Theveninimpedansen

Thevenin impedance, denoted Zth, is the impedance that a linear electrical network presents to external circuitry at a specified pair of terminals. It is part of Thevenin's theorem, which states that any linear network of sources and impedances can be replaced, from the viewpoint of the terminals, by a voltage source Vth in series with Zth. This Thevenin equivalent preserves the same terminal voltage and current for any external load.

To determine Zth, identify the two terminals of interest. If the network contains only independent sources, deactivate them: voltage sources become short circuits and current sources become open circuits. The impedance seen across the terminals in this reduced network is Zth. If the network contains dependent sources, they must remain active; in such cases a test source is applied at the terminals and Zth is computed as Zth = Vtest / Itest (or equivalently Itest / Vtest).

In AC analysis, impedances are generally complex and may depend on frequency, so Zth = Zth(ω) can vary for reactive components. Thevenin impedance is used to assess how a load interacts with a network, to perform impedance matching, and to simplify circuit design by allowing the network to be treated as a single source in series with Zth. Once Zth and Vth are known, the external load can be analyzed with standard voltage-divider or current-divider concepts as if the original network were replaced by the Thevenin equivalent.