Download Definitions for Thévenin`s Theorem

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1. Musfiqur Rahman
2. Shihab Omar
3. Shamir Shakir
4. Shahriar Islam
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Thevenin’s
Theorem


Thévenin's theorem is a popular theorem, used
often for analysis of electronic circuits. Using this
theorem, a model of the circuit can be developed
based on its output characteristic.
It was discovered in 1883 by
French telegraph engineer León Charles
Thévenin's.


Any two-terminal bilateral ac network can be
replaced by equivalent circuit consisting of a
voltage source and a series resistance.
VTh, in series with a resistor, RTh.


Linear circuit is a circuit where the voltage is
directly proportional to the current (i.e., Ohm’s
Law is followed).
Two terminals are the 2 nodes/2 wires that can
make a connection between the circuit to the
load.
Definitions for Thévenin’s Theorem
Open-circuit voltage Voc is the voltage, V, when the load
is an open circuit (i.e., RL = ∞W).
VOC  VTh

Input resistance is the resistance seen by the load when
VTh = 0V.

It is also the resistance of the linear circuit when the
load is a short circuit (RL = 0W).
Rin  RTh  VTh iSC

The four steps involved are:

1- First Remove the load.



2- Then determine the voltage seen by the
load (Vth).
3- Then replace the voltage source with a
short.
4- At the end determine the resistance seen
by the load (Rth).


4- Determine the resistance seen by the
load (Rth):
Find the total resistance from the open terminal
side which is called thevinin equivalent
resistance (Rth) by removing actual voltage
source from the circuit.
Find VX by first finding VTH and RTH to the left of A-B
4W
12 W
30 V +_
6W

A
+
2W
VX
_

B
Figure 1 : Circuit for
Example

First remove everything to the right of A-B
4W
12 W
_
30 V +

A
6W

B
Notice that there is no current flowing in the 4 W resistor
(A-B) is open. Thus there can be no voltage across the
resistor.
(30)(6)
VAB 
 10V
6  12
We now deactivate the sources to the left of A-B and find
the resistance seen looking in these terminals.
4 W
12 W

A
6 W

B
We see,
RTH = 12||6 + 4 = 8 W
After having found the Thevenin circuit, we connect this
to the load in order to find VX.
RTH
8W
VTH
+
_
10 V
A

+
2W
VX
_
B

Figure 1: Circuit of Example after connecting Thevenin
circuit.
(10)( 2)
VX 
 2V
28



In some cases it may become tedious to find
RTH by reducing
the resistive network with the sources
deactivated. Consider
the following:
R
TH
VTH
A

+
_
ISS
B

Figure 10.12: A Thevenin circuit with the
output shorted.
We see;
RTH
VTH

I SS
•
•
•
•
Certain circuits can be simplified by using
Thevenin’s Theorem.
Equivalent Resistance is essential for the calculation
of TEC.
Ohm’s law is useful in this theorem.
A linear network can be transferred into a voltage
source