Definition of Thevenin Theorem:
Thevenin theorem was named after M. L. Thevenin, a French engineer. Thenvin’s theorem is very useful in simplifying the voltages in a network. By Thevenin’s Theorem, many sources and components, no matter how they are interconnected, can be represented by an equivalent series circuit with respect to any pair of terminals in the network.
In figure (a) below, imagine that the block at the left contains a network connected to terminals A and B. Thevenin’s theorem states that the entire network connected to A and B can be replaced by a singe voltage source VTH in series with a single resistance RTH, connected to the same two terminals.
Voltage VTH is the open circuit voltage across terminals A and B. This means, find the voltage that the network produces across the two terminals with an open circuit between A and B. The polarity of VTH is such that it will produce current from A to B in the same direction as in the original network.
Resistance RTH is the open circuit resistance across terminals A and B, but with all the sources killed. This means, find the resistance looking back into the network from terminals A and B. Although the terminals are open, an ohmmeter across AB would read the value of RTH as the resistance of the remaining paths in the network, without any sources operating.