A power supply is an electrical device that supplies power to an electrical load. The power supply draws current from an input power source like main power lines, batteries, fuels cells, and generators, and delivers the correct voltage, current and frequency to the load.
Power supplies can be standalone, or built into the load they power. Standalone supplies like benchtop units, inverters and converters typically limit main line current to safe levels before delivering power to the load. Built-in units perform similar functions for desktop computers and other consumer electronics devices.
Power supply units can deliver AC or DC power, convert from one signal to the other, regulate current, voltage, and frequency, provide overload protection, and provide an uninterruptible source of power in the event of lost source power.