Thevenin’s theorem: Thervenin’s theorem states that any two terminal, linear, bilateral network can be replaced by an equivalent circuits consisting of a voltage source and series resistor with it. Where equivalent voltage (Vth) is voltage across the open circuited terminal pair and equivalent resistant(Rth) is a resistance seen from open circuited terminal, when all the independent sources are replaced by their internal impedances.
1. Procedure to calculate Vth: a. Identify the load and remove from the circuit. b. After getting the open circuited terminal pair, calculate the potential across that by any method of analysis. c. This open circuit voltage is Vth.
2. Rth calculation: Case I: Network with no dependent sources: a. Identify the load and remove from the network and get the open circuited terminal. b. Replace all the independent sources by their internal impedances. c. Calculate equivalent resistance across the open circuited terminal by using series/ Parallel/Star/delta simplification, that will be Rth . d. Draw the equivalent circuit.
Case II: Network consist dependent and independent source together: a. Identify the load and remove form the circuit. b. Replace all the independent sources by their internal impedances c. Apply a test source of ‘V’ and assume the current is ‘I’ outside from it. Or Apply a test source of ‘I’ and assume the voltage is ‘V’ across it d. Calculate ratio V/I . This will be Rth.