Now, we will discuss voltage and current sources
An electrical source is a device which converts mechanical, thermal, chemical or some other
form of energy into electrical energy. There are two types of sources, voltage sources and current sources
Voltage sources are further categorized as ideal voltage source and practical voltage source
Examples of voltage sources are batteries, dynamos, alternators, etc. Ideal voltage source is defined as the energy source which gives constant voltage across
its terminals irrespective of current drawn through its terminals. The symbol of ideal voltage source is shown here
In an ideal voltage source, the terminal voltage is independent of the load impedance, RL connected
Whatever is the voltage of the source, the same voltage is available across the load
terminals of RL i.e. VL equals VS under loading condition as shown here
There is no drop of voltage in the source supplying current to the load
The internal resistance of the source is therefore zero. In a practical voltage source, there will be a drop in voltage available across the
load due to voltage drop in the resistance of the source itself when a load is connected
as shown here. In certain applications, a constant current flow through the circuit is required
When the load resistance is connected between the output terminals, a constant current iL
will flow through the load. The examples of current sources are photoelectric cells, collector current in transistors, etc
The symbol of current source is shown here
Now we will come to practical voltage and current sources
A practical voltage source like a battery has the drooping load characteristics due
to some internal resistance. A voltage source has small internal resistance in series
On the other hand, a current source has some high internal resistance in parallel
For ideal voltage source, internal series resistance is zero. For ideal current source, parallel internal resistance is infinity
A practical voltage source is shown as an ideal voltage source in series with a resistance
This resistance is called the internal resistance of the source as shown here
A practical current source is shown as an ideal current source in parallel with its
internal resistance as shown here. From this figure, we can write VL at open circuit condition is equal to VS i.e. when
the load is removed, the circuit becomes an open circuit and the voltage across the source
becomes the same as the voltage across the load terminals. When the load is short circuited, the short circuit current IL will be VS by RSE where
RSE is the internal resistance of the source. In the same way, from this figure, we can write VL at open circuit equals to ISH into RSH
And IL at short circuit equals to IS
Hope you got the idea. Thank you