- Voltage Source Definition: A voltage source is defined as a device that delivers electric power to a connected circuit.
- Independent Voltage Source: Delivers a steady voltage and does not depend on other circuit elements.
- Direct and Alternating Voltage Sources: Direct voltage sources provide constant output, while alternating voltage sources change polarity at intervals.
- Dependent Voltage Source: The output depends on other quantities in the circuit, such as voltage or current elsewhere.
- Ideal vs. Real Voltage Source: Ideal sources have no internal resistance and are 100% efficient, whereas real sources have internal resistance, causing minor voltage drops.
What is a Voltage Source?
A voltage source is defined as a device that delivers electric power to a connected circuit. It drives electrons around a circuit much as a pump drives water through a pipe. Voltage sources power many everyday devices and systems.
A voltage source is usually a two-terminal device, with one connection where electrons leave the source and one where they return. That simple arrangement underlies daily electricity use, from mobile phones to kitchen appliances.
Types of Voltage Sources
The main types of voltage sources include:
- Independent Voltage Source: this type has two subtypes, Direct Voltage Source and Alternating Voltage Source.
- Dependent Voltage Source: this type has two subtypes, Voltage Controlled Voltage Source and Current Controlled Voltage Source.
Independent Voltage Source
An independent voltage source maintains a set voltage, fixed or variable over time, whose value does not depend on any other voltage or current in the circuit.
Direct Voltage Source or Time Invariant Voltage Source
A voltage source that produces a constant voltage output is termed a Direct Voltage Source. Electrons flow in one direction only, so the polarity never reverses, and the currents driven through the circuit keep their direction. The voltage value does not change with time. Examples include a DC generator, a battery and cells.
Alternating Voltage Source
An Alternating Voltage Source produces an output whose polarity changes at regular intervals, so the current flows first one way and then the other. Examples include DC to AC converters and alternators.
Dependent or Controlled Voltage Source
A dependent voltage source delivers an output whose value depends on another quantity, such as a voltage or current elsewhere in the circuit.
Its symbol has four terminals: an input pair that senses the controlling quantity and an output pair that delivers the controlled voltage. When the controlling quantity is a voltage in another part of the circuit, the element is called a Voltage Controlled Voltage Source (VCVS).
When it depends on a current elsewhere in the circuit, it is called a Current Controlled Voltage Source (CCVS), shown in the figure below.
Ideal Voltage Source
An ideal voltage source delivers constant voltage regardless of the current the circuit draws, so its behaviour matches an independent voltage source. Its internal resistance is zero, so no power is wasted inside the source.
No matter what load resistance or current the circuit presents, this source holds its terminal voltage steady, so the full source voltage appears across the load with no internal loss.
The circuit above shows why. The battery modelled here is an ideal voltage source delivering 1.7 V, with internal resistance RIN = 0 Ω and load resistance RLOAD = 7 Ω. Because nothing drops inside the source, the load receives the full 1.7 V of the battery.
Real or Practical Voltage Source
Now consider the same circuit supplied by a practical voltage source with an internal resistance of 1 Ω. That resistance causes a small voltage drop across RIN.
The output voltage therefore falls from 1.7 V to about 1.49 V: with 1.7/(1+7) ≈ 0.213 A flowing through both resistances, roughly 0.21 V drops inside the source. In any practical case the terminal voltage sits below the ideal value by that internal drop.
The ideal voltage source remains the design model, while real sources are built with the lowest feasible internal resistance so that the voltage source stays close to ideal with minimal power loss.
