ME 117 Electrical Engineering1 Chapter 4 Circuit Theorems

ME 117 Electrical Engineering(1) Chapter 4 Circuit Theorems 1

Circuit Theorems - Chapter 4 Source Transformation Thevenin’s Theorem 2

Source Transformation • It is the process of replacing a voltage source v. S in series with a resistor R by a current source i. S in parallel with a resistor R, or vice versa. 3

Source Transformation + + - - (a) Independent source transform (b) Dependent source transform • The source transformation is not possible when R = 0 for voltage source and R = ∞ for current source. 4

Thevenin’s Theorem It states that a linear two-terminal circuit (Fig. a) can be replaced by an equivalent circuit (Fig. b) consisting of a voltage source VTH in series with a resistor RTH, where • VTH is the open-circuit voltage at the terminals. • RTH is the equivalent resistance at the terminals when the independent sources are turned off. 5

EXAMPLE : Find the Thévenin equivalent circuit for the network in the shaded area of the network. Then find the current through RL for values of 2 Ω, 10 Ω, and 100 Ω Solution: Step 1: Remove the Load resistor RL Step 2: Replacing the voltage source E 1 with a short-circuit, then calculate Rth 6

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EXAMPLE : Find the Thévenin equivalent circuit for the network in the shaded area of the network Solution: Step 1: Remove the resistor R 3 Step 2: Replacing the current source I with a open -circuit, then calculate Rth 8

EXAMPLE : Find the Thévenin equivalent circuit for the network between point (a, b). 9

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Homework : Find the Thévenin equivalent circuit for the network between point (a, b). 11