Optoelectronics

CTR = Current Transfer Ratio = output collector current divided by input LED forward current, expressed as a percentage. PC817A: 80-160%, PC817B: 130-260%, PC817C: 200-400%, PC817D: 300-600%. Higher CTR means less LED drive current for a given output, which extends LED life but narrows dynamic range. PC817C is the industry standard for SMPS feedback applications.

Not recommended. The PC817 has about 100 kHz bandwidth with slow rise/fall edges, which fails at SPI speeds above a few hundred kHz and can cause I²C bus hang-ups due to the open-collector output's slow rise time. For digital bus isolation, use dedicated isolators like ISO7741 (SPI, 100 Mbps), ISO1541 (I²C) or ADuM1201 (general-purpose, 150 Mbps).

Under continuous 10 mA drive at 25 °C, the LED maintains more than 50% of its initial output for 100,000+ hours. At 20 mA and 70 °C, life drops to about 20,000 hours. For industrial designs expecting 10+ year life, derate the LED current to 5-10 mA and design the feedback loop to accommodate CTR degradation over time.

Digital isolators (ISO7721, Si8621, ADuM1201) use capacitive or magnetic coupling instead of an LED, offering 100× the bandwidth, tighter timing tolerance, and 40+ year life without LED degradation. They cost 3-5× more than a PC817 but eliminate the CTR-derating design margin. For new high-reliability designs, choose digital isolators. For cost-sensitive SMPS feedback where the PC817 topology is already proven, stick with PC817C.