PC817

The PC817 is Sharp's iconic 4-pin photocoupler: an infrared-emitting GaAs LED optically coupled to a silicon phototransistor inside a compact DIP-4 (or SOP-4) package. It provides 5,000 Vrms of isolation between input and output while passing DC signals up to a few hundred kilohertz, making it the default choice for breaking ground loops, isolating microcontroller logic from mains-referenced circuits, and closing the feedback loop in switch-mode power supplies.

Key parameters: forward current 50 mA max (typical drive 5-20 mA), collector-emitter voltage 80 V, current transfer ratio (CTR) 50% to 600% depending on the suffix (PC817A: 80-160%, PC817B: 130-260%, PC817C: 200-400%, PC817D: 300-600%), and rise/fall times around 4 µs / 5 µs with a 100 Ω load — which limits useful bandwidth to roughly 100 kHz.

The CTR range is wide and varies significantly with temperature and age, so designs must accommodate worst-case: design for minimum CTR at high temperature and end-of-life (typical derating factor 50% after 10 years at high LED current). For digital isolation this rarely matters, but for SMPS feedback loops it forces generous design margins.

Modern alternatives with better specs include the LTV-817 (Lite-On, direct clone), EL817 (Everlight), and — for higher bandwidth and lower CTR spread — the digital isolators like TI ISO7721 or Silicon Labs Si8621 (which use capacitive isolation and support 100+ Mbps).

Shenzhen stock: PC817C (CTR 200-400%, DIP-4) and PC817S (SOP-4 gull-wing) are the most common. MOQ 100 pcs loose or 1,000 pcs on tape for the SOP variant.