I Networks: Network Theorems: Thevenin's Theorem

Statement of Thevenin's Theorem:

A general statement of Thevenin's Theorem is that any linear active network consisting of independent and or dependent voltage and current source and linear bilateral network element can be replaced by an equivalent circuit consisting of a voltage source in series with a resistance, the voltage source is being the open circuited voltage across the open circuited load terminals and the resistance being the internal resistance of the source network looking through the open circuited load terminals. 

or we can say that

Any two terminal bilateral linear d.c. circuit can be replaced by an equivalent circuit consisting of a voltage source and a series resistor.

The theorem was first discovered by German scientist Hermann von Helmholtz in 1853, but was then rediscovered in 1883 by French telegraph engineer Léon Charles Thévenin (1857–1926).

Steps for Solving a Network Utilizing Thevenin's Theorem:

I. Remove the load resistance Rl and find the open circuit voltage Voc across the open circuited load terminals.

II. Deactivate the constant sources ( for voltage source, remove it by internal resistance which is usually zero, and for current source remove it by internal resistance which is usually infinite, so voltage source is shorted and current source is opened ) of the source side looking through the open circuited load terminals. Let this resistance be Rth.

III. Obtain Thevenin's equivalent circuit by placing Rth in series with Voc.

IV. Reconnect Rl across the load terminals.

The Load current I  can be given as:

Voc/(Rth+Rl)

http://en.wikipedia.org/wiki/Th%C3%A9venin's_theorem

http://www.allaboutcircuits.com/vol_1/chpt_10/8.html