Editing Equivalent circuit (section)

==Examples==

===Thévenin and Norton equivalents===
One of linear circuit theory's most surprising properties relates to the ability to treat any two-terminal circuit no matter how complex as behaving as only a source and an impedance, which have either of two simple equivalent circuit forms:<ref name="Johnson (2003a)"/><ref name="Johnson (2003b)">{{cite journal|last=Johnson|first=D.H.|title=Origins of the equivalent circuit concept: the current-source equivalent|journal=Proceedings of the IEEE|year=2003b|volume=91|issue=5|pages=817–821|doi=10.1109/JPROC.2003.811795|url=http://www.ece.rice.edu/~dhj/paper2.pdf}}</ref> 
*[[Thévenin's theorem|Thévenin equivalent]] – Any linear two-terminal circuit can be replaced by a single [[voltage source]] and a series impedance.
*[[Norton equivalent]] – Any linear two-terminal circuit can be replaced by a [[current source]] and a parallel impedance.
However, the single impedance can be of arbitrary complexity (as a function of frequency) and may be irreducible to a simpler form.

===DC and AC equivalent circuits===
In [[linear circuit]]s, due to the [[superposition principle]], the output of a circuit is equal to the sum of the output due to its DC sources alone, and the output from its AC sources alone.  Therefore, the DC and AC response of a circuit is often analyzed independently, using separate DC and AC equivalent circuits which have the same response as the original circuit to DC and AC currents respectively.  The composite response is calculated by adding the DC and AC responses:
*A DC equivalent of a circuit can be constructed by replacing all capacitances with open circuits, inductances with short circuits, and reducing AC sources to zero (replacing AC voltage sources by short circuits and AC current sources by open circuits.)
*An AC equivalent circuit can be constructed by reducing all DC sources to zero (replacing DC voltage sources with short circuits and DC current sources with open circuits)
This technique is often extended to [[Small-signal model|small-signal]] nonlinear circuits like tube and transistor circuits, by linearizing the circuit about the DC bias point [[Q-point]], using an AC equivalent circuit made by calculating the equivalent ''small signal'' AC resistance of the nonlinear components at the bias point.

===Two-port networks===
{{main|Two-port network}}
Linear four-terminal circuits in which a signal is applied to one pair of terminals and an output is taken from another, are often modeled as [[two-port network]]s.  These can be represented by simple equivalent circuits of impedances and dependent sources.  To be analyzed as a two port network the currents applied to the circuit must satisfy the [[Port (circuit theory)|''port condition'']]:  the current entering one terminal of a port must be equal to the current leaving the other terminal of the port.<ref name=Gray>
{{cite book 
|author1=P.R. Gray |author2=P.J. Hurst |author3=S.H. Lewis |author4=R.G. Meyer |title=Analysis and Design of Analog Integrated Circuits 
|year= 2001 
|edition=Fourth 
|publisher=Wiley
|location=New York 
|isbn=978-0-471-32168-2 
|pages=§3.2, p. 172
|url=http://worldcat.org/isbn/0471321680}}
</ref> By [[small-signal model|linearizing]] a nonlinear circuit about its [[Q-point|operating point]], such a two-port representation can be made for transistors: see [[hybrid-pi model|hybrid pi]] and [[Bipolar junction transistor#h-parameter model|h-parameter]] circuits.

===Delta and Wye circuits===
In [[Three-phase electric power|three phase power]] circuits, three phase sources and loads can be connected in two different ways, called a "delta" connection and a "wye" connection.  In analyzing circuits, sometimes it simplifies the analysis to convert between equivalent wye and delta circuits. This can be done with the [[wye-delta transform]].

===Li-ion batteries===
{{main|Equivalent circuit model for Li-ion cells}}

The electrical behavior of a [[Lithium-ion battery]] cell is often approximated by an [[Equivalent circuit model for Li-ion cells|equivalent circuit model]]. Such a [[Mathematical model|model]] consists of a voltage generator driven by the [[state of charge]], representing the [[open-circuit voltage]] of the cell, a resistor representing the [[internal resistance]] of the cell, and some [[RC circuit|RC parallels]] to simulate the dynamic voltage transients.