Batteries are often shown on a schematic diagram as the source of DC voltage but usually the actual DC voltage source is a power supply. A more reliable method of obtaining DC power is to transform, rectify, filter and regulate an AC line voltage. Power supplies make use of simple circuits which we will discuss presently.
DC power supplies are often constructed using a common inexpensive three-terminal regulator. These regulators are integrated circuits consisting of several solid state devices and are designed to provide the desirable attributes of temperature stability, output current limiting and thermal overload protection.
In power supply applications it is common to use a transformer to isolate the power supply from the 110 V AC line. A rectifier can be connected to the transformer secondary to generate a DC voltage with little AC ripple. The object of any power supply is to reduce the ripple which is the periodic variation in voltage about the steady value.