Question

Thevenin equivalent of a circuit consists of

• A. Current source and series resistance
• B. Voltage source and series capacitance
• C. Voltage source and series resistance
• D. Current source and parallel resistance

Hint:

To remember the distinction between the two primary network theorems: - Thevenin's Model: Voltage source ($V_{th}$) in Series with resistance ($R_{th}$). - Norton's Model: Current source ($I_N$) in Parallel with resistance ($R_N$). You can easily switch between them using a standard source transformation: $V_{th} = I_N \cdot R_{th}$ where $R_{th} = R_N$.

Answer 1

The Correct Option is C

Concept: Thevenin's Theorem is a fundamental network reduction theorem used in electrical engineering. It states that any linear, bilateral circuit containing independent sources, dependent sources, and resistors can be simplified across any pair of terminals into an equivalent circuit containing just one single ideal voltage source connected in series with a single equivalent resistor. Let us define the core components of this simplified network model:
• Thevenin Equivalent Voltage ($V_{th}$ or $V_{oc}$): This is the open-circuit voltage measured across the designated terminal pair when the external load impedance is entirely disconnected.
• Thevenin Equivalent Resistance ($R_{th}$): This is the equivalent internal input resistance looking back into the open terminal pair. It is calculated by deactivating all independent sources within the network:
• Independent ideal voltage sources are replaced by a short circuit (0V drop).
• Independent ideal current sources are replaced by an open circuit (0A current). Structural Arrangement: Thevenin's theorem specifies that the independent equivalent voltage source $V_{th}$ must be placed in a direct series loop connection with the internal equivalent resistance $R_{th}$. This simple series configuration ensures that when an external load resistance $R_L$ is attached, the current flowing through it matches the original complex circuit exactly: \[ I_L = \frac{V_{th}}{R_{th} + R_L} \] Analysis of the Options:
• Option 1: Describes a current source with a series resistance, which violates fundamental source topologies.
• Option 2: Mentions a capacitance, whereas Thevenin's theorem for resistive DC circuits relies purely on resistance.
• Option 3: Correctly states that the model consists of an ideal voltage source and series resistance.
• Option 4: Represents a Norton Equivalent Circuit (which uses an ideal current source in parallel with a resistor) rather than a Thevenin model. Hence, Option (3) is the correct structural description.