The time-varying part of a voltage that is ideally time-invariant.

Most electronic systems require a direct-current voltage for at least part of their operation.

An ideal direct-current voltage is available from a battery, but batteries are impractical for many applications.

To obtain a direct-current voltage from the alternating-current power mains requires using some type of power supply.

The voltage regulator is usually an electronic circuit that is specifically designed to provide a very stable direct current output voltage even if large variations occur in the input.

Nonlinear power supplies, which are often termed switching power supplies or switched-mode power supplies, are becoming increasingly popular as a practical alternative for producing a low-ripple direct current output.