Thévenin’s Theorems

Basic Electronics (211000)
Thévenin’s Theorems

1. Topic : Thévenin’s Theorems
Branch : Batch :

2. Name ENROLLMENT NO .

Abhishek Chokshi 140120109005
Himal Desai
Harsh Dedakia
140120109008
140120109012

4. A linear two-terminal circuit can be replaced with
an equivalent circuit of an ideal voltage source,
V
Th
, in series with a resistor, R
Th
.
◦V
Th is equal to the open-circuit voltage at the terminals.
◦R
Th
is the equivalent or input resistance when the
independent sources in the linear circuit are turned off.

6. 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.

7. Open-circuit voltage V
oc
is the voltage, V, when the load is an open
circuit (i.e., R
L
= ∞W).
+
V
oc
_
ThOC
VV=

8. Input resistance is the resistance seen by the
load when V
Th
= 0V.
It is also the resistance of the linear circuit when
the load is a short circuit (R
L
= 0W).
SCThThin
iVRR ==

9. 1.Identify the load, which may be a resistor or a part of
the circuit.
2.Replace the load with an open circuit .
3.Calculate V
OC. This is V
Th.
4.Turn off all independent voltage and currents sources
in the linear 2-terminal circuit.
5.Calculate the equivalent resistance of the circuit.
This is R
Th.
The current through and voltage across the load in series
with V
Th
and R
Th
is the load’s actual current and voltage in the
original circuit.

10. Example
=
R
Th
v
oc
W=
1
0i
R
Th

11. THEVENIN & NORTON
THEVENIN’S THEOREM:
Find V
X
by first finding V
TH
and R
TH
to the left of A-B.
1 2W 4W
6W 2W V
X3 0 V
+
_
+
_
A
B
·
·
Circuit for Example
First remove everything to the right of A-B.

12. THEVENIN & NORTON
THEVENIN’S THEOREM:
1 2W 4W
6W3 0 V
+
_
A
B
·
·
Circuit for finding V
TH
for Example
(30)(6)
10
6 12
AB
V V= =
+
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.

13. THEVENIN & NORTON
THEVENIN’S THEOREM:
We now deactivate the sources to the left of A-B and find
the resistance seen looking in these terminals.
1 2W 4W
6W
A
B
·
·
R
TH
Circuit for find R
TH
for Example
We see,
R
TH
= 12||6 + 4 = 8 W

14. THEVENIN & NORTON
THEVENIN’S THEOREM:
After having found the Thevenin circuit, we connect this
to the load in order to find V
X
.
8W
1 0 VV
T H
R
T H
2W V
X
+
_
+
_
A
B
·
·
after connecting Thevenin
circuit.
10 2
2
2 8
= =
+
( )( )
X
V V

15. THEVENIN & NORTON
THEVENIN’S THEOREM:
In some cases it may become tedious to find R
TH
by reducing
the resistive network with the sources deactivated. Consider
the following:
V
T H
R
T H
+
_
A
B
·
·
I
S S
A Thevenin circuit with the output shorted.
We see;
TH
TH
SS
V
R
I
=

16. circuits
Thevenin