PC817C

The PC817C from Sharp is a single-channel phototransistor optocoupler that provides galvanic isolation between input and output circuits using an infrared LED optically coupled to a phototransistor. The C-grade specifies a current transfer ratio (CTR) of 200% to 400% at IF=5mA, meaning the output phototransistor can sink up to 4× the LED current—a significant advantage over lower CTR grades when driving loads from small LED currents. The 5000Vrms isolation voltage provides robust safety isolation between high-voltage and low-voltage circuits, meeting IEC 60950 and medical equipment standards. The 80kHz cut-off frequency and 4µs/3µs rise/fall times make it suitable for low-frequency signal isolation but not for high-speed data communication. The PC817 is one of the most widely produced optocouplers, available from Sharp, Everlight, Lite-on, Cosmo, and many other manufacturers, ensuring multi-source availability. The C suffix denotes the CTR grade; A (50-160%), B (130-260%), and D (300-600%) grades are also available.

The PC817C consists of an infrared LED and an NPN phototransistor die, separated by a transparent insulating dielectric that provides 5000Vrms electrical isolation. When current flows through the LED (pins 1-2), it emits infrared light that passes through the dielectric and illuminates the base region of the phototransistor. The absorbed photons generate base current in the phototransistor, which is amplified by the transistor’s current gain (hFE) to produce a much larger collector current. The ratio of collector current to LED current is the current transfer ratio (CTR). For the PC817C, CTR = 200-400% at IF=5mA, meaning IC = CTR × IF = 200-400% × 5mA = 10-20mA. The phototransistor operates like a standard NPN transistor, but the base is optically driven rather than electrically connected. With no LED current, only a tiny dark current (100nA max) flows through the phototransistor. The VCE(sat) of 0.1V typical means the phototransistor can saturate like a normal transistor, pulling the collector very close to the emitter voltage when driven hard enough. The CTR degrades over time and with temperature: at 100°C, CTR may drop to 50% of its 25°C value, and after 50,000 hours of operation at IF=20mA, CTR may degrade by 30-50%. This aging must be factored into long-life designs such as SMPS feedback loops.

PC817C pin assignment (DIP-4, top view, dot marks Pin 1): Pin 1 (top-left) = Anode, Pin 2 (top-right) = Cathode, Pin 3 (bottom-right) = Emitter, Pin 4 (bottom-left) = Collector. Pins 1-2 form the LED input side, Pins 3-4 form the phototransistor output side. The isolation barrier between sides provides 5000Vrms minimum insulation. CTR grade selection: A=80-160%, B=130-240%, C=200-400%, D=300-600%.

• SMPS Feedback: LED driven by TL431 via optocoupler; collector connects to PWM controller FB pin; 200% CTR provides ample loop gain
• MCU I/O Isolation: LED from MCU GPIO through 330Ω resistor; collector pulled up to isolated 5V; emitter to isolated GND; signal crosses isolation barrier
• Zero-Crossing: LED driven from AC mains through 100kΩ resistor and bridge rectifier; collector output pulses at 100/120Hz for TRIAC triggering
• Relay Driver: LED from 3.3V MCU; collector drives relay coil through NPN transistor; 5kV isolation protects MCU from relay transients
• Solid-State Relay: PC817C drives TRIAC gate through buffer transistor; optically isolated AC switching

PC817C is a 4-pin phototransistor optocoupler with 200-400% CTR (C-grade) and 5kVrms isolation. It is the most widely used optocoupler for feedback loops in switching power supplies and TRIAC triggering. A/B/D grades offer different CTR ranges. EL817 is a direct replacement. For high-speed digital isolation, TLP2710 provides 5Mbps logic output. For safety-critical applications, VO618A offers 5.3kVrms with high CTR.