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Encyclopedia of Electrical Engineering
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HOME TABLE OF CONTENTS
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Encyclopedia of Electrical Engineering
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Voltage Sources

What is a Voltage Source?

A voltage source is a two-terminal device which can maintain a fixed voltage. An ideal voltage source can maintain the fixed voltage independent of the load resistance or the output current. However, a real-world voltage source cannot supply unlimited current. A voltage source is the dual of a current source.
Batteries and generators are two of the most common sources of voltage that power our daily lives.
Voltage source is in fact a active element which can create a continuous force for the movement of electrons through the wire in which it is connected. It is usually a two terminal device.

Types of Voltage Source

Independent Voltage Source: They are of two types - Direct Voltage Source and Alternating Voltage Source.
Dependent Voltage Source: They are of two types - Voltage Controlled Voltage Source and Current Controlled Voltage Source.

Independent Voltage Source

The voltage source which can deliver steady voltage (fixed or variable with time) to the circuit and it does not depend on any other elements or quantity in the circuit.

Direct Voltage Source or Time Invariant Voltage Source

The voltage source which can produce or deliver constant voltage as output is termed as Direct Voltage Source. The flow of electrons will be in one direction that is polarity will be always same. The movement of electrons or current will be in one direction always. The value of voltage will not alter with time. Example: DC generator, battery, Cells etc.

Alternating Voltage Source

The voltage source which can produce or deliver alternating voltage as output is termed as Alternating Voltage Source. Here, the polarity gets reversed at regular intervals. This voltage causes the current to flow in a direction for a time and after that in a different direction for another time. That means it is time varying. Example: DC to AC converter, Alternator etc.

Dependent or Controlled Voltage Source

The voltage source which delivers an output voltage which is not steady or fixed and it always depends on other quantities such as voltage or current in any other part of the circuit is termed as dependent voltage source. They have four terminals. When the voltage source depends on voltage in any other part of the circuit, then it is called Voltage Controlled Voltage Source (VCVS). When the voltage source depends on current in any other part of the circuit, then it is called Current Controlled Voltage Source (CCVS).

Ideal Voltage Source

The voltage source which can deliver constant voltage to the circuit and it is also referred as independent voltage source as it is independent of the current that the circuit draws. The value of internal resistance is zero here. That is, no power is wasted owing to internal resistance. In spite of the load resistance or current in the circuit, this voltage source will give steady voltage. It performs as a 100% efficient voltage source. All of its voltage of the ideal voltage source can drop perfectly to the load in the circuit.

Real or Practical Voltage Source

Next, we can consider a circuit with practical voltage source having an internal resistance. Due to the internal resistance, there will be small amount of voltage drop, which will reduce output voltage to some extent. So in practical cases there will be reduction in source voltage due to the internal resistance.