from the 24 VDC PLC input side — the reinforced insulation margin covers the transient overvoltage category.
Switching speed and drive limits
Rise and fall times are 1 µs and 2 µs typical, with turn-on/turn-off times of 2 µs and 3 µs. This limits the practical switching frequency to a few tens of kHz — fine for reading a 24 VDC digital input or driving a gate driver at a few kHz, but too slow for a high-frequency SMPS feedback loop. The output can sink 50 mA continuous at up to 70 V, and the Vce(sat) is clamped at 300 mV max, keeping the logic-low level well below the 0.8 V threshold of a 5 V TTL input.
Lifecycle reality — obsolete, but sourced
onsemi lists the CNY17F3S as Obsolete. The operating temperature range of -55°C to 100°C covers industrial and most automotive under-hood environments, so a last-time buy for a long-life product is viable if the board spin is not an option.