Thevenin’s Theorem

Thevenin’s theorem states that any two terminal linear network or circuit can be represented with an equivalent network or circuit, which consists of a voltage source in series with a resistor. It is known as Thevenin’s equivalent circuit. A linear circuit may contain independent sources, dependent sources, and resistors.

If the circuit contains multiple independent sources, dependent sources, and resistors, then the response in an element can be easily found by replacing the entire network to the left of that element with a Thevenin’s equivalent circuit.

The response in an element can be the voltage across that element, current flowing through that element, or power dissipated across that element.

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Methods of Finding Thevenin’s Equivalent Circuit

There are three methods for finding a Thevenin’s equivalent circuit. Based on the type of sources that are present in the network, we can choose one of these three methods. Now, let us discuss two methods one by one. We will discuss the third method in the next chapter.

Follow these steps in order to find the Thevenin’s equivalent circuit, when only the sources of independent type are present.

• Step 1 − Consider the circuit diagram by opening the terminals with respect to which the Thevenin’s equivalent circuit is to be found.

• Step 2 − Find Thevenin’s voltage VTh across the open terminals of the above circuit.

• Step 3 − Find Thevenin’s resistance RTh across the open terminals of the above circuit by eliminating the independent sources present in it.

• Step 4 − Draw the Thevenin’s equivalent circuit by connecting a Thevenin’s voltage VTh in series with a Thevenin’s resistance RTh.

Now, we can find the response in an element that lies to the right side of Thevenin’s equivalent circuit.

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Follow these steps in order to find the Thevenin’s equivalent circuit, when the sources of both independent type and dependent type are present.

• Step 1 − Consider the circuit diagram by opening the terminals with respect to which, the Thevenin’s equivalent circuit is to be found.

• Step 2 − Find Thevenin’s voltage VTh across the open terminals of the above circuit.

• Step 3 − Find the short circuit current ISC by shorting the two opened terminals of the above circuit.

• Step 4 − Find Thevenin’s resistance RTh by using the following formula.

 

RTh=VThISCRTh=VThISC

Step 5 − Draw the Thevenin’s equivalent circuit by connecting a Thevenin’s voltage VTh in series with a Thevenin’s resistance RTh.

Now, we can find the response in an element that lies to the right side of the Thevenin’s equivalent circuit.