Electrical engineers use electricity to perform useful tasks by designing circuits in which voltages and currents are controlled, adjusted and applied to a wide range of components. However, these voltages and currents represent energy. A circuit is a dynamic system in which voltage acts as potential energy and current acts like kinetic energy. The components we incorporate into the electrical circuit cannot control and use electricity unless there is something that also generates electricity, and therefore the circuit has voltage and current source. We can call this an electrical source because it can supply electrical energy to the circuit needed for proper operation. So, today I will discuss electrical sources explicitly.

## What Is Electrical Source?

The source that produces electricity as energy is called an “electrical source.” Examples of common energy sources include generators, photovoltaic cells, thermocouples, and primary batteries. These devices generate electrical voltage, which in turn drives electrical current to flow into the circuit. In an electrical circuit, single or more electrical sources can be available. It depends on our requirements. For every source, we get two quantities namely voltage and current. Depending on the type of source one quantity remains constant and another varies. Whatever, it is used for producing electrical power in the form of voltage and current.

## Classification of Electrical Source

Electrical sources are mainly classified into two types according to the component. After that, they are also classified into different types. These classifications are as follows.

### 1. Voltage Source

A voltage source, such as a battery or generator, provides a potential difference (voltage) between two points within a circuit, allowing current to flow around it. Remember that voltage can exist without current. The battery is the most common example of a voltage source for an electrical circuit, and the voltage that appears on the positive and negative terminals of the battery is called the terminal voltage.

#### Characteristics of Voltage Sources

Fig: Characteristics of Voltage Source

The voltage source creates a potential difference at its two terminals. When the two terminals are connected in a network of interconnected components that form a continuous conductive path, the current flows.

Simple electrical circuits usually consist of a single voltage source connected to various components. In these cases, the negative terminal of the source is set out as a reference node with zero voltage, and therefore the node connected to the positive terminal of the source has a voltage equal to the value of the source.

Fig: Characteristics of Voltage Source Without Zero Volt Negative Terminal

However, it is important to note that voltage sources may occupy different positions in the electrical circuit and, as a result, the negative terminal is not always at 0 V. For example:

Therefore, the value of the source does not always reflect the voltage at the positive terminal; however, the source value indicates the voltage difference between the positive terminal and the negative terminal of the source.

#### Properties of Voltage Source

1. The ideal voltage source always supplies a constant voltage despite the current consumption.
2. The ideal voltage has zero internal resistance, but in the real world there is no ideal voltage source, so a practical voltage source has a certain resistance, but it is very low compared to the load resistance.
3. In a practical voltage source, the terminal voltage is always lower than the actual voltage generated by the source due to its internal resistance.
4. In a practical voltage source, the voltage level changes with the current it supplies.
5. Because the voltage source always supplies a constant voltage, the actual consumption of the source depends on the external circuit or charge, and any amount of current can be drawn from the voltage source.
6. The voltage drop depends on the current drawn by the load and the resistance of the load or circuit elements.

#### Classification of Voltage Sources

Voltage source is classified into different types. They are:

##### (a) Ideal Voltage Source

The voltage source can supply a constant voltage to the circuit and is also called an independent voltage source because it is independent of the current drawn by the circuit. The value of the internal resistance is zero here. This means that no energy is wasted by internal resistance. Despite the resistance of the charge as a current in the circuit, this voltage source will provide a constant voltage. It works as a 100% efficient voltage source. All its voltage on an ideal voltage source can completely drop on the circuit load.

Fig: Ideal Voltage Source & Its Characteristics

To understand the ideal voltage source, we can take an example from the above circuit. The battery shown here is an ideal voltage source that supplies 1.7V. Internal resistance RIN= 0Ω. Circuit load resistance RLOAD = 7Ω. Here we see that the load receives a total of 1.7V of the battery.

##### (b) Practical Voltage Source

Furthermore, we can consider a circuit with a practical voltage source with an internal resistance of 1Ω in the same circuit described above. Due to the internal resistance, the RIN voltage decreases slightly. Therefore, the output voltage is reduced to 1.49V from 1.7V. So, in practical cases, the source voltage is reduced due to the internal resistance.

Fig: Practical Voltage Source

We can now conclude that the ideal voltage source is stored as models and the practical voltage source is made with minimal internal resistance so that the voltage source approaches the ideal with minimal power loss.

##### (c) Controlled Voltage Source

A controlled voltage source or dependent voltage source provides a voltage whose magnitude depends on the other voltage across or on the current flowing through other circuit elements. The voltage-dependent source is indicated by the rectangle shape and is used as an equivalent power source for many electronic devices, such as transistors, operational amplifiers and so on.

As stated, the output voltage of a controlled voltage source depends on the current or voltage of the circuit. Based on this, there are two types of dependent voltage sources:

###### (i) Voltage Controlled Voltage Source (VCVS)

It is defined as a source whose output voltage depends on the voltage of some circuit element. This output voltage is a scalar multiple of the input voltage. VCVS is indicated as shown below.

Fig: Voltage Controlled Voltage Source

The voltage Vo is dependent on input voltage  Vi. The output voltage Vo is k times the input voltage Vi. Here, “k” is a constant. The value of k = (Vo/ Vi). Thus, this scaling factor k is a dimensionless quantity.

The ideal transformer is an example of a VCVS. The output voltage of an ideal transformer is the secondary voltage (V2) and the input control voltage is the primary voltage (V1). Using the conversion ratio

V2 = (N2/N1)V1

= kV1

Where k = (N2 / N1) is the turn ratio. The output voltage is controlled by the input voltage and therefore, it is a voltage-controlled voltage source.

###### (ii) Current Controlled Voltage Source (CCVS)

It is defined as a source of electricity whose power depends on the value of the current in the circuit. The output voltage of the CCVS is a scalar multiple of the input current. It is also a four-terminal device with two input terminals and two output terminals. CCVS is shown below.

Fig: Current Controlled Voltage Source

As can be seen from the diagram above, the output voltage of the CCVS is linearly dependent on the input current. It is measured by some constant α. Output voltage (Vo) is α times of input current (I). This scaling factor, α = (Vo/i). Therefore, the unit of scaling factor in CCVS is volts/amperes.

### 2. Current Source

A current source is an electrical source that always has a constant current, regardless of its voltage or without changing the voltage level. There are also practical and ideal concepts that can be used as a voltage source. An ideal current source has infinite internal resistance but it is almost impossible for a practical current source. The practical current source has a very high internal resistance, but it is not infinite. The current source is always only be closed because no current will flow or the current will be zero when the circuit is open.

#### Characteristics of Current Sources

Fig: Characteristics of Current Sources

The current source produces a certain amount of energy in part of the circuit. The value of the current source is the magnitude of the current generated by the source and the symbol contains an arrow indicating the direction of the current.

If you see a current source in the circuit diagram, you know that the leading path connected to the current source has a current equal to the value of the source. If this path separates different branches, as shown in the figure below, you must perform a circuit analysis to determine the proportion of resources currently supplied to each branch.

Fig: Characteristics of Current Sources

#### Properties of Current Source

1. An ideal current source has infinite internal resistance, while a practical current source has a very high internal resistance.
2. The current source is dual of a voltage source.
3. The voltage of the current source depends on the external circuit and the load.
4. Because the current source always supplies a constant current, the voltage drop as a power consumption depends on the resistance of the load and the circuit components.

#### Classification of Current Sources

The current source is classified into different types. They are:

##### (a) Ideal Current Source

The ideal current source is a two-terminal device that provides constant current despite load resistance. The value of current can be constant in relation to the time and resistance of the load. This means that the ability to supply power is infinite for this source.

Fig: Ideal Current Source With Its Characteristics

The ideal current source has an infinite parallel resistor connected. Therefore, the output current is independent of the voltage at the source terminal. There is no such current source in the world, it is just a concept. However, each electrical source is designed to be closer to the ideal.

Fig: Ideal Current Source With Parallel Resistor

##### (b) Practical Current Source

A practical current source is a two-terminal device with a certain resistor connected to its terminals. Unlike an ideal current source, the output current of a practical source depends on the source voltage. The higher this voltage, the lower the current. For a better understanding, let’s look at a practical resource, as shown below.

Fig: Practical Current source And Its Characteristics

From the above connection, it is very clear that the voltage of the source is equal to the voltage drop across the resistor R. This voltage drop is given as V = iR, where i is the current through the resistor R. The output current Io = (I – i).

So, if V is more, it means I is more, and therefore the output current (Io) will be smaller.

Characteristics of Practical Current Source:

First, we get the relationship between the source voltage and the output current.

V = iR

i = V/R

Therefore,

Io = (I – i)

= (I – V/R)

The above equation represents a straight line with slope (-1/R). Therefore, the characteristics of practical current sources can be obtained, as shown in the figure.

##### (c) Controlled Current Source

A controlled or dependent current source, on the other hand, varies its available current depending on the voltage connected to some other element of the circuit or the current passing through it. In other words, the output of the controlled current source is controlled by another voltage or current.

Today, dependent current sources behave the same as the current sources we have seen so far, both ideal (independent) and practical. The difference at this point is that the current-dependent source can be regulated by the input voltage or current. In this case, there are two types of dependent current sources:

###### (i) Voltage Controlled Current Source (VCCS)

The ideal voltage-controlled current source VCCS maintains an output current IOUT proportional to the control input voltage VIN. In other words, the output current “depends” on the value of the input voltage, making it a dependent current source.

Fig: Voltage Controlled Current Source

Then the VCCS output current is defined by the following formula: IOUT = α VIN. The SI unit of this multiplicative constant α (alpha) is mhos, ℧ (inverted ohm sign), and since α = IOUT/ VIN, its units are amperes/volts.

###### (ii) Current Controlled Current Source (CCCS)

An ideal current-controlled current source, CCCS, maintains an output current proportional to the control input current. The output current then “depends” on the value of the input current, again making it a dependent current source.

Fig: Current Controlled Current Source

As the controlling current, IIN determines the magnitude of the output current, IOUT is multiplied by the amplification constant β (beta), and the output current of the CCCS element is determined by the following formula: IOUT = β IIN. Note that the multiplicative constant β is a dimensionless scaling factor because β = IOUT/ IIN, so its units are amps/amps.