Norton's theorem

Norton's theorem is Thevenin's theorem told from the current point of view. Where Thevenin says any linear two-terminal network looks like a voltage source in series with a resistor, Norton says the very same network looks like a current source in parallel with a resistor. They are two costumes for the identical box, and which you reach for is just a matter of which is more convenient for the problem in front of you.

The two Norton numbers are the Norton current and the Norton resistance. The Norton current In is the short-circuit current, the current that flows if you connect a wire straight across the two terminals. The Norton resistance Rn is found exactly as the Thevenin resistance, by zeroing the internal sources and looking back in, so Rn equals Rth. The two equivalents convert into each other with a single relation, Vth = In times Rn, which is just Ohm's law applied to that shared resistance.

Norton's form is the natural choice when you are thinking in terms of currents, for instance when a circuit is driven by current sources or when you want the short-circuit current directly, such as in fault analysis. It is also the form transistor and op-amp output stages often take. Because Norton and Thevenin carry exactly the same information, you freely flip between them mid-problem to whichever makes the next step easier.