**Norton’s theorem** says that a linear two-terminal electric circuit may be exchanged with a Norton equivalent circuit consisting of a current source, I_{N}, in parallel with a Norton resistor R_{N}. Where I_{N} is the short-circuit current through the terminals load resistor and R_{N} is equivalent resistance at the terminals when all the independent sources are turned off.

Just like Thevenin Theorem, Norton Theorem is also useful to analysis single resistor which changes frequently and the rest of the circuit remain same. In contrast to Thevenin Theorem, the Norton Theorem reduce circuit to the single current source instead of the voltage source. An American Engineer E. L. Norton, in 1926, proposed that a circuit can be reduced to the single current source, two resistors; R_{N}, parallel with the current source and load resistor, R_{L}, frequently changing resistor.

Norton Resistor R_{N}, is exactly same as Thevenin’s resistor R_{th} and Norton Current I_{N} is Vth/R_{N}. So, this is the implies that Norton Theorem is not more than source transformation of Thevenin Theorem.

## Norton Equivalent Circuit :

According to the Norton theorem, any linear circuit can be reduced to single current source in parallel with R_{N}, which is calculated from load resistor end keeping all dependent sources.

And the current of current source can be calculated from load resistor terminal voltage keeping it open. The generic Norton Equivalent circuit is shown in the diagram below.

## Example :

Suppose we want to connect some variable resistor at terminal A-B and find Norton equivalent circuit to the following example for the terminals.

### Solution:

#### Find Norton Resistance:

First of all, we have to replace all independent voltage source with the closed circuit and current sources with an open circuit, as shown in the figure below.

Now we will find equivalent resistance from terminal A-B point of view, the direction shown by the arrow in the diagram above. So, Norton resistor R_{N} will be:

$R_{N}=5||( 8+4+8)=\frac{ 20\times 5}{20+5}$

$R_{N}=4\Omega $

### Find Norton Voltage:

To find I_{N}, first we find out the voltage at terminal A-B, while it is open circuited. So the circuit becomes something like this:

You can use any method you want, here I am using mesh analysis and writing equations as below:

$I_{1}=3A \\ I_{2} (4+8+5+8)-4 I_{1}-12=0 \\ 25 I_{2}-4 I_{1}=12 \\ I_{2}=0.8 A$

The current I_{2} is flowing through 5Ω, so the voltage across the resistor will be:

$V_{th}=5\times 0.8 \\ V_{th}=4 v$

### Find Norton Current:

Now we have both resistance R_{N} and voltage V_{th}, so we can find out the Norton current I_{N} as follow:

$I_{N}=\frac{V_{th}}{R_{N}}=\frac{4}{4}=1A$

The Norton equivalent circuit for the terminal A-B will be as follow:

Where at terminal A-B we can put any resistor we need. We can analyze the resistor current through current divider rule instead of going into complex circuit calculation.