There are two principal types of source, namely voltage source and current source. Sources can be either independent or dependent upon some other quantities.
An independent voltage source maintains a voltage (fixed or varying with time) which is not affected by any other quantity. Similarly an independent current source maintains a current (fixed or time-varying) which is unaffected by any other quantity. The usual symbols are shown in figure 1.3.
Figure 1.3: Symbols for independent sources
Some voltage (current) sources have their voltage (current) values varying with some other variables. They are called dependent voltage (current) sources or controlled voltage (current) sources , and their usual symbols are shown in figure 1.4.
Remarks -- It is not possible to force an independent voltage source to take up a voltage which is different from its defined value. Likewise, it is not possible to force an independent current source to take up a current which is different from its defined value. Two particular examples are short-circuiting an independent voltage source and open-circuiting an independent current source. Both are not permitted.
Figure 1.4: Symbols for dependent sources. Variables in brackets are the controlling variables whose values affect the value of the source.