Why Does a Current Source Have Internal Resistance in Parallel?

[electroboy12]

hello.. this is my first post..forgive me if my question is wrong.. Why does a current source is modeled in such a way that it has its internal resistance in parallel ? Why can't it have its internal resistance in series ? The question may seem awkward , but please answer me.. thanks in advance..

 

[sophiecentaur]

This may not be intuitive but Thevenin and Norton theorems tell you that any network of emf / current sources and resistances can be characterised by either an emf in series with or a current source in parallel with an appropriate resistance. That is not to say that it would be the best way to produce either, in practice! You could be making a lot of smoke if you chose the inappropriate one to use.

 

[otto9K9otto]

A current source is defined as, well, a source of current that has zero ohms source resistance. Electrically, this current source, when it is paralleled with a resistor, behaves identically to a voltage source that is in series with that same resistor. The amount of equivalent voltage is the product of the current (source) times the paralleled resistor.

 

[sophiecentaur]

Not a correct definition for an Ideal Source. An ideal current source has infinite resistance - in the same way that an ideal voltage source has zero resistance.

A real current source will have some Conductance (i.e. not infinite resistance) so it can actually be replaced by a suitable combination of ideal voltage or current source with a resistor.

A current source will provide just enough volts to drive the required current into the load and a voltage source will provide just enough current to develop the required voltage across the load. In practical terms this means that a current source (Anode, for instance) looks like a high resistance whilst a voltage source (Transistor emitter, for instance) looks like a low resistance.