PC Power supply wiring

Operation Basics

Starting a Standalone Unit

To start a standalone power supply unit (PSU) outside of a PC case, short the PS_ON# line to the common. This will initiate full power output; otherwise, only the 5V standby voltage will be present.

Connector Types and Specifications

Main Connector P1

• Old 20-pin connector: Used in original ATX systems.
• New 24-pin connector: Introduced with PCI Express® to provide additional wattage.
• Molex Housing Mini-Fit Jr. P/N#: 39-01-2240 (new), 5557-24R (old).
• Contacts: 44476-1112.
• Mating motherboard header: Molex 44206-0007.
• Old socket: 39-01-2200, mating header 39-29-9202.

12V Connectors

• Standard P4: 4 pins, Molex p/n 39-01-2040.
• EPS and high-power systems: 8 or more pins.

Peripheral Connectors

• Molex connector: Rated current 6A per contact.
• SATA connector: Three pins for each voltage, one pin for pre-charge.

Additional Connectors

• 2x3 or 2x4 connectors: Used in PSUs over 450W for high-end graphics cards.
• PCI Express® receptacle: Molex p/n 0455590.

Voltage and Current Ratings

Main Molex Connector

• 3.3V: 18A maximum.
• 5V: 24A maximum.
• 12V: Increased importance due to voltage regulation modules (VRM) for CPU and motherboard components.

Auxiliary Power Cable

• Removed from ATX12V spec v2.0: Due to extra wires added to P1.

Proprietary PSU Designs

Brand-Specific PSU Designs

• Apple Power Mac, Dell, Compaq, HP: Use proprietary boards with different pin designations.

Testing and Measurement

Voltage Measurement

• Common reference: All voltages are referenced to the same common.
• Voltmeter connection: Connect the return lead to any COM contact.

Additional Features

Optional 2x3 Socket

• Functions: Fan monitoring and control, IEEE-1394 power source, remote sense of 3.3V.

High-Power Systems

• PSU over 450W: Additional 2x3 or 2x4 connectors for high-end graphics cards.