As an introduction, we can go through the Electrical source. It is nothing but a device which can deliver electric power to a connected circuit. They can be a current source or a **Voltage source**. Here, we can discuss about the voltage source which is most commonly used. Voltage source is in fact a passive 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) (shown in figure below).

## 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.

For understanding the ideal voltage source, we can take an example of a circuit shown above. The battery shown here is an ideal voltage source which delivers 1.7V. The internal resistance R_{IN} = 0Ω. The resistance load in the circuit R_{LOAD} = 7Ω. Here, we can see the load will receives all of the 1.7V of the battery.

## Real or Practical Voltage Source

Next, we can consider a circuit with practical voltage source having an internal resistance of 1Ω in the similar circuit which is explained above. Due to the internal resistance, there will be small amount of voltage drop in the R_{IN}. So, the output voltage will be reduced to 1.49V from 1.7V. So in practical cases there will be reduction in source voltage due to the internal resistance.

We can now conclude that the ideal voltage source is kept as models and the real voltage source is made with minimum internal resistance to get the **voltage source** close to the ideal one with minimum power loss.