Virtually every electronic device gets its power from a direct current (DC) supply. Computers, cell phones and music players cannot run directly on the alternating current (AC) from the household electrical outlet, so they use a circuit that converts AC to DC. Smaller gadgets rely on a DC adapter built into the AC plug. Larger equipment, such as computers, stereos and televisions, have a DC supply inside the unit's case.
The AC electricity that arrives at your home has a voltage of 110 to 120 volts. In the late 1800s, engineers decided this voltage was high enough to power a light bulb or run an electric motor while being low enough to prevent bad shocks. Modern electronics, however, use voltages ranging from 1.5 to 24 volts. A transformer, a device with two coils surrounded by an iron core, efficiently reduces voltage from 110 to the level the circuits need. Transformers work only on AC, so a power supply reduces the incoming voltage before converting it to DC.
After reducing the voltage, a power supply converts AC to DC with a device called a rectifier. In AC, current flows back and forth in two directions, whereas with DC, it flows one way. The diodes inside a rectifier act as fast one-way switches, reversing the current when the AC flows backwards. If you looked at the current from a rectifier, you could safely say it was DC, though because of the rapid switching action, it has a lot of electrical noise.
To remove the electrical noise that rectifiers produce, DC power supplies use one or more large capacitors. These components absorb the noise's energy, smoothing it out. The capacitors also act as an energy reservoir, providing additional electrical current for brief periods of greater demand. For example, an audio amplifier uses more current during loud passages of music, especially for drums and bass-frequency sound.
The more sensitive an electronic device, the more it needs a precise voltage from its DC supply. While filtering removes the electrical noise from DC, its voltage will decrease when a device uses more current, then increase when demand eases up. The voltage will also wander when the household AC voltage changes during periods of peak demand. A regulator solves this problem by holding the voltage to a fixed value. A good regulator will hold voltage steady to within a few percent of its nominal rating.