# Thevenin’s Theorem

## Thevenin’s Theorem

Thevenin’s theorem was formulated for resistive networks by French physicist M. Leon. Thevenin, who proposed it in 1883. It may be enunciated as follows

Any two terminal networks consisting of linear impedance and emf sources may be replaced by a single voltage source with a equivalent series resistance. It makes the solution of complicated networks quite quick and simple.

## Explanation of Thevenin Theorem

Consider a simple circuit, to determine the current through load resistance RL, we will proceed as under

Step-1

Remove the resistance RL in which current is to be determined thus creating an open circuit between terminals A and B.

Step-2

Calculate the open circuit voltage VOC (Thevenin voltage Eth) which appears across terminals A and B when they are open (i.e when RL is removed.

Step-3

Replace the source (Battery) by its internal resistance r. When seen from the terminals A and B, the circuit consists of two parallel paths: one containing R2 and the other containing (R1+r). The equivalent resistance Rth of the network, as viewed from these terminals is given as

The equivalent resistance is also called Thevenin resistance.

Step-4

Replace the entire network by a single source (Called Thevenin voltage) source having an emf Eth and internal resistance Rth.  RL (Load Resistance) is now connected back to its terminals A and B from where it was removed.

Determine current flowing through the load resistance RL by applying ohm’s law.

## Applications of Thevenin’s Theorem

• To calculate the current in particular branch in the networks Thevenin Theorem is used.
• Designing of electronic circuits.

The above applications are practical applications of thevenin theorem

## Limitations of Thevenin’s Theorem

• Thevenin’s theorem cannot be applied to a networks which contains non-linear elements. This theorem is applicable only linear circuits or networks.
• Thevenin’s theorem cannot be used for determining the efficiency of the circuit.