What is Thevenin’s theorem ?

In circuit analysis, Thevenin’s theorem for linear electrical network states that any combination of voltage sources, current sources, and resistors with two terminals may be considered as electrically equivalent to a single voltage source V with a single series resistor R. Getting confused ? Let’s make it simpler:

Suppose we have to calculate the current through any branch in a circuit& this branch is connected with of the circuit by its two terminals. As because there are active sources in the circuit so there should be a potential difference between the terminals of the branch & by the potential difference a current is passing through the branch. So, rest of the circuit may be assumed as a single voltage source, whose voltage is the open circuit voltage between the terminals of the said branch & the internal resistance of the circuit is nothing but the equivalent resistance of the circuit looking from into the terminals towards the circuit.

So the Thevenin’s theorem can also be stated as,

• When a particular branch is removed from a circuit, the open circuit voltage appears across the terminals of that circuit, called Thevenin's equivalent voltage.

• The equivalent resistance of the circuit looking from the terminal towards the circuit, is called Thevenin's equivalent resistance.

• Ultimately when we replace the rest of the circuit by a single voltage source, then the voltage of the source would be Thevenin equivalent voltage and internal resistance of the voltage source would be Thevenin's equivalent resistance which would be connected in series.

Thevenin’s theorem is especially useful in analyzing power systems and other circuits where one particular resistor in the circuit (called the “load” resistor) is subject to change, and calculation of the circuit is necessary with each trial value of load resistance, to determine voltage across it and current through it.