Thevenin’s Theorem states that “Any linear circuit containing several voltages and resistances can be replaced by just one single voltage in series with a single resistance connected across the load“. As far as the load resistor RL is concerned, any complex “one-port” network consisting of multiple resistive circuit elements and energy sources can be replaced by one single equivalent resistance Rth and one single equivalent voltage Vth. Rth is the source resistance value looking back into the circuit and Vth is the open circuit voltage at the terminals.
Thevenin’s equivalent circuit: A series combination of Thevenin’s equivalent voltage source Vth and Thevenin’s equivalent resistance Rth forms Thevenin’s equivalent circuit as shown below.