Thevinen Theorem Notes

Statement : Any combination of linear bilateral circuit elements and active sources, regardless of the connection or complexity, connected to a given load RL, may be replaced a simple two terminal network consisting of a single voltage source of volts and a sing resistance RTH in series with the voltage source, across the two terminals of the load voltage VTH is the open circuit voltage measured at the two terminals of interest, with resistance RL removed. This voltage is also called Thevenin’s equivalent voltage. The RTH is equivalent resistance of the given network as viewed through the terminals where connected, but with RL removed and all the active sources are replaced by their internal resistances. If the internal resistances are not known then independent voltage sources are to be replaced by the short circuit while the independent current sources must be replaced the open circuit.

I V
R R L
TH
TH L
=
When the circuit is replaced by Thevenin’s equivalent across the load resistance, then the load current can be obtained as
71

Steps to Apply Thevenin’s Theorem

Step 1: Remove the branch resistance through which current is to be calculated.

Step 2 : Calculate the voltage across these open circuited terminals, by using any of the network simplification techniques. This is VTH

Step 3: Calculate RTH as viewed through the two terminals of the branch from current is to be calculated by removing that branch resistance and replacing all
independent sources by their internal resistances. If the internal resistances are not known then replace independent voltage sources by short circuits and independent
current sources by open circuits.

Step 4 : Draw the Thevenin’s equivalent showing source VTH, with the resistance RTH in series with it, across the terminals of branch of interest.

Step 5 : Reconnect the branch resistance. Let it be RL.

Limitations of Thevenin’s Theorem

The limitations of Thevenin’s theorem are,

1. Not applicable to the circuits consisting of nonlinear elements.
2. Not applicable to unilateral networks.
3. There should not be magnetic coupling between the load and circuit to be replaced by Thevenin’s theorem.
4. In the load side, there should not be controlled sources, controlled from some other part of the circuit.