PC817 pins: how to test an optocoupler

The small part that carries the output information to the mains side with light. Its LED is on the safe side and its transistor on the mains side, with no electrical connection between them. Tap a pin.

Where you'll meet it Almost every charger, adapter, TV and PC power supply (equivalents: EL817, LTV817, FOD817)

SupplyControl / signalGroundDrive output

Pin 1 · Supply

ANOT

The anode of the internal LED, usually tied to the output voltage through a resistor on the safe side. The dot on the body marks this pin.

🔧 How to measure

DC against safe-side ground, with the board running.

Expected on a working boardClose to the output, or a little below through the resistor

On the scope

⚠ If the reading is wrong0 V means the LED's series resistor is open: no feedback, the output rises and protection shuts down.

📋 What order to measure in?

Out of circuit: does 1–2 show ≈1.0–1.2 V one way and OL the other on diode mode?
Out of circuit: is 3–4 OL both ways? (If shorted, the transistor is faulty.)
Quick test: feed 5 V to 1–2 through 1 kΩ; the resistance across 3–4 should drop to a few hundred ohms.
With the board running, is there ≈1.1 V across the LED (1–2)?
When replacing, use the same code (and CTR grade, e.g. PC817C).

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