Thevenin's Theorem

Thevenin's theorem, for dc and ac steady state systems, allows you to replace a complex network of voltage sources and resistances with a single voltage source and single resistance (Thevenin equivalent circuit).

The complex network needs to only have one "port" for the theorem to work. A "port" is usually two terminals such as a ground/common/cold and a signal or "hot" of some kind.

To calculate the Thevenin equivalent circuit, voltage sources have to be short circuited (i.e. resistance = 0) and then all resistors are simplified by the rules of series and parallel resistance.