The electrical power is almost exclusively generated, transmitted and distributed in the form of ac because of economical consideration but for operation of most of the electronic devices and circuits, dc supply is required. Dry cells and batteries can be used for this purpose. No doubt, they have the advantages of being portable and ripple free but their voltages are low, they need frequent replacement and are expensive in comparison to conventional dc power supplies.

Now a days, almost all electronic equipment include a circuit that converts ac supply into dc supply. The figure show the basic block diagram of DC supply.

Transformer:

The transformer steps up or steps down the input line voltage and isolates the power supply from the power line.

Rectifier: 

RECTIFIER section converts the alternating current input signal to a pulsating direct current.

Filter:

pulsating dc is not desirable for this reason a FILTER section is used to convert pulsating dc to a purer, more desirable form of dc voltage.

Regulator:

The final section, the REGULATOR, does just what the name implies. It maintains the output of the power supply at a constant level in spite of large changes in load current or input line voltages. Now that you know what each section does, let's trace an ac signal through the power supply.

Constant voltage and current Modes of operation:

Voltage mode and current mode are the two regulating conditions that control the output of the supply. Most applications call for a supply to be used as a voltage source. A voltage source provides a constant output voltage as current is drawn from 0 to full rated current of the supply. In these applications, the power supply runs in voltage mode, maintaining a constant output voltage while providing the required current to the load. A voltage source is generally modelled as providing a low output impedance of the supply. 

Current mode works in a similar fashion, except it limits and regulates the output current of the supply to the desired level. When the supply runs in current mode, the supply provides a constant current into a variety of load voltage conditions including a short circuit. A current source is generally modelled as providing a very high output impedance of the supply. 

These two regulating modes work together to provide continuous control of the supply, but with only one mode regulating at a time. These are fast acting electronic regulating circuits, so automatic crossover between voltage mode to current mode is inherent in the design. With the programming of the voltage mode and current mode set points available to the customer, the maximum output voltage and current of the supply can be controlled under all operating conditions.