MAXIMUM POWER TRANSFER
THEOREM
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In many electrical and electronic applications, we
are interested in the amount of power received by a
particular load (speaker, electric motor, antenna)
Electric systems are a source of power and a load
connected to that source
Sources – Amplifiers, generators, power supplies
All linearly constructed sources can be reduced to
their Thevenin equivalent
In DC circuits, the load can be represented by a
resistance RL
Maximum Power Transfer
Theorem
1
POWER DELIVERED TO LOAD
T hevenin equivalent
circuit
RTH
IL
+
ETH
VL
RL
Load
Source
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The source develops a voltage VL across the load
and enables current IL to flow into it
PL  V L I L  I L R L  V L R L
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2
The power delivered to the load resistance (RL)
depends on the value of RL
Maximum Power Transfer
Theorem
2
MAXIMUM POWER, CURRENT
AND VOLTAGE CONDITIONS
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Maximum current IL occurs when RL = 0 (shorted
terminals)
The maximum voltage VL occurs when RL =  (open
circuited terminals)
Yet load power PL = 0 for both cases
PL is maximum when RL equals the Thevenin
equivalent resistance of the source, I.e. when RL =
RTH
The maximum power transfer theorem is thus:
Maximum power is developed in a load when the
load resistance equals the Thevenin resistance
of the source to which it is connected
Maximum power is delivered when VL = ETH/2
Thus
2
PL (max) 
VL
RL
2

E TH
4 R TH
Maximum Power Transfer
Theorem
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