A computer power supply, usually called a power supply unit or PSU, converts electrical energy from the mains supply into the regulated electrical power required by the computer's electronic components. A typical desktop PSU accepts alternating current from a wall outlet and converts it into several low-voltage direct-current supplies that can be distributed to the motherboard, processor, graphics card, storage devices, fans, USB-connected hardware and other components.
The PSU therefore performs much more than simply supplying electricity: it converts voltage, regulates output, filters unwanted electrical variation, manages changing loads and incorporates protection mechanisms intended to prevent abnormal electrical conditions from damaging the system. Without an appropriate power supply, the computer's components cannot receive the stable voltages and currents required for reliable operation.
Most desktop PSUs are switched-mode power supplies. Incoming mains AC is filtered and rectified, then electronic switching circuitry operates at high frequency and transfers energy through appropriate conversion stages, with transformers, inductors, capacitors and control circuits used to produce regulated DC outputs. The exact circuit topology varies among PSU designs and generations, but the fundamental purpose is to transform comparatively high-voltage AC into stable low-voltage DC.
Modern PSUs commonly provide a principal 12-volt output together with 5-volt and 3.3-volt supplies, although individual components often use onboard voltage regulators to convert these rails further. The motherboard and graphics card, for example, can use voltage-regulator modules to create the very low and tightly controlled voltages required by modern processors and memory.
The PSU distributes power through several types of connectors. The motherboard normally receives power through the large ATX main connector and additional CPU power connectors, while graphics cards can receive dedicated PCI Express power connectors when the electrical power available through their expansion slot is insufficient. SATA power connectors supply internal storage devices and some accessories, while other connectors may be present depending on the PSU design.
The connectors are physically keyed to reduce incorrect insertion, but visually similar modular PSU connectors should not be assumed to have identical electrical pinouts. A modular PSU allows selected cables to be attached as needed, whereas a non-modular PSU has its output cables permanently attached.
A PSU must handle changing electrical demand. A computer does not consume a fixed amount of power at all times: processors, graphics cards, storage devices and fans can move from low-power idle states to substantially higher consumption during demanding workloads. Graphics processors in particular can create rapid transient changes in power demand. The PSU's regulation circuitry and energy-storage components must respond to these changes while maintaining acceptable output voltage and stability. A PSU with a nominal wattage rating that appears adequate on paper may nevertheless be unsuitable if its design cannot handle the system's sustained or transient requirements. Correct power-supply selection therefore considers the entire system rather than simply adding a few component wattages together.
Electrical protection is another major PSU function. Depending on its design, a PSU can include over-voltage protection, under-voltage protection, over-current protection, short-circuit protection, over-power protection and over-temperature protection. These mechanisms monitor operating conditions and can shut down or otherwise limit the power supply when dangerous conditions are detected. Input filtering can also reduce conducted electrical interference, while power-factor correction can improve how the PSU draws current from the mains supply. Protection circuitry cannot guarantee that a computer will survive every external electrical event, however, so surge protection, appropriate mains infrastructure and an uninterruptible power supply can provide additional layers of protection where required.
PSU efficiency describes how much of the electrical power drawn from the mains is converted into useful output power rather than being dissipated as heat. A PSU operating at 90 percent efficiency, for example, delivers approximately 90 units of useful output power for every 100 units drawn under the relevant test conditions, with the remainder becoming heat. Efficiency varies with load and operating conditions. Certification programmes such as 80 PLUS classify efficiency performance under specified conditions, but an efficiency certification should not be interpreted as a complete measure of overall PSU quality. Electrical regulation, ripple, transient response, protection, component quality, thermal design and reliability are separate characteristics.
PSU wattage is the maximum rated output capability under specified conditions rather than the amount of electricity the computer automatically consumes. A 750-watt PSU does not continuously force 750 watts into a computer; components draw the power they require. The PSU's capacity should provide adequate headroom for normal operation and expected transient demands without being unnecessarily excessive.
The 12-volt output is particularly important in modern PCs because processors and graphics cards ultimately derive much of their operating power from it through local voltage-regulation circuitry. Manufacturers may publish recommended PSU capacities for graphics cards, but system builders should also consider CPU consumption, drives, cooling devices, USB peripherals and the characteristics of the specific PSU.
The power supply is therefore an essential part of the computer's electrical infrastructure rather than a performance component in the same sense as the CPU or GPU. Its job is to take available electrical energy and make it suitable for sensitive electronic circuitry by converting, regulating, filtering and distributing it while providing protection against specified abnormal conditions.
A PSU does not normally store significant energy for continued operation during a mains failure; that is a function associated with batteries and UPS systems. Laptop computers generally use an external AC adapter together with internal power-management circuitry and a battery rather than a conventional desktop PSU. Understanding what a PSU does explains why its electrical characteristics, connectors, capacity, efficiency and protection features all matter to the stability and safety of a computer.
