TL431 pins: what to measure in the feedback loop

The small part on the safe side of almost every switch-mode supply that measures the output and reports it to the mains side through the optocoupler.

Where you'll meet it The feedback circuit of chargers, adapters, TV and PC power supplies

Control / signalGroundDrive output

Pin 1 · Control / signal

REF

The reference input, fed from the output by a voltage divider; the TL431 draws cathode current so that this pin sits at 2.495 V. Output voltage = 2.495 × (1 + R1 / R2).

🔧 How to measure

DC voltage against the anode (cold-side ground), with the board running.

Expected on a working board2.495 V (2.47–2.52 V)

On the scope

⚠ If the reading is wrongIf REF is clearly above 2.5 V and the output is high, the TL431 isn't sinking current (faulty). If REF is below 2.5 V, measure the divider resistors.

📋 What order to measure in?

With the board running, is REF–ANODE 2.495 V?
Measure the divider resistors: does output = 2.495 × (1 + R1/R2) hold?
Is the CATHODE below the output voltage (is it sinking current)?
Is there ≈1.0–1.2 V across the optocoupler LED?
Out of circuit: in diode mode you should see ≈0.7 V from ANODE to CATHODE (its internal protection diode) and OL the other way.

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