A20112 Power Supply Schematic Guide

The AC mains enter through a standard IEC connector. This stage features a safety fuse (typically a slow-blow 2A fuse) and a Metal Oxide Varistor (MOV) placed across the live and neutral lines to suppress high-voltage transient surges. A step-down transformer reduces the mains voltage to an intermediate AC level (e.g., 18V AC). Stage 2: Rectification Bridge

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3. Power Factor Correction (PFC) Stage (The R2A20112 Engine)

| Symptom | Likely Failed Component | Schematic Area to Inspect | |--------|------------------------|----------------------------| | No output, no LED | Fuse, MOV, Bridge rectifier | AC input, DB1, C1 | | Output low or oscillating | TL431, optocoupler, output caps | Feedback loop, secondary filter | | PSU ticks / squeals | Rsense, PWM controller, auxiliary supply | CS pin circuit, VCC capacitor | | Output high (overvoltage) | Shorted optocoupler or TL431 | Feedback divider R12/R13 | | MOSFET shorted | Q1, Rsense, U1 | Primary switching, gate drive |

Check the voltage across the reference Zener diode. If the Zener is shorted, the regulator cannot bias properly.

AC Live --- Fuse --- EMI Choke --- Bridge Diode (+) --- Bulk Cap (+) --- Transformer Primary (Pin 1) | | GND MOSFET (Drain) | Source --- Current Sense Resistor --- GND Primary | PWM Controller (CS pin)