To support 75W PCI Express® requirements, in the new systems the old 20-pin main power connector has been replaced by a 24-pin connector. See the diagram for pinouts of the old and new connectors. The colors represent recommended colors of the wires in the PSU power cables. Main connector uses Molex Housing Mini-Fit Jr. P/N# 39-01-2240 or equivalent (old part number 5557-24R), contacts: Molex 44476-1112 or equivalent, mating motherboard connector is Molex 44206-0007. The old 20-pin connector was Molex 39-01-2200 or equivalent, mating motherboard part was Molex 39-29-9202.
Also see our guide on connecting 20-pin PSU to 24-pin motherboard.
To power up a stand alone PSU for testing, you need to short PS_ON pin with one of the common pins. Normally, PS_ON is activated when you press and release the computer power button while it is in standby mode.
The current rating of the main Molex connector is 6A/pin. In early 2000's, some motherboards with 3.3V >18A and 5V >24A (mainly dual CPU AMD systems) used an auxiliary 6-pin power cable. It was removed from ATX12V spec v2.0 in 2003 because extra pins were added to the main connector.
When the industry began using voltage regulation modules (VRM) running off 12V2 to power CPU and other motherboard components, the bulk of the power shifted to 12 volt bus. Most of today's motherboards power their CPU with a separate 12 volt cable, which has 4 pins for ATX style (sometimes called P4) or 8 or more pins for EPS and non-standard high-power systems. Some PSUs may have three or four 12 volt 4-pin connectors. The standard 4-pin connector is Molex 39-01-2040 or equivalent.
4-pin peripheral power connects go to disk drives, fans, and other smaller devices. The floppy drive power connector obviously powers the floppy drive.
Pin numbers and wire numbers in Serial Power ATA (SATA) connector are not 1:1. There are three power pins for each voltage. One pin from each voltage is used for pre-charge in the backplane. Mating serial connector of ATA devices contains both signal and power segments.
Some units may also have an optional 2x3 connector that can be used for ancillary functions, such as fan monitoring and control, IEEE-1394 power source, and a remote sense of 3.3 V.
The 6-pin PCI Express® power connector is Molex part number 0455590002.
