The ATX power supply specification defines the standard for desktop computer power delivery, establishing a reliable framework for converting household current into the precise voltages required by modern components. This specification evolved from earlier AT designs, introducing features like soft power switching and standardized output rails to support increasingly complex hardware. Understanding the ATX power supply spec is essential for selecting a unit that provides stable power, supports your system architecture, and operates efficiently under varying loads. This guide breaks down the key aspects of the specification, from wattage and rails to efficiency and safety certifications.
Core Voltage Rails and Their Purpose The ATX specification defines several key voltage rails, each serving specific components within a PC. The primary rails include +3.3V, +5V, and +12V, which must adhere to strict tolerance levels to ensure component safety and performance. Modern power supplies also provide secondary rails such as -12V and +5VSB, with the latter maintaining power to the motherboard and RAM during sleep states or wake-on-LAN functionality. The +12V rail is particularly critical, as it delivers the bulk of power to the CPU and graphics card, making its stability and amperage capacity central to the ATX power supply spec. Wattage, Amperage, and Headroom
The ATX specification defines several key voltage rails, each serving specific components within a PC. The primary rails include +3.3V, +5V, and +12V, which must adhere to strict tolerance levels to ensure component safety and performance. Modern power supplies also provide secondary rails such as -12V and +5VSB, with the latter maintaining power to the motherboard and RAM during sleep states or wake-on-LAN functionality. The +12V rail is particularly critical, as it delivers the bulk of power to the CPU and graphics card, making its stability and amperage capacity central to the ATX power supply spec.
Total wattage is a common metric for comparing power supplies, representing the maximum continuous power the unit can deliver under specified conditions. However, the distribution of current across the +12V rail is often more important than the overall wattage figure for system stability. The ATX specification does not mandate a minimum +12V output, so high-quality units emphasize robust 12V rail capacity to handle modern processors and GPUs. When selecting a power supply, focusing on sufficient +12V amperage and headroom for future upgrades ensures compliance with the underlying intent of the ATX power supply spec.
PS-ON and Control Signals
The PS-ON signal is a fundamental control line in the ATX power supply spec, allowing the motherboard to remotely turn the power supply on and off. When the user presses the power button, the motherboard sends a low signal to PS-ON, enabling the power supply to energize the main rails. The system can then initiate a software-controlled shutdown, sending a turn-off command that the power supply respects. This mechanism replaces the older hard-wired power switch, providing a standardized method for safe power management that aligns with the ATX specification's design goals.
Efficiency, PFC, and Active Components
Power factor correction (PFC) is a critical feature in modern ATX power supplies, improving how the unit draws current from the AC line and reducing electrical noise. Active PFC circuits, typically implemented as boost converters, correct the phase angle of the input current, achieving power factors above 0.9 and complying with regulations in many regions. Efficiency ratings, such as 80 Plus Bronze, Silver, Gold, Platinum, and Titanium, indicate how much of the input power is converted to usable DC output with minimal heat loss. These ratings are not part of the original ATX power supply spec but have become a key quality indicator, guiding consumers toward greener and more cost-effective power solutions.
Physical Connectors and Motherboard Integration
The 24-pin main power connector and the 4-pin or 8-pin CPU power connector are standardized outputs defined by the ATX power supply spec, ensuring compatibility between motherboards and power supplies. Additional peripheral connectors, such as SATA power cables and PCIe connectors for graphics cards, deliver power to storage and expansion cards through standardized pinouts. Modular and semi-modular designs allow users to attach only the cables they need, improving airflow and cable management within the chassis while maintaining full adherence to the specification's electrical requirements.
