1. THEVENIN’S THEOREM ENGR. VIKRAM KUMAR B.E (ELECTRONICS)
M.E (ELECTRONICS SYSTEM ENGG:)
MUET JAMSHORO

2. Thevenin's Theorem :
This theorem was first discovered by German scientist Hermann von Helmholtz in 1853, but was then rediscovered in 1883 by French telegraph engineer Léon Charles Thévenin ( ).
This theorem states that any linear bilateral circuit with combination of voltage sources , current sources and resistors with two terminals is electrically equivalent to a single voltage source VTh (Thevenin Voltage) and a single series resistor RTh (Thevenin Resistance).This equivalent is called Thevenin Equivalent.

4. Thévenin’s Theorem
Any two-terminal dc network can be replaced by an equivalent circuit consisting of a voltage source and a series resistor. 5

5. Thévenin’s Theorem Thévenin’s theorem can be used to:
Analyze networks with sources that are not in series or parallel.
Reduce the number of components required to establish the same characteristics at the output terminals.
Investigate the effect of changing a particular component on the behavior of a network without having to analyze the entire network after each change. 6

6. Thévenin’s Theorem
Procedure to determine the proper values of RTh and ETh
Preliminary
Remove that portion of the network across which the Thévenin equation circuit is to be found. In the figure below, this requires that the load resistor RL be temporarily removed from the network. 7

7. Thévenin’s Theorem RTh:
Mark the terminals of the remaining two-terminal network. (The importance of this step will become obvious as we progress through some complex networks.)
RTh:
Calculate RTh by first setting all sources to zero (voltage sources are replaced by short circuits, and current sources by open circuits) and then finding the resultant resistance between the two marked terminals. (If the internal resistance of the voltage and/or current sources is included in the original network, it must remain when the sources are set to zero.) 8

8. Thévenin’s Theorem ETh:
Calculate ETh by first returning all sources to their original position and finding the open-circuit voltage between the marked terminals. (This step is invariably the one that will lead to the most confusion and errors. In all cases, keep in mind that it is the open-circuit potential between the two terminals marked in step 2.) 9

9. Thévenin’s Theorem Conclusion:
Draw the Thévenin equivalent circuit with the portion of the circuit previously removed replaced between the terminals of the equivalent circuit. This step is indicated by the placement of the resistor RL between the terminals of the Thévenin equivalent circuit.
Insert Figure 9.26(b) 10

10. Calculating the Thevenin equivalent
To calculate the equivalent circuit, one needs a resistance and a voltage - two unknowns. And so, one needs two equations. These two equations are usually obtained by using the following steps, but any conditions one places on the terminals of the circuit should also work:
1. Calculate the output voltage, VAB, when in open circuit condition (no load resistor - meaning infinite resistance). This is VTh.
2. Calculate the output current, IAB, when those leads are short circuited (load resistance is 0) RTh equals VTh divided by this IAB. 14

11. Calculating the Thevenin equivalent
2a. Now replace voltage sources with short circuits and current sources with open circuits.
2b. Replace the load circuit with an imaginary ohm meter and measure the total resistance, R, "looking back" into the circuit. This is RTh. 14

12. Calculating the Thevenin equivalent
The Thevenin-equivalent voltage is the voltage at the output terminals of the original circuit. When calculating a Thevenin-equivalent voltage, the voltage divider principle is often useful, by declaring one terminal to be Vout and the other terminal to be at the ground point.
The Thevenin-equivalent resistance is the resistance measured across points A and B "looking back" into the circuit. It is important to first replace all voltage- and current-sources with their internal resistances. 14

13. Calculating the Thevenin equivalent
For an ideal voltage source, this means replace the voltage source with a short circuit.
For an ideal current source, this means replace the current source with an open circuit.
Resistance can then be calculated across the terminals using the formulae for series and parallel circuits. 14