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1.5.2 
Ideal Independent current Source
An ideal independent current source delivers or absorbs a current at its terminals that is a specified 
function of time. Rest of the circuit in which it is connected decides its terminal voltage. The current 
delivered or absorbed by it does not depend on the voltage that appears across its terminals.
Practical current sources will have a parallel resistance and capacitance at its terminals representing 
the effect of finite conductivity within the source and charge distribution on its surface and terminals. 
These parasitic components are neglected in the ‘ideal independent current source’ model.
The symbol of a constant ideal independent current source (that is, a DC source) is shown in (a) 
of Fig. 1.5-3 and that of a time-varying ideal independent current source is shown in (b) of Fig. 1.5-3.
Fig. 1.5-1 

Approximate
equivalentcircuitof
apracticalvoltage
source
+
+
–
–
V
(
t
)
E
(
t
)
L
i
R
i
C
i
Fig. 1.5-2 

Idealindependent
voltagesource
–
–
+
+
(b)
i
(
t
)
v
(
t
)
E
(
t
)
–
–
+
–
–
+
+
+
E
E
OR
(a)
V
V
i
(
t
)
i
(
t
)
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PowerandEnergyRelationsforTwo-TerminalElements
 
 
1.29
An ideal independent current source is specified by the 
following equations:
i(t) 
=
I
s
(t) , a specified function of time.
v(t) 
=
Arbitrary, decided by the rest of the circuit in which 
this source is connected.
There are no ideal independent voltage sources and ideal 
independent current sources in practice. These are only models 
of practical sources that give reasonably accurate results 
provided they are not applied under extreme loading conditions. 
The ideal model will undoubtedly fail for a practical voltage source that is shorted or for a practical 
current source that is open-circuited. The short-circuit current in a DC voltage source is limited by 
its internal resistance while that of a time-varying voltage source is limited by internal resistance and 
internal inductance. Similarly, the open-circuit voltage that appears across a practical current source 
is limited by its internal resistance in the case of a DC source. It is limited by internal capacitance and 
resistance in the case of a time-varying current source.
In fact, ideal model for a voltage source models a practical voltage source accurately only when the 
current delivered/absorbed by it is a small fraction of its short-circuit current. Similarly, ideal model 
for a current source models a practical current source accurately only when the voltage appearing 
across its terminals is a small fraction of its open-circuit voltage.