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A computer power supply unit (PSU) is an internal device that converts the building's AC power into low-voltage DC power for the computer's components. It acts as the "heart" of the computer, converting the \(120V\) AC from a wall outlet into the specific voltages, such as \(+12V\), \(+5V\), and \(+3.3V\). This DC power is then distributed to parts like the motherboard, processor, and storage drives. 

Key functions and features

AC to DC conversion: The primary function is to convert high-voltage alternating current (AC) from the wall into the stable, low-voltage direct current (DC) that computer components need to operate.

Voltage regulation: It maintains a stable output voltage, preventing fluctuating power from damaging sensitive hardware.

Power distribution: It supplies different components with the precise voltages they require through multiple output rails (e.g., \(+12V\) for fans and \(+5V\) for network cards).

Safety features: Modern PSUs include protection circuits like overcurrent protection (OCP) and short circuit protection (SCP) to safeguard the system from power surges and other issues.

Standby power: Many PSUs provide a small amount of constant power (e.g., \(5VSB\)) to the motherboard for functions like turning the computer on, even when it's "off".

Form factor: PSUs come in different sizes, such as ATX for desktops and smaller options for mini-PCs, with laptops often using an external power "brick".