The ATX (Advanced Technology eXtended) power supply standard, introduced by Intel in 1995, defines both the physical layout of power supplies and the electrical specifications required for personal computers. One of its most important aspects is the connector pinout, which guarantees compatibility between motherboards, PSUs, and peripherals.

Understanding ATX Power Supply Pinout and Specifications

The ATX (Advanced Technology eXtended) power supply standard, introduced by Intel in 1995, defines both the physical layout of power supplies and the electrical specifications required for personal computers. One of its most important aspects is the connector pinout, which guarantees compatibility between motherboards, PSUs, and peripherals.

Electrical Outputs of ATX Power Supplies

ATX PSUs convert AC mains voltage (110/220 V depending on the region) into regulated DC voltages. The standard rails are:

• +3.3 V: Logic circuits, chipsets, RAM.
• +5 V: Older logic circuits, USB, some drives.
• +12 V: CPU, GPU, fans, hard disks, optical drives.
• -12 V: Legacy ports (RS-232, some network cards).
• +5 VSB (Standby): Keeps certain circuits active even when the PC is off, enabling wake-on-LAN or USB charging.

24-Pin ATX Connector Pinout

Modern ATX motherboards use a 24-pin main connector (earlier versions had 20 pins). Each pin has a fixed role. Below is a simplified color-coded diagram that reflects the standard wiring:

+----+----+----+----+----+----+----+----+----+----+----+----+
|  1 |  2 |  3 |  4 |  5 |  6 |  7 |  8 |  9 | 10 | 11 | 12 |
|3.3 |3.3 | G  |5V  | G  |5V  | G  |PWR |5SB |12V |12V |12V |
|Org |Org |Blk |Red |Blk |Red |Blk |Gray|Pur |Yel |Yel |Yel |
+-----------------------------------------------------------+
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 |
|3.3 |-12 | G  |PS# | G  | G  | G  |-5* |5V  |5V  |5V  |3.3 |
|Org |Blu |Blk |Grn |Blk |Blk |Blk |Wh* |Red |Red |Red |Org |
+----+----+----+----+----+----+----+----+----+----+----+----+

Legend

• Org = Orange (+3.3 V)
• Red = +5 V
• Yel = +12 V
• Blk = Ground
• Pur = +5 VSB
• Grn = PS_ON# (active low)
• Gray = PWR_OK
• Blu = -12 V
• Wh = -5 V (obsolete in modern ATX)

*The -5 V rail is no longer present in the ATX12V 1.3 and later specifications.

Control Signals

• PS_ON# (Pin 16): Turns on the power supply when grounded by the motherboard.
• PWR_OK (Pin 8): Sent high once voltages are stable and within tolerance.
• +5 VSB (Pin 9): Always powered as long as the PSU is connected to mains.

These signals allow soft power control, system stability checks, and energy-saving features.

Engineering Tolerances

ATX specifies strict tolerances:

• ±5% for +5 V and +3.3 V rails.
• ±10% for +12 V and -12 V rails.

Protection mechanisms include over-voltage, over-current, and short-circuit safeguards, ensuring both user safety and hardware longevity.

Market and Evolution

The ATX standard has evolved into derivatives such as:

• ATX12V: Focused on +12 V rail for modern CPUs/GPUs.
• EPS12V: Used in servers, with 8-pin CPU connectors.
• SFX/TFX: Compact PSUs for small form factor systems.

Despite evolutions, the 24-pin main connector remains a cornerstone of desktop PC power design.

References

• ATX Standard – Wikipedia
• Power Supply Unit (Computer) – Wikipedia
• Institute of Electrical and Electronics Engineers (IEEE)
• PCI-SIG – Peripheral Component Interconnect Special Interest Group

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