Current Source

A current source is an integral part of electronic circuits which is used for supplying current in the circuit. In this article, we will discuss these current sources along with their types. We will also learn about their nature through some characteristics. These characteristics when put to use can be applied at various places which have been discussed through the applications. We will conclude the article with some points and list some frequently asked questions for reference.

What is Current Source?

A current source can be defined as any device in a circuit which either delivers or absorbs the current in the circuit. The voltage across the current source is independent of voltage developed across it. It is necessary to maintain a constant current in that branch of circuit even when load is changed.

Symbol and Representation

A current source is represented with a circle with a symbol of arrow inside to indicate the direction of current flow. There is no indication of voltage in the representation since current is independent of voltage developed across it.

Current Source Symbol

Some examples of current sources are diodes, operational amplifiers, photoelectric cells, collector currents of transistors.

Types of Current Source

Types of Current Source can be classified on the basis of the field as we have given below accordingly.

Current Source on the Basis of the Circuit Dependency can be categorized as :

• Independent Current Source
• Dependent Current Source

Current sources can be divided on the basis of Internal Resistance as:

• Idea Current Source
• Practical Current Source

Current sources can be divided on the basis of variation in supply as:

• Fixed Current Source
• Variable Current Source

Types of Current Source According To Circuit Dependency

Current Source on the Basis of the Circuit Dependency can be categorized as :

1. Independent Current Source
2. Dependent Current Source

Independent Current Source

In this type of the current Source the Output current is constant regardless of any circuit variables. It maintains specified current levels providing constant current to the components connected through it.

Independent Current Source

It can be Further Classified as :

• Direct Current Source: The Direct Current Sources which are also Referred as Dc Source are the type of Independent Current Source which provides constant Current at any point of Time.
• Alternating Current Source: Alternating Current Source, Also Know as AC source, is an electrical power source that generates an alternating current, oscillating periodically in both magnitude and direction over time.

Dependent Current Source

In this type of Current Source the Output Current Depends on the Circuit Variables, as the Value to the Output Current is Controlled by the other voltage or current of the circuit. So, it is Also called Controlled Current Sources.

Dependent Current Source

It can be Further Classified as :

Voltage Controlled Current Source (VCCS):When the Output Current of the Current Source is Controlled by the Voltage Present in Some other Branch, then it is known as Voltage Controlled Current Source.

The Output Current of the Voltage Controlled Current Source (VCCS) can be Written as

[Tex]I_{out}=\mu V{in}[/Tex]

Where [Tex]\mu[/Tex] is the Coefficient of Current Source

Current Controlled Current Source (CCCS) :When the Output Current of the Current Source is Controlled by the Current Present in Some other Branch, then it is known as Current Controlled Current Source.

The Output Current of the Current Controlled Current Source (VCCS) can be Written as

[Tex]I_{out}=\mu V{in}[/Tex]

Where [Tex]\mu[/Tex] is the Coefficient of Current Source

Types of Current Source According To Internal Resistance

Current sources can be divided on the basis of Internal Resistance as:

• Ideal Current Source: Current Sources which have infinite resistance are termed as this source. An infinite resistance means they have zero conductance. It is not practically feasible so it is just a concept. It provides the same current to any load resistance and this current remains constant even when load resistance is changed.
• Practical Current Source: A practical Current source is similar to an ideal current source in parallel. This source has large but finite resistance and very low conductance. Unlike ideal current source, the output current of practical source depends on the voltage of the source. The current can vary by varying the load resistance due to the internal resistance.
  
  

  Voltage Source

Types of Current Source According To Variation in Supply

Current sources can be divided on the basis of variation in supply as:

• Fixed Current Source: A current source which supplies a fixed amount of current in the circuit is a fixed current source. This is used when we want constant current despite variation in load.
• Variable Current Source: A current source which supplies a variable amount of current in the circuit is a variable current source. This is used when we want to vary current depending on our needs.

Characteristics of Current Source

To discuss about various properties of current sources, we will enlist some characteristics of Current Sources

• Current Source supply a constant current in circuit to ensure a fixed current despite any variations in load or any change in voltage across source.
• They generally have infinite or very large resistances. This is to ensure that output current remains same even in case when load resistance varies. Having infinite impedance is just a concept since it is not possible so we try to achieve maximum impedance.
• Good current sources are insensitive to variation in load resistances. No matter load resistance increases or decreases, current source should ensure constant current supply.
• Current Sources are adjustable and stable devices since they can withstand variations in temperature, pressure i.e. they can operate in a range of environment conditions.
• Current Source show linearity. This is to ensure that the output generated is directly proportional to the input values.

Difference Between Current Source and Voltage Source

Given Below is the Table for the Comparison of the Current Source and Voltage Source

Current Source	Voltage Source
It provides fixed current in circuit independent of voltage across it.	It provides fixed Voltage in circuit independent of current being drawn.
It has very high resistance almost infinite	It usually has small negligible resistance
load resistance is always lower than the internal resistance.	load resistance is always higher than the internal resistance.
Examples include Photovoltaic cells, current regulator ICs and diodes, HID & LED driver	Examples include Batteries, Generators, Alternators and Single Cell with internal resistance connected in series etc.

Applications of Current Source

Current sources are used largely at various places. Here are some of the applications:

• Current Sources are used extensively for trial and testing. When it is required to test a product under certain conditions ,current sources become useful as we can adjust current range in it and test how devices like ammeter are working.
• Medical Industry needs current sources since they work as simulators. In Biomedical, these can be used for neurovascular and muscle simulation in patients with light electrical signals.
• In practical applications like electroplating, it becomes necessary to use constant current source. This is done to ensure that the layer being deposited is uniform and of high-quality with no inconsistency due to current fluctuations.
• It is also used to check the performance levels of various LEDs ,batteries and semiconductor devices when a stable current is supplied to them.
• Scientific equipment like electron microscopes also require constant current source to control the intensity of electron beam and thereby generating the required image of samples.

Current Source in Electronic Devices

Current Source is a two-terminal element that allows the flow of current and also indicates the direction in which current flows. The current flowing is denoted in Ampere(A).

We can define a relationship between current sources and voltage variables around a network is given by Ohm’s law. This is denoted by :

Now , since current is constant then we can define the voltage developed for a resistance R across the terminals as

V=IR

This voltage will become larger for larger values of R because the current remains independent of R.

Current Source in Power Supplies

In a Power Supply circuit, current source determines the output current to a certain level. These are used especially in LED lightning. Since the current in such circuits is constant ,the voltage developed is directly proportional to resistance across it.

The power generated can be denoted by

P=I²R

Due to constant current, we can say that with increasing R the power delivered by the current source increases. This can also become infinite if resistance becomes infinite. To avoid this , we need to ensure that our circuit is not an open circuit.

Conclusion

We have seen that current sources have a wide range of applications due to their property of delivering constant current independent of voltage or load in the circuit. Modern industrial and consumer products have an ardent need of such products. Thereby it becomes necessary to make these products rough and adjustable to different environment conditions. Advancements have been seen in this field and new technologies are coming up.

Current Source – FAQs

What is the difference between a current source and a voltage source?

A constant current source delivers a constant current irrespective of the load applied whereas a voltage source delivers a constant voltage irrespective of the load applied.

Is a current source active or passive element?

Since current source performs the task of delivering constant current, it is an active device in the circuit.

What is the reason for high resistance in current sources?

A current source has high impedance to ensure that even when external load is increased or decreased , it has negligible effect on resistance thereby ensuring a constant current generation.