Thévenin's theorem

Fig. 1. Any black box containing only resistances, voltage sources and current sources, can be replaced by a Thévenin equivalent circuit consisting of an equivalent voltage source in series connection with an equivalent resistance. As originally stated in terms of direct-current resistive circuits only, Thévenin's theorem states that "Any linear electrical network containing only voltage sources, current sources and resistances can be replaced at terminals A–B by an equivalent combination of a voltage source Vth in a series connection with a resistance Rth."

The equivalent voltage Vth is the voltage obtained at terminals A–B of the network with terminals A–B open circuited.