Vth

Vth, or Thevenin voltage, is a parameter used in Thevenin's theorem. It represents the open-circuit voltage that would appear at the terminals of a linear bilateral network when no external load is connected.

To obtain the Thevenin equivalent, identify two terminals of interest. The Thevenin voltage Vth equals Voc, the voltage across the terminals with the load removed. The Thevenin resistance Rth is the equivalent resistance seen from the terminals with all independent sources turned off (voltage sources replaced by short circuits, current sources by open circuits). If a network contains dependent sources, Rth must be found by alternative methods such as a test source, rather than simply turning off sources.

The Thevenin equivalent circuit consists of a voltage source of value Vth in series with a resistance Rth. Any linear network connected to the same two terminals can be replaced by this equivalent for the purpose of analyzing the load. The Norton equivalent, consisting of a current source In in parallel with a resistor Rn, is related by In = Vth / Rth and Rn = Rth.

Vth is widely used in circuit analysis to simplify calculations of power, current, and voltage across loads. It is applicable to linear networks with independent or dependent sources; non-linear or time-varying networks may not have a valid Thevenin equivalent at all times. The concept is named after Léon Charles Thévenin, a French physicist and engineer.