What are the Differences between a Current and Voltage sources?

Current and Voltage sources or power sources are the most essential elements in the circuit. They supply the necessary power to drive the other components in the circuit. The concept of current and voltage sources are really helpful for analysing the performance of the circuit. Let’s start with some basics-

Active and passive elements

• Active elements: Active elements or components generally supply energy to the circuit. They do not need any external power to operate.

Please note Diode, Transistor etc are also called active elements as they can amplify or rectify the ac power meaning they have a gain factor associated with them. 

We replace the active elements(diode, transistor) by equivalent circuits consisting of current and voltage sources for analysing the performance of some electric circuits. 

• Passive elements: Passive elements need an external supply to operate. Ex- resistor,capacitor etc.

Differences between a Current and Voltage sources

Independent Sources

If the output of any source does not depend on circuit parameters, then these kinds of sources are called Independent Sources. There are two types of independent sources. 

Independent Voltage sources

If the output voltage of any source is independent of current delivered by the source then the source is called Independent Voltage Source.

 

Independent Current sources

If the output current of any source is independent of voltage delivered by the source then the source is called Independent Current Source. 

Dependent or Controlled Source

If the out current or voltage depends on the circuit parameters then these types of sources are called dependent or controlled sources. 

Dependent sources are of four types-

• Voltage Controlled Voltage Sources(VCVS)

If the output voltage of a source depends on the voltage elsewhere in the circuit  then the source is called voltage-controlled voltage source. 

• Voltage Controlled Current Sources(VCCS)

In this case output current depends on the voltage elsewhere in the circuit elsewhere. 

• Current Controlled Voltage Sources(CCVS)

Here output voltage depends on the current elsewhere in the circuit.

• Current Controlled Current Sources (CCCS)

In this case output current depends on the current elsewhere in the circuit.

There are also two types of sources- Ideal sources and real sources. 

Ideal Sources

An ideal source is a source whose circuit output value is independent of current or  voltage elsewhere in the circuit with respect to time. They are of two types- ideal current and voltage source. 

• Ideal voltage Sources

Ideal voltage source produces voltage independent of current delivered by the source. In case of an ideal voltage source, the internal impedance must be zero. So, the voltage remains the same no matter what amount of current is drawn from the source. 

Now, if the load resistance connected across it tends to zero, the load current tends to infinity. 

• Ideal Current Sources

Ideal Current source produces current independent of voltage delivered by the source. For an ideal current source, the internal impedance must be infinite.

Now, if the load resistance connected across it tends to be infinite, the load voltage  must be infinite in order to maintain the current constant. 

Practical Sources

• Practical Voltage Sources

There is no ideal voltage source. In reality, every voltage source has a minimum amount of internal impedance. Thus the voltage is not always the same as the ideal one.

 

There is always a current rating on the voltage source available in the market. The current rating is the indicator for the maximum amount of current drawn from the source. 

• Practical Current Sources

There is no existence of an ideal current source. The current differs at least in a small amount from the ideal current source. 

Note: 

• Voltage source– Load impedance is very large compared to the internal impedance.

• Current source– Load impedance is very small compared to the internal impedance.