Electronics Engineering And Circuit Design

To derive an electronic circuitry there has to be source of energy. A voltage source and a current source is an example of energy source. An electrical energy source is a device that is capable of converting non-electrical energy into electrical energy. For example a battery which converts chemical energy into electrical energy. A dynamo which converts mechanical energy into electrical energy. It delivers power to rest of the circuit elements which are passive in nature. Our aim in this article is to explain some important concepts regarding voltage sources and current sources.

Key Concepts

Ideal Vs practical sources (voltage and current sources)
Independent voltage and current sources
Dependent voltage and current sources

Concept Of An Ideal Voltage Source:

As the name implies it is responsible for delivering voltage across the circuit. It is a two terminal device. There are various definitions found for an ideal voltage source. An ideal voltage source is the one which maintains prescribed/constant voltage across its terminals regardless of the load connected to its terminals. Or we can say an ideal voltage source maintains a constant voltage across its terminals regardless of the current flowing through its terminals. I will explain it with the help of Ohm's law. Or in other words an ideal voltage source has 'zero’ internal resistance. If zero internal resistance then you can definitely have no voltage drop across the terminals.

According to Ohm's law V=IR

Consider

V=12V, and R=10Ω (load resistor) it can not maintain unlimited current as per Ohm's law. As the load varies current varies accordingly
V=12V, R=0Ω (internal resistance) I = 12/0 = ∞ , it was is capable of supplying infinite current
For ideal voltage source terminal voltage (V) remains same regardless of the load connected. E=V, where E=EMF and V=Terminal Voltage.

Concept Of A Practical Voltage Source:

Do you think an ideal voltage source exists?? Ideal voltage sources are not practical. For a practical voltage source there's a finite internal resistance that's associated with it's internal connection and terminals. This causes voltage drop across the terminals.

All practical voltage sources have small internal resistance Ri
Internal resistance Ri is connected in series with any load resistance RL
There's a voltage drop of I*Ri because of this internal resistance
Terminal voltage (V) which appears across load is V=E-I*Ri.
Because of this small internal resistance load voltage is always less than the source voltage

Concept Of An Ideal Current Source:

As the name suggests it is the source of current, that supplies constant current to the circuit. An ideal current source has infinite internal resistance. Or an ideal current source maintains a constant current flow regardless of the voltage drop across its terminals.

It provides constant current to the load irrespective of other conditions in the circuit
It has 100% efficiency
Because of infinite internal resistance all the current supplies to the load
Ri = internal resistance = ∞ From Ohm's law I=V/Ri , because of such high internal resistance all the current will flow through the load irrespective of the voltage

Concept Of A Practical Current Source:

As the ideal voltage source doesn't exist, in the same way ideal current source also doesn't exist. No current source can maintain constant current. We have seen a practical voltage source can be modelled by a voltage source and a small resistance in series with it. Whereas a practical current source is represented a shunt resistor (Rs) connected in parallel with an ideal current source. Rs should be very large. It should be large enough so that small load resistances have no effect on the load current.

Reason for larger value of Rs

Do you remember, the current finds the lowest resistance path. The path which has lowest resistance, maximum current will flow through that path. From Ohm's law, I=V/R lower the resistance higher will be the current value.

Rs should be large enough so that all current flows through the load resistor

Independent Voltage Source:

Independent voltage sources that have voltage (fixed or time variant) which is not affected by any other current or voltage elsewhere in the circuit.

Independent Current Sources:

Independent current sources that delivers or absorbs current (fixed or time variant) at its terminals which is not affected by any other voltage or current elsewhere in the circuit.

Dependent Voltage Sources:

Dependent sources are also called controlled sources. A dependent voltage source can either be controlled by voltage or current elsewhere in the circuit. Input and output are linearly dependent. The equation for current and voltage is a linear equation.

There are two possible types of dependent voltage sources: