This document discusses the maximum power transfer theorem. It begins by providing examples of circuits where maximum power transfer to a load is important, such as stereo amplifiers and radio transmitters. It then states the maximum power transfer theorem - that a load will receive maximum power when its resistance equals the internal resistance of the driving source. The document outlines the procedure for finding this resistance matching condition. It also provides examples of maximum power transfer calculations and experiments demonstrating lower power transfer when the load resistance does not match the source resistance.

1. ELECTRONICS
Fall 2020
Lec10
Dr. Mona M.Soliman
Dr. Walid M. Ibrahim
IT Dept.

2. In some applications, the purpose of a circuit is to
provide maximum power to a load. Some
examples:
 Stereo amplifiers
 Radio transmitters
 Communications equipment
Our question is: If you have a system, what load
should you connect to the system in order that the
load receives the maximum power that the system
can deliver?
2
Maximum Power Transfer
Theorem

3. 3
Maximum Power Transfer
Theorem
 Principle
 Maximum power absorbed by the load
resistor.
–
+
VTh
RTh
RL
iL
+
vL
–
a
b
Thévenin equivalent circuit

4. 4
Maximum Power Transfer
 Procedure
L
2
L
Th
Th
L
2
L R
R
R
V
R
i
p 










Power absorbed by load resistor:
–
+
VTh
RTh
RL
iL
+
vL
–
a
b
Thévenin equivalent circuit

5. 5
Maximum Power Transfer
 Procedure
   
 
   
L
Th
L
Th
L
2
L
Th
4
L
Th
L
Th
L
2
L
Th
2
Th
0
2
0
2
R
R
R
R
R
R
R
R
R
R
R
R
R
R
V
dR
dp
L





















To find the value of RL for which p is maximum, set to 0:
L
dR
dp
L
2
L
Th
Th
L
2
L R
R
R
V
R
i
p 










Power absorbed by load resistor:

6. 6
Maximum Power Transfer
 Procedure
A resistive load receives maximum power from a circuit if the
load resistance equals the Thévenin resistance of the circuit.
L
Th R
R 

L
2
L
Th
Th
L
2
L R
R
R
V
R
i
p 










Power absorbed by load resistor:

7. 7
Maximum Power Transfer
 Procedure
 
Th
2
Th
L
2
L
Th
2
Th
Th
Th
L
2
L
4R
V
R
i
p
R
R
R
V
R
i
p













Maximum Power Transferred
–
+
VTh
RTh
RL
iL
+
vL
–
a
b
Thévenin equivalent circuit

8. Maximum Power Transfer
 Maximum power
transfer theorem is
used frequently to
insure that the
greatest power can
be transferred from a
power source to a
load.
 The total power
delivered by a supply
such as VTh is
absorbed by both the
Thévenin equivalent
resistance and the
load resistance. Any
power delivered by
the source that does
not get to the load is
lost to the Thévenin
resistance. 8
–
+
VTh
RTh
RL
iL
+
vL
–
a
b

9.  If the load resistance is lower or higher than the
Thevenin/Norton resistance of the source
network, its dissipated power will be less than
maximum.
 A load resistance that is too high will result in
low power output. A load resistance that is too
low will not only result in low power output, but
possibly overheating of the circuit due to the
power dissipated in its internal (Thevenin or
Norton) resistance.
9

10. Maximum Power Transfer
 Sometimes in engineering we are asked to design a
circuit that will transfer the maximum power to a load
from a given source.
 If the source circuit is already in the form of a Thevenin
or Norton equivalent circuit then the solution is simple.
 If the circuit is not in the form of a Thevenin or Norton
equivalent circuit, we must first
use Thevenin’s or Norton’s theorem to obtain the
equivalent circuit.
10

11. Maximum Power Transfer
Experiment (RLoad=Rth)
11

12. Maximum Power Transfer
Experiment (Rload< Rth)
12
Rload=0.5

13. Maximum Power Transfer
Experiment (Rload>Rth)
13
Rload=1.1

14. 14
Maximum Power Transfer
Example-1
What is the value of R2 to recieve a maximum
power. What is this power
2
2
0.8
/ 4
(11.2) / 4 0.8 25.0
L th
MAX th L
MAX
R R
P V R
P X W
 

 

15. 15
Maximum Power Transfer
Example-2
For the following circuit, find the exact value of R to
guarantee that the resistor of 5 K Ω receives the
maximum power.
7,3
5 is the load
R 7 3 10
:10 ||
5 10 / (10 )
10
th L
th
R
R R
R R
R R
R
 
  

 
 