A source is an important electrical network or circuit element that supply electrical energy. A source converts mechanical, thermal, chemical, or some other form of energy into electrical energy. Every electrical and electronic circuit requires a source of supply of voltage or current for its operation.
Some examples of sources of electrical energy are photoelectric cells, batteries, generators, alternators, collector currents of transistors, etc. All the energy sources present in the electrical circuit are voltage and current sources. Voltage and current sources are considered active elements of electrical circuits. In this article, let us learn about the current source and different types of current sources.
What is a Current Source?
Electric current simply means the flow of free electrons or charge. Generally, the flow of electrons or current is from the negative terminal to the positive terminal through the circuit. But the conventional current is assumed to flow from the positive to the negative terminal via the circuit.
A source of energy that provides current in an electrical circuit is termed a current source. In other words, a current source delivers a specified current regardless of other circuit parameters.
A current source is the dual of a voltage source. The below figure shows the symbol of a current source. The arrow indicates the direction of the current supplied by the current source and it also determines the polarity of the corresponding voltage. The rating of current sources is given in Amperes. Let us see the different types of current sources.
Types of Current Sources :
Based on the dependency on other circuit parameters current sources are divided into,- Independent current sources, and
- Dependent current sources.
Independent Current Source :
An independent current source is one whose output current does not depend upon any other circuit elements. The current delivered by the independent current source is not influenced by any other circuit variables. There are two types of independent current sources,- Direct current sources, and
- Alternating current sources.
Direct Current Source :
A direct current source is also referred to as a dc source which delivers a constant flow of electrons i.e., the magnitude of the current supplied remains constant at each instant of time. Examples of direct current sources are batteries, solar panels, rectifiers, etc. The below shows the symbol of a direct current source and its waveform.
The current supplied by the direct current source is unidirectional which means the current always flows in a single direction from the positive terminal to the negative terminal.
Alternating Current Source :
An alternating current source is one whose current changes its magnitude and polarity at regular intervals of time (periodically). In this type of current source, the free electrons flow in the forward as well as in the backward direction which means the direction of current flow in the circuit will be reversed back and forth constantly.
In the above current waveform, it can be seen that the current swings between positive and negative maximum values constantly. The number of times the current changes its direction is determined by frequency.
If the frequency of the current source is 50 Hz, then the current changes its direction 50 times in one second. Examples of alternating current sources are alternators, inverters, crystal oscillators, etc.
Dependent Current Source :
A current source whose output depends upon other circuit variables like voltage or current is called a dependent current source. Since the current supplied by the current source is controlled by another voltage or current of the circuit, dependent current sources are also called controlled current sources.
Dependent current sources are divided into two types,- Voltage-controlled current source (VCCS).
- Current controlled current source (CCCS).
Voltage Controlled Current Source (VCCS) :
When the output current of the dependent current source is controlled by voltage in some other branch of the circuit then the dependent current source is called voltage controlled current source. The below diagram shows the voltage-controlled current source.
In the above figure, the output current Iout of the dependent current source is controlled by voltage Vin. The output current delivered by voltage controlled current source is expressed as,
Where µ is the coefficient of the current source that multiplies with the controlling voltage. MOSFET is the practical example of voltage controlled current source where drain current varies with variation in gate voltage.
Current Controlled Current Source (VCCS) :
When the output current of the dependent current source is controlled by the current in some other part of the circuit then the dependent current source is called the current-controlled current source. The below diagram shows the current controlled current source.
In the above figure, the output current Iout of the dependent current source is controlled by the current Iin. The output current delivered by current controlled current source is expressed as,
Where µ is the coefficient of the current source that multiplies with the controlling current. A BJT is an example of a current-controlled current source where the base current controls the collector current.
Current sources are also categorized based on internal resistance, they are,- Ideal current source, and
- Practical current source.
Ideal Current Source :
An ideal current source is the same as an ideal voltage source that delivers a constant and fixed current in the circuit. The current supplied by an ideal current source is completely independent of the amount of current drawn by the load and other circuit parameters.
An ideal current source has infinite internal resistance because of which no current loss takes place and the whole of the source current is supplied to the load. The voltage across an ideal current source is completely determined by the circuit to which it is connected.
The symbol and V-I characteristics of an ideal current source are shown below. The arrow represents the direction of the current delivered by the current source.
Therefore, an ideal current source supplies the same current at any load connected to it and it is clear that voltage across the ideal current source varies according to the load. For example, if an ideal current source of 4A is connected to a 20Ω resistor, the terminal voltage of the current source is V = 4A × 20Ω = 80 volts. Suppose if the load resistor is changed to 50Ω, the terminal voltage of the current source is V = 4A × 50Ω = 200 volts.
Practical Current Source :
Above we have seen an ideal current source that supplies a constant current. However, in practice, it is impossible to have an ideal current source. A practical or real current source has some considerable or finite internal resistance. This internal resistance draws some part of the current due to which the output of the current source will no longer remain constant.
Therefore, due to internal resistance, the output current of a practical current source is different from that of the actual current that is to be delivered. All practical or real current sources are represented by an ideal current source in parallel with a resistance whose value is equal to its internal resistance as shown below.
If RL is the load resistance connected to the terminals of the practical current source. The load current IL delivered to the load is given by,
Where I is the ideal current source value and Rint is the parallel internal resistance of the current source and V is the voltage across the terminals. From the graph shown above we can say that the less the internal resistance of the current source, the less will be the drop, and thus current delivered will be closer to the ideal current source.
Current Sources in Parallel :
When two current sources are connected in parallel aiding i.e. when the current supplied by both current sources is in the same direction (indicated by their arrows) as shown below. The net current will be equal to the sum of the currents of both current sources.
In the above figure we can see that two current sources 2A and 3A are connected in parallel and currents delivered by the two sources are in the same direction as indicated by arrows. The net current of two current sources will be equal to,
If two current sources are connected in parallel opposing i.e. when the current supplied by both current sources are in opposite directions (indicated by their arrows) as shown below. The net current will be equal to the difference between the currents of two current sources.
In the above figure, we can see that two current sources 2A and 3A are connected in parallel, but currents delivered by the two sources are in opposite directions as indicated by arrows. The resultant current of two current sources will be equal to,
In the parallel opposing connection of current sources, the resulting direction of the current will be equal to the source with the greater current rating. Therefore, in the parallel configuration of current sources, the resultant current will be the algebraic sum of the individual current. The parallel configuration of current sources can increase the current in the circuit.
Current Sources in Series :
Generally, a series connection of current sources is not allowed. When two current sources of the same rating are connected in a series aiding direction the net current remains same in the loop. But if two current sources of different ratings are connected in series, the source with a lower rating will act upon the circuit. The other source will become redundant.
