The CMOS battery provides ~3V to the PCH to keep time and BIOS settings.
This triggers the internal relays of the PSU to turn on the main, high-current voltage rails: . Phase 3: Main Voltage Rail Generation
The PCH now knows every single power rail on the desktop motherboard is stable and clean.
The computer is fully powered on, active, and operational. desktop motherboard power sequence pdf exclusive
Mastering this sequence is the difference between randomly swapping parts and precisely pinpointing a faulty component. It transforms troubleshooting from an art into a science.
For more information on the desktop motherboard power sequence, please refer to the following resources:
Power supply fan does not turn on when motherboard is triggered. Failed SIO or short circuit on main rails. VCORE 0.8 V - 1.4 V Fans spin at maximum speed; no display; CPU remains cold. Shorted CPU VRM Mosfets / Dead PWM Controller. 8 PLTRST# / CPURST# The CMOS battery provides ~3V to the PCH
The +5V_SB rail is stepped down by Linear Drop-Out regulators to create +3.3V_ALW (Always) and +3.3V_SUS (Suspend) rails.
To help you better understand the desktop motherboard power sequence, we have created a comprehensive PDF guide that provides detailed information on the power sequence, including:
Standby power (Purple wire) provided as soon as the PSU is plugged in. The computer is fully powered on, active, and operational
This chip monitors motherboard temperatures, voltages, fan speeds, and handles front panel buttons.
Understanding this sequence is essential for diagnosing "no power" or "no display" faults, as a failure at any specific step points directly to the malfunctioning component (e.g., SIO, PCH, or VRM). ⚡ The 8-Step Power Sequence
The power supply unit (PSU) is turned off or unplugged. There is zero power running through the board except for the CMOS battery.
(Internal Link Placeholder) Summary Table for Fast Diagnosis Signal Name Destination Normal State