Isolation barrier and CTR — the fit decision
The onsemi CNY17F3W is a single-channel optoisolator in a 6-DIP through-hole package (0.400-inch lead spacing, 10.16 mm body width). The 5300 Vrms isolation rating qualifies it for reinforced insulation in mains-referenced circuits — the barrier withstands the 250 V working voltage common to industrial PSU feedback loops and PLC digital inputs without derating. Current transfer ratio is guaranteed between 100% and 200% at If = 10 mA. That means the output transistor saturates with 100 µA of base drive for every 100 µA of LED current — enough margin to drive a 5 V logic input through a 10 kΩ pull-up without an external buffer. The 300 mV Vce(sat) max ensures the output pulls below the TTL low threshold across the full 70 V collector rating.
Switching speed and temperature envelope
Typical turn-on time is 2 µs, turn-off 3 µs, with rise/fall of 1 µs / 2 µs. This places the useful data rate around 100 kbps — adequate for 24 VDC sensor polling, relay coil feedback, and isolated serial links where the bus edge rate is not the bottleneck. Above 250 kbps the propagation delay skew starts to eat into the bit-cell window. The -55°C to +100°C operating range covers industrial cabinet ambient and outdoor telecom enclosures. At the cold end the LED forward voltage rises about 2 mV/°C below 25°C, so the drive current stays within the 100 mA abs-max limit even at -55°C with a 3.3 V logic supply and a 220 Ω series resistor.
onsemi lists the CNY17F3W as obsolete. No official successor or pin-compatible replacement is recorded from the original manufacturer. For new designs, consider a current-production optoisolator in the same 6-DIP footprint with equivalent or higher CTR (e.g., 400% or 600% bin) to maintain the same pull-up resistor value.