The ATX power supply specification defines the standard interface and electrical behavior for desktop computer power supplies, establishing a reliable foundation for system stability and component protection. First introduced in 1995 by Intel, the ATX form factor has evolved through numerous revisions to accommodate advances in processor technology, graphics performance, and energy efficiency requirements. Modern implementations dictate not only physical dimensions and mounting points but also precise voltage rails, ripple tolerances, and safety certifications that ensure compatibility across a vast ecosystem of motherboards, graphics cards, and storage devices.
Evolution and Historical Context of the ATX PSU Standard
Before the ATX specification, desktop computers relied on AT and Baby AT power supplies, which featured awkwardly placed connectors and lacked true shutdown sequencing controlled by the motherboard. The introduction of ATX brought a reversible connector, software-controlled power states, and standardized dimensions that allowed for more efficient internal layouts. Subsequent iterations, such as ATX12V and EPS12V, addressed the growing power demands of high-performance processors by introducing dedicated 12V rails and stricter regulation, directly influencing the design of today’s high-wattage units.
Key Electrical Specifications and Voltage Rails
An ATX power supply must deliver several distinct voltage rails with tight tolerances to meet the specification. The primary rails include +3.3V, +5V, +12V, -12V, and +5VSB (standby), each serving specific components such as chipset, memory, CPU, and legacy peripherals. The +12V rail is particularly critical, as it powers the CPU and graphics card, and the specification defines maximum current capacity, load regulation, and ripple noise to prevent instability or data corruption under varying loads.
Form Factor, Dimensions, and Mechanical Compatibility
The standard ATX power supply measures 150 mm wide by 86 mm high by 140 mm deep, although slimline variants exist for smaller cases. Mounting holes follow a consistent pattern to ensure compatibility with ATX towers, mid-towers, and full-tower chassis. Proper alignment of the I/O shroud and screw holes is essential to avoid clearance issues with motherboard backplates and to maintain optimal airflow around the unit, which directly impacts thermal performance and longevity.
Power Good Signal and System Control A core feature of the ATX specification is the Power Good (PG) signal, which indicates when the power supply has achieved stable voltages within defined limits. The motherboard awaits this signal before sending power to the CPU and other components, preventing potential damage from inrush current or voltage sag. Additionally, the PS_ON# pin allows the operating system to initiate soft shutdowns, after which the supply enters a low-power state while maintaining +5VSB for wake-on-LAN and other standby functions. Efficiency, Certification, and Environmental Considerations
A core feature of the ATX specification is the Power Good (PG) signal, which indicates when the power supply has achieved stable voltages within defined limits. The motherboard awaits this signal before sending power to the CPU and other components, preventing potential damage from inrush current or voltage sag. Additionally, the PS_ON# pin allows the operating system to initiate soft shutdowns, after which the supply enters a low-power state while maintaining +5VSB for wake-on-LAN and other standby functions.
