Start-up resistor and auxiliary winding: how the IC gets its power

The IC runs the transformer, and the transformer powers the IC. So where does the first energy come from? The commonest adapter fault hides right here.

What does it do? When plugged in, the start-up resistor slowly charges the VCC capacitor. When VCC reaches the start threshold the IC runs; shortly after, the transformer's auxiliary winding takes over powering it.

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

The time scale here is different: each slice is hundreds of milliseconds. The IC waits in slice 1, starts on Cvcc's energy in slice 2 and runs from the auxiliary winding in slice 3.

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

Phase 1/3: 1. Plugged in: Cvcc charges

The IC isn't running yet. 310 V slowly charges Cvcc through the high-value start-up resistor (hundreds of milliseconds).

PWM output: LOW (0 V, switch off)

1. VCC (healthy circuit)

Rises slowly to 16 V, dips a little when the IC starts, then settles at ≈14 V once the auxiliary winding takes over.

On the scope

Multimeter≈14 V while running

🔍 How to spot it on a board

•High-value resistors in series from the 310 V rail to the IC's VCC pin (inside the IC on some newer parts)
•A small 10–47 µF electrolytic on the VCC pin
•A fast diode fed from a separate winding on the primary side of the transformer

🔧 How to measure it

•Unplugged: measure the start-up resistors one by one.
•Remove the VCC capacitor and measure capacitance/ESR.
•Plugged in, watch VCC during the first seconds: it should settle, not cycle.

⚠ Common faults

•Start-up resistor open: VCC never rises, the device is dead.
•VCC capacitor dried out: VCC cycles between 10 and 16 V and the adapter ticks.
•Dvcc open or a cracked auxiliary winding joint: the same ticking.

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Go further

Where it's used: the real flyback circuit UC3842: the VCC pin and thresholds Repair Workshop: charger