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Our simple circuit models for independent voltage and current sources are not good
enough: each of them implies that any source can supply infinite power, which is
certainly not true. Also, current sources and voltage sources seem to be worlds apart.
Better models for independent sources include source resistances: in series with a
voltage source, and in parallel to a current source.
These models limit the power that
can be transferred to the load, and lead to the concept of
equivalent sources
which can
be swapped without any effect on the load.
A more general idea is to
replace an entire circuit with
a single source and a resistor:
the Thevenin equivalent circuit, which includes a voltage source
V
T
with its
resistance
R
T
in series; or the Norton equivalent circuit, which includes a current
source
I
N
with its resistance
R
T
in parallel.
The Thevenin and Norton circuits are
equivalent to each other if their source resistances
R
T
are the same, and
V
T
= R
T
⋅
I
N
As a result of this analysis, the maximal power is transferred to the load when the load
resistance equals the source resistance
R
T
, and that maximal power equals
P
MAX
=
V
T
( )
2
4
"
R
T
=
I
N
( )
2
4
"
R
T
These ideas apply to any circuit containing linear elements such as sources and resistors.
The circuit supplying power can be as simple as a single battery – or as complicated as
your audio amplifier or an entire power plant. In this Chapter you will learn the strategies
to calculate the equivalent parameters
V
T
, R
T
,
and
I
N
for the given circuit.
Making sense of EE // 2nd edition
Max Power Transfer
© 2008 A. Ganago
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Max Power Transfer
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Max Power Transfer
© 2008 A. Ganago
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View Full Document© 2008 A. Ganago
Thevenin and Norton
Equivalent Circuits
The powerful idea of equivalent sources
can be extended: a whole circuit can be
replaced with the equivalent source and
resistance. The Thevenin equivalent circuit
includes voltage source
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This note was uploaded on 12/06/2010 for the course EECS 314 taught by Professor Ganago during the Spring '07 term at University of Michigan.
 Spring '07
 Ganago
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