Page 156 - Basic Electrical Engineering

source is the open-circuit voltage between the terminals caused by the active
network. The series resistance is the equivalent resistance of the whole
circuit across the terminals looking to the circuit from the two terminals with

all the sources of EMF short circuited.
The steps involved in applying Thevenin’s theorem in calculating current

in a circuit component are as follows:

1. Remove the resistance from the circuit terminals through which current is to be determined.
2. Determine the open-circuit voltage that would be appearing across the circuit terminals
wherefrom the resistance has been removed. This is called V OC .

3. Calculate the equivalent resistance of the whole circuit across the terminals after replacing the
sources of EMFs by their internal resistances (or by simply short–circuiting them if internal
resistance is not provided or not known) and by keeping the current sources open–circuited
(i.e., considering having infinite resistance).
4. Draw the Thevenins equivalent circuit with V OC as the voltage source, R as the internal
eq
resistance of the voltage source, and R is the load resistance connected across the voltage
source.
5. Calculate the current through R using the relation.

Example 2.22 Using Thevenin’s theorem calculate the range of current
flowing through the resistance R when its value is varied from 6 Ω to 36 Ω.

Figure 2.51

Solution: