ATX Style

  • In the ATX type power supply the fan should be mounted along the inner side of the supply drawing air in from the rear of the chassis and blow it inside across the motherboard. This kind of airflow runs in the opposite direction as most standard supplies, which exhaust air out the back of the supply through a hole in the case where the fan is mounted. The idea was that the reverse flow design could cool the system more efficiently with only a single fan, eliminating the need for a fan (active) heat sink on the CPU.

ATX Power supply and Connectors

  • Power supplies used with Baby-AT boards have two connectors that plug into the motherboard. If these connectors are inserted out of their normal sequence, the motherboard will burn out. Most responsible system manufacturers “key” the motherboard and power supply connectors so that you cannot install them backward or out of sequence. However, some vendors of cheaper systems do not feature this keying on the boards or supplies they use.
  • The ATX form factor includes different power plugs for the motherboard to prevent users from plugging in their power supplies incorrectly. The ATX design features up to three motherboard power connectors that are definitively keyed, making plugging them in backward virtually impossible.
  • The new ATX connectors also supply +3.3V, reducing the need for voltage regulators on the motherboard to power the chipset, DIMMs, and other +3.3V circuits.
  • Besides the new +3.3V outputs, another set of outputs is furnished by an ATX power supply that is not normally seen on standard power supplies. The set consists of the Power_On (PS_ON) and 5V_Standby (5VSB) outputs mentioned earlier, known collectively as Soft Power. This enables features to be implemented, such as Wake on Ring or Wake on LAN, in which a signal from a modem or network adapter can actually cause a PC to wake up and power ON. Many such systems also have the option of setting a wake-up time, at which the PC can automatically turn itself ON to perform scheduled tasks. These signals also can enable the optional use of the keyboard to power the system ON. Users can enable these features because the 5V Standby power is always active, giving the motherboard a limited source of power even when OFF.


 Motherboard Power Connectors

Every PC power supply has special connectors that attach to the motherboard, giving power to the system processor, memory, and all slotted add-on boards (ISA, PCI, and AGP). Attaching these connectors improperly can have a devastating effect on your PC, including burning up both your power supply and motherboard.

ATX Main Power Connector

power connector terminal side view.

ATX Main Power Supply Connector Pin-out (Wire Side View)

Red+5V199+5VSB (Standby)Purple
ATX12V Connector

Power for the processor comes from a device called the voltage regulator module (VRM), which is built into most modern motherboards. This device senses the CPU voltage requirements (usually via sense pins on the processor) and calibrates itself to provide the proper voltage to run the CPU. VRM is mostly designed to use 12V for input power.

Peripheral Power Connectors

In addition to the motherboard power connectors, power supplies include a variety of peripheral power connectors for everything from floppy and hard drives to internal case fans. The following sections discuss the various types of connectors you’re likely to find in your PC.

Peripheral and Floppy Drive Power Connectors

The disk drive connectors on power supplies are fairly universal with regard to pin configuration and even wire color. The picture below shows the peripheral and floppy power connectors.

Peripheral Power Connector Pin-out (Large Drive Power Connector)


Peripheral Power Connector Pin out (Small Drive Power Connector)


Family of ATX, SATA

ATX, introduced in late 1995, defined three types of power connectors:

  • 4-pin “Molex connector” — transferred directly from AT standard: +5 V and +12 V for P-ATA hard disks, CD-ROMs, 5.25 inch floppy drives and other peripherals.
  • 4-pin Berg floppy connector — transferred directly from AT standard: +5 V and +12 V for 3.5 inch floppy drives and other peripherals.
  • 20-pin Molex Mini-fit Jr. main motherboard connector — new to the ATX standard.
  • A supplemental 6-pin AUX connector providing additional 3.3 V and 5 V supplies to the motherboard, if needed. This was used to power the CPU in motherboards with CPU voltage regulator modules which required 3.3 volt and/or 5 volt rails and could not get enough power through the regular 20-pin header.

SATA, short for Serial ATA, is an IDE standard for connecting devices like optical and drives to the motherboard.

The term SATA generally refers to the types of cables and connections that follow this standard. Serial ATA replaces Parallel ATA as the IDE standard of choice for connecting storage devices inside of a computer. SATA storage devices can transmit data to and from the rest of the computer over twice as fast as an otherwise similar PATA device.

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