Concept of Current Source

 Concept of Current Source

Concept of Current Source

Like a constant voltage source, there may be a constant current source - A Source that supplies a constant current to a load even if its impedance varies. Ideally, the current supplied by It should remain constant, no matter what the load impedance is.

 

A symbolic representation of such an ideal current source is shown below :

The arrow inside the circle indicates the  direction in which current will flow in the circuit when a load is connected to the source. Following figure show the V-I characteristics of an ideal current source: .

Let us connect a variable load impedance Z_L  to a constant current source as shown below :

As stated above, the current supplied by the source should remain current at I_S  for all values of load impedance. It means even if Z_L is made infinity, the current through this should remain I_S. Now, we must see if any practical current source could satisfy this condition.

 

The load impedance Z_L = ∞ means no conducting path, external to the source, exists between the terminals A and B. hence, it is a physical impossibility for current to flow between  terminals A and B.

If the source could maintain a current I_S through an infinitely large load impedance, there would have been an infinitely large voltage drop across the load.

It would then have consumed infinite power from the source. Of Course, no practical source could even supply infinite power.

A practical current source supplies current I_S to a short circuit (I.e., when Z_L = 0).

This is why, the current I_S is called a short-circuit current. But,when we increase the load impedance, the current falls below I_s .

When the load impedance Z_L is made infinite (I.e., the terminals A and B are open-circuited), the load current reduces to zero. It means there should be some path (inside the source itself) through which the current I_S can flow.

When some infinite load impedance is connected, only a part of this current I_S flows through the load. The remaining current goes through the path inside the source.

 

The inside path has an impedance Z_S , and is called the internal impedance.

 The symbolic representation of such a practical current source is shown below:.

Now, if the terminal A and B are open-circuited (Z_L= ∞), The terminal voltage does not have to be infinite. It is now an infinite value, V_T = I_S Z_S.

 It means that the source does not have to supply infinite power.