Question

Write the steps for finding the current through an element by Thevenin’s theorem.

Answer 1

Steps for finding Branch Current by Thevenin’s Theorem: 

i) Identify the load branch whose current is to be found. 

ii) Redraw the circuit with load branch separated from the rest of the circuit such that the the load branch appears between terminals, say A and B, which are connected to rest of the circuit by two wires. 

iii) Remove the load branch from terminals A and B, so that the rest of the circuit appears between these terminals A and B. 

iv) Find the open circuit voltage appearing across the terminals A and B (after load removal) due to internal independent sources, using any circuit analysis technique. Let this open circuit voltage be VTh. 

v) Determine the equivalent impedance of the circuit seen between the open terminals A and B, while looking back into the circuit, with all internal independent voltage sources replaced by short-circuit and all internal independent current sources replaced by open-circuit. Let this equivalent impedance be ZTh. 

vi) The circuit appearing between open circuited terminals A and B (due to removal of load branch) is now represented by simple circuit consisting of a voltage source, having magnitude VTh in series with an impedance ZTh. 

vii) If the load impedance is ZL, then connecting it between terminals A and B gives rise to load current IL = VTh / (ZTh + ZL)

Answer 2

Steps to find Thevenin’s equivalent circuit, taking an example is as follows: 

1. From the given circuit (fig.a),Remove RL from the terminals A and B and redraw the circuit as shown in Fig.b.

2. Calculate the open-circuit voltage Voc which appears across terminals A and B.

As seen, Voc = drop across R2 = IR2 where I is the circuit current when A and B are open.

3.Now, imagine the battery to be removed from the circuit, leaving its internal resistance r behind and redraw the circuit, as shown in Fig.(c). When viewed inwards from terminals A and B, the equivalent resistance is given as, 

This is called Thevenin’s equivalent resistance Rth.

4.Connect RL back across terminals A and B (fig.d)from where it was temporarily removed earlier. 

Current flowing through RL is given by