Optocoupler

Isolates two circuits by carrying the signal with light.

Schematic letter: U / OC
Unit / key rating: Isolation voltage (V), CTR (%)

Pinout

1 anode, 2 cathode: The LED side (the dot on the body marks pin 1).
3 emitter, 4 collector: The transistor side.

Real photos

What is it for?

An optocoupler holds an LED and a phototransistor facing it. When the LED lights, the transistor conducts; there's no electrical connection between the sides, only light.

It carries output information to the mains side in power supplies and isolates processors from high-voltage circuits. The most common one is the PC817.

Things to watch out for

⚠ The LED side should read ≈1.1 V on diode mode.
⚠ When replacing, use the same code and CTR grade (e.g. PC817C).

🔒 This content is in the repair pack

The repair pack unlocks all interactive content in one go: diagrams and current flows, the Repair Workshop, simulations, concepts, IC guides, the Component Library and comparison guides. One sample in every section is free.

Repair pack: 5,000 ₺, members get 10% off → 4,500 ₺. Or 1-week access for 1,000 ₺.