one of the most important topics in the analysis of electric circuits is the concept of an **equivalent circuit.**

thevenin theorem show that it is always possible to view even a very complicated circuit in terms of much simpler equivalent source and load circuits, and that the transformations leading to equivalent circuits are easily managed, with a little practice. In studying node voltage and mesh current analysis, you may have observed that there is a certain correspondence (called duality) between current sources and voltage sources, on one hand, and parallel and series circuits, on the other. This duality appears again very clearly in the analysis of equivalent circuits: It will shortly be shown that equivalent circuits fall into one of two classes, involving either voltage or current sources and (respectively) either series or parallel resistors, reflecting this same principle of duality. The discussion of equivalent circuits begins with the statement of two very important theorems:

**thevenin theorem**- Norton Theorem

The Thevenin Theorem :When viewed from the load, any network composed of ideal voltage and current sources, and of linear resistors, may be represented by an equivalent circuit consisting of an ideal voltage source vT in series with an equivalent resistance RT .

#### Determination of Thevenin Equivalent Resistance

The first step in computing a Thevenin equivalent circuit consists of finding the equivalent resistance presented by the circuit at its terminals. This is done by setting all sources in the circuit equal to zero and computing the effective resistance between terminals. The voltage and current sources present in the circuit are set to zero by the same technique used with the principle of superposition: Voltage sources are replaced by short circuits; current sources, by open circuits.

#### Computing the Thevenin Voltage

The Thévenin equivalent voltage is defined as follows:

The equivalent (Thévenin) source voltage is equal to the open-circuit voltage present at the load terminals (with the load removed).

This states that to compute vT , it is sufficient to remove the load and to compute the open-circuit voltage at the one-port terminals.