---
title: "Thevenin's theorem and steps"
subject: "Basic Electronics Engineering"
chapter: 1
key: "semester_2/basic_electronics_engineering:thevenin-steps"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/thevenin-steps
site: BCT Engineering
---

# Thevenin's theorem and steps

Basic Electronics Engineering · Chapter 1: Basic Circuits Concepts

- Asked in: 2071 Magh, 1+4 marks
- Video: [Thevenin's Theorem](https://www.youtube.com/watch?v=veAFVTIpKyM) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/thevenin-steps
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-1

## Question

State Thevenin's Theorem. Write down the steps for determining Vth and Rth with necessary circuit diagrams.

## Short answer

Thevenin's theorem: any linear two-terminal network can be replaced by a voltage source Vth in series with resistance Rth. Steps: remove load, find open-circuit voltage Vth; zero independent sources (short V, open I) and find Rth; reconnect load, I = Vth/(Rth+RL).

## Answer

**Thevenin's theorem:** any linear, bilateral two-terminal network containing independent sources and resistors can be replaced by an equivalent circuit consisting of a single voltage source Vth in series with a single resistance Rth, connected across the same two terminals. The load current is then I = Vth/(Rth + RL).

#### Steps to find Vth and Rth
1. Identify the load (or the two terminals of interest) and remove it from the circuit, leaving terminals a and b open.
2. **To find Vth:** calculate the open-circuit voltage between a and b using KVL, KCL, a voltage divider or nodal analysis.
3. **To find Rth:** set all independent sources to zero (replace voltage sources by short circuits and current sources by open circuits). Find the equivalent resistance seen looking into terminals a and b by series/parallel reduction.
4. Draw the Thevenin equivalent: Vth in series with Rth.
5. Reconnect the load RL and calculate I = Vth/(Rth + RL).

```text
 (a) Original circuit            (b) Load removed: find Vth
     +-----------+ a                 +-----------+ a
     |  Linear   |-----+             |  Linear   |----o +
     |  network  |    [RL]           |  network  |      Vth = Voc
     |           |-----+             |           |----o -
     +-----------+ b                 +-----------+ b

 (c) Sources zeroed: find Rth    (d) Thevenin equivalent
     +-----------+ a                     Rth
     | V -> short|----o             +--/\/\/\--+ a
     | I -> open |      <-- Rth     |          |
     |           |----o            Vth       [RL]
     +-----------+ b                |          |
                                    +----------+ b
```

Rth can also be found as Vth/Isc, where Isc is the short-circuit current between a and b. If a dependent source is present, it is not set to zero; instead, apply a test source at the terminals and find Rth = Vtest/Itest.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the first step in applying Thevenin's theorem?
   A. Short all sources
   B. Remove the load and leave terminals a and b open
   C. Find the short-circuit current
   D. Reconnect the load
   Answer: B. First identify the load and remove it, leaving the two terminals open. (Key point: First step)

2. How is Rth found?
   A. Voltage sources opened and current sources shorted
   B. All independent sources set to zero (voltage sources shorted, current sources opened), then the resistance at the terminals is found
   C. Measure Vth divided by RL
   D. Add all resistors in series
   Answer: B. Set independent sources to zero and reduce the network seen from a and b. (Key point: Finding Rth)

3. Which is another way to find Rth?
   A. Rth = Vth × Isc
   B. Rth = Vth/Isc
   C. Rth = Isc/Vth
   D. Rth = Vth + Isc
   Answer: B. Rth equals the open-circuit voltage divided by the short-circuit current between a and b. (Key point: Alternative for Rth)

4. If the circuit contains a dependent source, how is Rth found?
   A. Set the dependent source to zero
   B. Apply a test source at the terminals and take Rth = Vtest/Itest
   C. Open the dependent source
   D. Ignore Rth
   Answer: B. Dependent sources are not zeroed; a test source gives Rth = Vtest/Itest. (Key point: Dependent sources)

## Related in Basic Electronics Engineering

- [Thevenin: Vth, Rth and 5 kΩ current](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Athevenin-48v-12k) — Basic Electronics Engineering, chapter 1
- [Thevenin: 90, 50, 130 ohm circuit](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Athevenin-90-50-130) — Basic Electronics Engineering, chapter 1, asked 3×
- [Thevenin: 100, 60, 150 ohm circuit](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Athevenin-100-60-150) — Basic Electronics Engineering, chapter 1, asked 2×

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