Source Tranformation, Thevenin and Norton's Theorem

SOURCE TRANSFORMATION

Performing a source transformation consists of using Ohm’s Law. One can be derived from the other. This method can be performed on a circuit involving capacitors and inductors, as long as the circuit is first put into the frequency domain.

An ac voltage source V in series with an impedance Z can be replaced with an ac current source using this formula: (I=V/Z) connected in parallel with the impedance Z. An ac current source I in parallel with an impedance Z can be replaced with an ac voltage source of value using this formula:  (V=IZ) in series with the impedance Z.

TRANSFORMATION:

  

NOTE:

Using this method, we must keep these things in mind. First, apply Ohm's Law Second, impedances remain the same. Third, the arrow of the current source is directed toward the positive terminal of the voltage source.

I've learned that in source transformation, we used to get the equivalent circuit whose voltage-current characteristics are identical with the original circuit.  Also, dependent voltage source in series with impedance can be transformed to dependent current source in parallel with the impedance. For personal insights, source transformation is the easiest among the methods discussed because it is less confusing and you can easily understand it very well.

THEVENIN AND NORTON’S THEOREM

The application of Thevenin and Norton’s theorem to ac network is just the same as that outlined for dc networks. The only difference is that the quantities being used in dc are all real values while in ac are phasors and complex numbers. These theorems enable one to replace a complicated circuit with a simple equivalent circuit. These are used to isolate a portion of the circuit then the remaining portion of the circuit is transformed into an equivalent circuit.

THEVENIN’S THEOREM

This theorem has an equivalent circuit containing only a voltage source and a series connected impedance. The V_Th is also known as an open circuit voltage.

See figure below:

A Thevenin equivalent circuit is one that produces the same voltage and current to a given load as the original circuit that it replaces.

HOW TO CALCULATE THE THEVENIN EQUIVALENT CIRCUIT???

·       . Calculate Z_Th

Ø  Set all sources to zero or simply replace voltage sources by short circuits and current sources by open circuits.

Ø  Then find the total impedance between the two terminals.

·       Calculate V_Th.

Ø  Find the open circuit voltage between the terminals. 

NOTE:

Don’t deadened dependent sources because they are controlled by circuit variables.

NORTON’S THEOREM

               This theorem has an equivalent circuit containing a current source and a parallel connected impedance. The I_N is also known as a short circuit current.

See figure below: 

          The current source in Norton equivalent circuit is usually dependent on the electric signals applied to the input terminals.
 
HOW TO CALCULATE THE NORTON EQUIVALENT CIRCUIT???

·       . Calculate Z_N

Ø  Set all sources to zero or simply replace voltage sources by short circuits and current sources by open circuits.

Ø  Then find the total impedance between the two terminals.

·       Calculate I_N. 

Ø  Find the short circuit current between the terminals. 

          Through the discussions in our class and the informations I've read, I've learned that Norton’s theorem is just the derivation of the Thevenin’s theorem. They both have equal magnitude of impedance and also, their equivalent circuit can easily be interchanged by just applying Ohm’s Law. Most importantly, these theorems provide a method of reducing a more complex circuit to a simpler one and more manageable form for analysis. Lastly,I've learned that these theorems are very important from both theoritical and practical viewpoints since it is useful in calculating and simulating changes for large scale circuits.