Source of power is essential for the operation of all the electrical or electronic circuits. These sources are classified into two categories as
- Voltage source
- Current source
The voltage and current sources can be further
classified as dependent sources and independent sources.
Independent Sources
The terminal voltage of source is not dependent on any other voltage source or current source is known as independent sources. The independent voltage sources can be further classified as:
- Ideal voltage sources
- Practical voltage sources
Ideal Voltage Source
The circuit symbol for an ideal voltage
source is as shown in the below figure. As shown in the below figure, the polarities
of the voltage are indicated inside the circle and terminal voltage is V volts.
The source resistance of an ideal voltage source is zero.
Therefore, the terminal voltage remains
constant and the magnitude is equal to V volts without load (RL =
infinite) or with as shown in the below figure. Thus, for an ideal voltage
source terminal voltage is independent of load. The graph voltage-current is as
shown in the above figure.
When the load is not connected with source, the current passes through the circuit is zero. But, when the load is connected, the load current (I) will flow from the positive end of the source, through load back to the negative end. This is the conventional flow of current.
Practical Voltage Source
In practice, the ideal voltage source does
not exist. All the sources practically existing has some amount of internal
resistance.
The circuit symbol for a practical voltage source is as shown in the below figure. It is clear from the below figure that the practical voltage source has an internal series resistance Rs.
When there is no load connected (RL
= infinite), the current passes through the circuit is zero. Therefore, there
is no voltage drop across Rs and the terminal voltage VL = V volts.
When the load is connected, a finite
current start flowing through the circuit. Hence, some voltage will drop across
the source and the terminal voltage will be slight less than V.
With increase in the load current I, the terminal voltage on decreasing as shown in the above figure.
Current Sources
An electric current is defined as rate of flow of electrons. This rate is not constant for voltage sources. It is necessary to have a constant current to the load in certain applications. Therefore, a current source is used instead of a voltage source.
Just like the voltage sources, the current sources also are classified as independent and dependent current sources.
Independent Current Sources
For an independent current source, the
current supplied by the source does not depend on any other voltage source or
current source. The independent current sources are further classified as;
- Ideal current sources
- Practical current sources
Ideal Current Sources
The symbol of an ideal current source is
shown in the below figure. The arrow indicates the direction of conventional current
flow. Note that the internal shunt resistance is infinite for the ideal current
source. (RL= infinite)
When the load resistance is connected between the output terminals as shown in the below figure. a constant current will flow through the load i.e. IL = I. The voltage across the load i.e. VL = I RL
As shown in the above graph, the current
remains constant irrespective of change in load. But the voltage will change
according to the change in load.
The current sources are derived from the
voltage source. It is not directly available. The current source contains a
voltage source with other electronics elements like inductor.
Practical Current Sources
The practical current source will have a finite internal shunt resistance as shown in the below figure.
When the load RL is connected as shown in figure, the load current I will start flowing through the circuit. But due to the presence of shunt internal resistance Rsh, the source current I gets divided between Rsh and RL. Hence the current through RL is less than I.
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