Opto-coupled feedback: how is the output held steady?

An adapter holds its output at 5 V whatever comes from the socket and whatever the load. A small loop that sends news from the safe side to the mains side with light makes it happen.

What does it do? The voltage divider scales the output for the TL431; the TL431 compares it with its 2.495 V reference and turns the difference into opto LED current. The light crosses the isolation barrier, and the mains-side IC shortens or lengthens the pulse accordingly.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

The loop always works in reverse: if the output rises, the pulse shortens; if it falls, the pulse lengthens. That's why the output stays at 5 V whatever the load.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/2: Output low (load increased)

When the output dips slightly below 5 V, REF falls below 2.495 V and the TL431 sinks less current. The opto LED dims, its transistor pulls COMP down less; COMP rises and the IC gives longer pulses. The output climbs back to 5 V.

1. Output

Every time the load changes the output deviates for a moment and the feedback corrects it.

On the scope

Multimeter5.0 V

🔍 How to spot it on a board

•A 3-pin TL431 on the safe side with two resistors (the divider) going to it
•A 4-pin optocoupler (PC817-type) sitting right over the isolation line
•The other side of the optocoupler going to the IC's COMP/FB pin

🔧 How to measure it

•Running, TL431 REF–anode is 2.495 V.
•The opto LED reads ≈1.1 V on diode mode.
•COMP should change with load; if it's always at maximum, there's no feedback.

⚠ Common faults

•Output high: the divider's top resistor is open, the TL431 isn't sinking, or the opto LED is weak.
•No or very low output: the TL431 is shorted or the opto transistor is shorted.
•Output rises then shuts off (over-voltage protection): a break in the feedback path.

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Go further

TL431 pins PC817 pins The real flyback circuit