---
title: "Ideal current and voltage sources"
subject: "Basic Electrical Engineering"
chapter: 1
key: "semester_1/basic_electrical_engineering:ideal-practical-sources"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/ideal-practical-sources
site: BCT Engineering
---

# Ideal current and voltage sources

Basic Electrical Engineering · Chapter 1: General Electric System

- Asked in: 2072 Kartik, 4 marks; 2072 Chaitra, 5 marks; 2067 Asoj, 4 marks (asked 3 times)
- Video: [2. Voltage and Current Sources - Ideal & Practical with diagrams |BEE|](https://www.youtube.com/watch?v=88d62PeqrL0) — Trouble- Free
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/ideal-practical-sources
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-1

## Question

Explain ideal current and voltage sources.

## Short answer

An ideal voltage source maintains constant voltage regardless of current, with zero internal resistance. An ideal current source delivers constant current regardless of voltage, with infinite internal resistance. Practical sources have small series resistance (voltage) or large parallel resistance (current), causing output to drop under load.

## Answer

An **ideal voltage source** keeps a constant terminal voltage whatever current is drawn from it. An **ideal current source** delivers a constant current whatever voltage appears across it. Practical sources deviate from these ideals because of their internal resistance.

#### Ideal voltage source
- Internal resistance is zero, so the terminal voltage *V = E* for every load current.
- The V-I characteristic is a horizontal line.
- It can supply infinite current when short-circuited, so it does not exist in practice.

#### Ideal current source
- Internal resistance is infinite, so the current *I = Is* for every load voltage.
- The V-I characteristic is a vertical line.
- With an open circuit, the voltage across it would be infinite, so it does not exist in practice.

#### Practical sources
- A practical voltage source is an ideal voltage source *E* in series with an internal resistance *Rs*. Terminal voltage *V = E − I·Rs*, so it falls as load current rises.
- A practical current source is an ideal current source *Is* in parallel with an internal resistance *Rp*. Load current *I = Is − V/Rp*, so it falls as terminal voltage rises.

```text
 Ideal V source      Practical V source    Practical I source
   o---+              o--[Rs]--+            o---+------+
      (E)               (E)    |              (Is)    [Rp]
   o---+              o--------+            o---+------+
```

| Point | Ideal | Practical |
|---|---|---|
| Voltage source Rs | 0 | small, finite |
| Current source Rp | ∞ | large, finite |
| Output | constant | drops with load |

A good voltage source has a very small *Rs*; a good current source has a very large *Rp*.

## Check yourself

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

1. What is the internal resistance of an ideal voltage source, and what does its terminal voltage do as load current changes?
   A. Zero; the terminal voltage stays constant at E
   B. Infinite; the terminal voltage stays constant at E
   C. Zero; the terminal voltage falls linearly with current
   D. Infinite; the terminal voltage rises with current
   Answer: A. With Rs = 0 there is no internal drop, so V = E for every load current. (Key point: Ideal voltage source)

2. Which statement about an ideal current source is correct?
   A. It has zero internal resistance and a horizontal V-I line
   B. It has infinite internal resistance and a vertical V-I line
   C. It has infinite internal resistance and a horizontal V-I line
   D. It has zero internal resistance and a vertical V-I line
   Answer: B. An ideal current source has Rp = infinity, so I = Is for any voltage, which plots as a vertical line. (Key point: Ideal current source)

3. A practical voltage source is modelled as an ideal source E with internal resistance Rs. How is Rs connected, and what is the terminal voltage?
   A. In parallel; V = E - V/Rs
   B. In series; V = E + I*Rs
   C. In parallel; V = E - I*Rs
   D. In series; V = E - I*Rs
   Answer: D. Rs is in series with E, so the internal drop I*Rs is subtracted: V = E - I*Rs. (Key point: Practical voltage source model)

4. For a good (nearly ideal) practical current source, the internal parallel resistance Rp should be:
   A. Very small
   B. Equal to the load resistance
   C. Very large
   D. Exactly zero
   Answer: C. A large Rp means little current is diverted internally, so the current stays close to Is. (Key point: Good practical sources)

## Related in Basic Electrical Engineering

- [Voltage and current sources, terminal voltage](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Avoltage-current-sources-terminal-voltage) — Basic Electrical Engineering, chapter 1
- [Ideal and practical sources, conversion](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Aideal-practical-sources-conversion) — Basic Electrical Engineering, chapter 2

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