About Norton and Thevenin Theorrems

There is two ways to examine common circuits



Thévenin's theorem

Replace voltage sources with short circuits, and current sources with open circuits.
Calculate the resistance between terminals A and B. This is RTh.

Norton's theorem

Replace independent voltage sources with short circuits and independent current sources with open circuits.
The total resistance across the output port is the Norton impedance RNo.
or
Use a given Thevenin resistance: as the two are equal.


A Norton equivalent circuit is related to the Thévenin equivalent by the following equations:

R_{Th} = R_{No}
V_{Th} = I_{No} R_{No}
V_{Th} / R_{Th} = I_{No}


Thévenin's Theorem

Any linear voltage network which may be viewed from two terminals can be replaced
by a voltage-source equivalent circuit comprising a single voltage source E and a single
series impedance Z. The voltage E is the open-circuit voltage between the two terminals
and the impedance Z is the impedance of the network viewed from the terminals with all
voltage sources replaced by their internal impedances.

Norton's Theorem

Any linear current network which may be viewed from two terminals can be replaced by a
current-source equivalent circuit comprising a single current source I and a single shunt
admittance Y. The current I is the short-circuit current between the two terminals and
the admittance Y is the admittance of the network viewed from the terminals with all current
sources replaced by their internal admittances.

Thévenin and Norton Equivalence

The open circuit, short circuit and load conditions of the Thévenin model are:

Voc = E
Isc = E / Z
Vload = E - IloadZ
Iload = E / (Z + Zload)

The open circuit, short circuit and load conditions of the Norton model are:

Voc = I / Y
Isc = I
Vload = I / (Y + Yload)
Iload = I - VloadY

Thévenin model from Norton model
Voltage = Current / Admittance
Impedance = 1 / Admittance
E = I / Y
Z = Y -1

Norton model from Thévenin model
Current = Voltage / Impedance
Admittance = 1 / Impedance
I = E / Z
Y = Z -1

When performing network reduction for a Thévenin or Norton model, note that:
- nodes with zero voltage difference may be short-circuited with no effect on the network current distribution,
- branches carrying zero current may be open-circuited with no effect on the network voltage distribution.



Read more...............................

Thévenin's theorem - Wikipedia, the free encyclopedia

Norton's theorem - Wikipedia, the free encyclopedia

Electronics/Thevenin/Norton Equivalents - Wikibooks, open books for an open world

Voltage and Current Sources