Alternating Current (AC) is a type of electrical current, in which the direction of the flow of electrons switches back and forth at regular intervals or cycles. Current flowing in power lines and normal household electricity that comes from a wall outlet is alternating current. The standard current used in the U.S. is 60 cycles per second (i.e. a frequency of 60 Hz); in Europe and most other parts of the world it is 50 cycles per second (i.e. a frequency of 50 Hz.).
Direct current (DC) is electrical current which flows consistently in one direction. Thecurrent that flows in a flashlight or another appliance running on batteries is direct current.
One advantage of alternating current is that it is relatively cheap to change the voltage of the current. Furthermore, the inevitable loss of energy that occurs when current is carried over long distances is far smaller with alternating current than with direct current.
DC power supplies are categorized by the mechanism used to convert and transfer the input power to the output power. There are three main categories:
Linear power supplies accept AC inputs and provide one or more DC outputs for a wide variety of computer and industrial applications. They use an active element (normally a power transistor) operating in its linear region to generate a desired voltage. The output voltage is regulated by dropping excess input power in ohmic losses (heat) in a series dissipative component (resistor) or a transistor. Linear power supplies provide excellent regulation, very small ripple, and very little output noise.
Switching power supplies use a switching element or regulator (normally a power transistor) to generate the desired voltage. They are also called switch-mode products or switching mode power supplies (SMPSs). These power supplies incorporate electronic components that continuously switch ON and OFF at a very high frequency. This switching action connects and disconnects energy-storing devices (inductors or capacitors) to and from the input source voltage or the output load. A SMPS design results in high power density (smaller size for the same power output) and reduced power consumption (higher efficiency) in comparison to linear power supplies.
SCR power supplies use silicon controlled rectifier (SCR) topology to provide well-regulated voltage and current output. Silicon controlled rectifiers are four-layer thyristors with an input control terminal, an output terminal, and a cathode or terminal that is common to both the input and output terminals. An SCR circuit is commonly used in applications involving high voltages and currents.