Read the following statements regarding Thevenin’s equivalent circuit : (a) The Thevenin’s voltage is calculated across the short circuit terminals. (b) The Thevenin’s voltage is calculated at the open circuit terminals. (c) The connection in the circuit is open if any voltage source is present. (d) The connection in the circuit is shorted if any voltage source is present. Which of the above statements are incorrect ?
(a) and (c)
Read the question carefully — it asks which statements are incorrect, so the true ones must be rejected. The statements come in two contradictory pairs, so exactly one of each pair is wrong.
Pair 1 — how VTh is found. The Thevenin voltage is the voltage that appears across the terminals when the load is removed, i.e. the open-circuit voltage:
\(V_{Th}=V_{OC}\)
So (b) is correct, and (a) is incorrect. Measuring across short-circuited terminals would always give zero volts — what a short circuit gives you is the Norton current, \(I_N=I_{SC}\), and the two are linked by \(R_{Th}=V_{OC}/I_{SC}\).
Pair 2 — how sources are treated when finding RTh. Every independent source is replaced by its internal resistance:
| Source | Replaced by | Because its ideal internal resistance is |
|---|---|---|
| Voltage source | short circuit | zero |
| Current source | open circuit | infinite |
So (d) is correct and (c) is incorrect — it applies the current-source rule to a voltage source.
The incorrect statements are therefore (a) and (c).
Why the rules are what they are. Deactivating a source means forcing its contribution to zero while leaving its resistance in place. Setting a voltage source to zero volts is exactly what a short circuit does; setting a current source to zero amps is exactly what an open circuit does. Getting this backwards is the single most common error in Thevenin problems.
One caution. These substitutions apply only to independent sources. A dependent source must be left in the circuit; RTh is then found by applying a 1 V test source at the terminals and computing \(R_{Th}=V_{test}/I_{test}\).
Hence, the incorrect statements are (a) and (c).
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