Glossary term
Thevenin Equivalent
A simplified linear circuit model represented by a voltage source and series impedance.
Definition
modelA Thevenin equivalent represents a linear two-terminal network as an ideal voltage source in series with an equivalent impedance.
Thevenin equivalence reduces a complex linear circuit to the voltage and impedance seen at a chosen pair of terminals. It is used for load analysis, source modelling, maximum power transfer, fault studies, small-signal electronics, battery models, sensors, and interface design.
For a linear two-terminal network, the Thevenin equivalent has an open-circuit voltage V_\text{th} and a series impedance Z_\text{th}. A load connected to the terminals sees the same terminal voltage and current as it would with the original network, as long as the assumptions behind the equivalent remain valid.
The open-circuit terminal voltage is:
The equivalent resistance or impedance can be found by deactivating independent sources and looking into the terminals, or by using a test source when dependent sources are present. It can also be inferred from open-circuit voltage and short-circuit current:
Engineering use
Thevenin equivalents simplify load matching, voltage-divider analysis, sensor interfacing, source impedance estimation, power-system fault calculations, and small-signal amplifier modelling. In AC analysis, Z_\text{th} may be complex and frequency dependent. In electronics, a nonlinear device can be represented by a local small-signal Thevenin model around a specified operating point.
The terminal pair must be stated. The same circuit can have different Thevenin equivalents depending on which terminals are observed and which sources, frequency range, and operating point are included.
Common mistakes
A common mistake is applying a Thevenin equivalent outside the operating point or frequency range where the source network is linear. Another is turning off dependent sources while calculating equivalent impedance; dependent sources must remain active and usually require a test source method. A strong Thevenin review states the terminal pair, operating point, source treatment, frequency, equivalent voltage, equivalent impedance, and whether the load perturbs the assumptions.