Norton Theorem states that a linear two-terminal circuit can be replaced by an equivalent circuit consisting of a current source In in parallel with a resistor Rn, where In is the short circuit current through the terminals and Rn is the input or equivalent resistance at the terminals when the independent sources are turned off.
We find Rn in the same way we find Rth. In fact from what we know about source transformation, the Thevenin and Norton resistances are equal; that is, Rn = Rth
Norton current In: we determine the short-circuit current flowing from terminal a to b in both circuits above. It is evident that the short-circuit current in figure b is In. This must be the same short-circuit current from terminal a to b in figure a. Since the two circuits are equivalent. Thus, In = Isc.
Norton current In: we determine the short-circuit current flowing from terminal a to b in both circuits above. It is evident that the short-circuit current in figure b is In. This must be the same short-circuit current from terminal a to b in figure a. Since the two circuits are equivalent. Thus, In = Isc.