Inside Collection: Circuits

Summary: Introduction of Norton equivalent circuits.

As you might expect, equivalent circuits come in two forms: the voltage-source oriented Thévenin equivalent and the current-source oriented Norton equivalent (see figure).

To derive the latter, the *v-i* relation for
the Thévenin equivalent can be written as

Find the Norton equivalent circuit for the circuit below.

Equivalent circuits can be used in two basic
ways. The first is to simplify the analysis of a complicated
circuit by realizing the *any *portion of a
circuit can be described by either a Thévenin or Norton
equivalent. Which one is used depends on whether what is
attached to the terminals is a series configuration (making the
Thévenin equivalent the best) or a parallel one (making
Norton the best).

Another application is modeling. When we buy a
flashlight battery, either equivalent circuit can accurately
describe it. These models help us understand the limitations of
a battery. Since batteries are labeled with a voltage
specification, they should serve as voltage sources and the
Thévenin equivalent serves as the natural choice. If
a load resistance
*neither
*as a voltage source or a current course. Thus, when
you buy a battery, you get a voltage source if its equivalent
resistance is much *smaller* than the
equivalent resistance of the circuit you attach it to. On the
other hand, if you attach it to a circuit having a small
equivalent resistance, you bought a current source.

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