There are several sources of direct current, capable of supplying voltages or currents, regardless of the specifications of the load to which they are connected. Among these various sources is the DC voltage, which has the versatility of being divided basically into three categories, they are batteries, generators and power supplies.
The power supplies are the most common to be found in laboratories, where they use the grinding processes, so that it is feasible to convert a voltage with variation in time into a voltage of fixed magnitude. Even in a DC current source is ideal, even if there are variations in voltage, the source can always provide a fixed current to any electronic system.
Batteries consist of the most common DC power source. They produce electricity by converting chemical energy and are made by secondary cells, rechargeable cells, or by primary cells, which can not be replenished. Generators are capable of converting mechanical energy into electrical energy and in the case of direct current, are also called dynamos and, when they are alternating current, alternators.