State Thevenin’s Theorem:
According to thevenin’s theorem, the current flowing through a load resistance Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL)
In electrical circuit theory, Thevenin’s theorem for linear electrical networks states that any combination of voltage sources, current sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems, the theorem can also be applied to general impedances, not just resistors. Any complex network can be reduced to a Thevenin’s equivalent circuit consist of a single voltage source and series resistance connected to a load.
To calculate the equivalent circuit, one needs a resistance and some voltage – two unknowns. Thus two equations are needed. These two equations are usually obtained by using the following steps, but any conditions one places on the terminals of the circuit should also work:
Calculate the output voltage, VAB, when in open circuit condition (no load resistor – meaning infinite resistance). This is VTh.
Calculate the output current, IAB, when the output terminals are short circuited (load resistance is 0). RTh equals VTh divided by IAB.
The Thevenin-equivalent voltage is the voltage at the output terminals of the original circuit. When calculating a Thévenin-equivalent voltage, the voltage divider principle is useful, by declaring one terminal to be Vout and the other terminal to be at the ground point.