As originally stated in terms of direct-current resistive circuits only, Thévenin's theorem states that "Any linear electrical network containing only voltage sources, current sources and resistances can be replaced at terminals A–B by an equivalent combination of a voltage source V_th in a series connection with a resistance R_th."

• The equivalent voltage V_th is the voltage obtained at terminals A–B of the network with terminals A–B open circuited.
• The equivalent resistance R_th is the resistance that the circuit between terminals A and B would have if all ideal voltage sources in the circuit were replaced by a short circuit and all ideal current sources were replaced by an open circuit.
• If terminals A and B are connected to one another, the current flowing from A and B will be ${\displaystyle {\tfrac {V_{\mathrm {th} }}{R_{\mathrm {th} }}}.}$ This means that R_th could alternatively be calculated as V_th divided by the short-circuit current between A and B when they are connected together.

In circuit theory terms, the theorem allows any one-port network to be reduced to a single voltage source and a single impedance.

The theorem also applies to frequency domain AC circuits consisting of reactive (inductive and capacitive) and resistive impedances. It means the theorem applies for AC in an exactly same way to DC except that resistances are generalized to impedances.

The theorem was independently derived in 1853 by the German scientist Hermann von Helmholtz and in 1883 by Léon Charles Thévenin (1857–1926), an electrical engineer with France's national Postes et Télégraphes telecommunications organization.^{[1][2][3][4][5][6][7]}

Thévenin's theorem and its dual, Norton's theorem, are widely used to make circuit analysis simpler and to study a circuit's initial-condition and steady-state response.^[8][9] Thévenin's theorem can be used to convert any circuit's sources and impedances to a Thévenin equivalent; use of the theorem may in some cases be more convenient than use of Kirchhoff's circuit laws.^[7][10]

Various proofs have been given of Thévenin's theorem. Perhaps the simplest of these was the proof in Thévenin's original paper.^[3] Not only is that proof elegant and easy to understand, but a consensus exists^[4] that Thévenin's proof is both correct and general in its applicability. The proof goes as follows:

Consider an active network containing impedances, (constant-) voltage sources and (constant-) current sources. The conguration of the network can be anything. Access to the network is provided by a pair of terminals. Designate the voltage measured between the terminals as V_θ, as shown in the box on the left side of Figure 2.

Suppose that the voltage sources within the box are replaced by short circuits, and the current sources by open circuits. If this is done, no voltage appears across the terminals, and it is possible to measure the impedance between the terminals. Call this impedance Z_θ.

Now suppose that one attaches some linear network to the terminals of the box, having impedance Z_e, as in Figure 2a. We wish to find the current I through Z_e. The answer is not obvious, since the terminal voltage will not be V_θ after Z_e is connected.

Instead, we imagine that we attach, in series with impedance Z_e, a source with electromotive force E equal to V_θ but directed to oppose V_θ, as shown in Figure 2b. No current will then flow through Z_e since E balances V_θ.

Next, we insert another source of electromotive force, E₁, in series with Z_e, where E₁ has the same magnitude as E but is opposed in direction (see Figure 2c). The current, I₁, can be determined as follows: it is the current that would result from E₁ acting alone, with all other sources (within the active network and the external network) set to zero. This current is, therefore,

$${\displaystyle I_{1}={\frac {E_{1}}{Z_{e}+Z_{\theta }}}={\frac {V_{\theta }}{Z_{e}+Z_{\theta }}}}$$

because Z_e is the impedance external to the box and Z_θ looking into the box when its sources are zero.

Finally, we note that E and E₁ can be removed together without changing the current, and when they are removed, we are back to Figure 2a. Therefore I₁ is the current, I, that we are seeking, i.e.

$${\displaystyle I={\frac {V_{\theta }}{Z_{e}+Z_{\theta }}}}$$

thus completing the proof. Figure 2d shows the Thévenin equivalent circuit.

The equivalent circuit is a voltage source with voltage V_th in series with a resistance R_th.

The Thévenin-equivalent voltage V_th is the open-circuit voltage at the output terminals of the original circuit. When calculating a Thévenin-equivalent voltage, the voltage divider principle is often useful, by declaring one terminal to be V_out and the other terminal to be at the ground point.

The Thévenin-equivalent resistance R_Th is the resistance measured across points A and B "looking back" into the circuit. The resistance is measured after replacing all voltage- and current-sources with their internal resistances. That means an ideal voltage source is replaced with a short circuit, and an ideal current source is replaced with an open circuit. Resistance can then be calculated across the terminals using the formulae for series and parallel circuits. This method is valid only for circuits with independent sources. If there are dependent sources in the circuit, another method must be used such as connecting a test source across A and B and calculating the voltage across or current through the test source.

As a mnemonic, the Thevenin replacements for voltage and current sources can be remembered as the sources' values (meaning their voltage or current) are set to zero. A zero valued voltage source would create a potential difference of zero volts between its terminals, just like an ideal short circuit would do, with two leads touching; therefore the source is replaced with a short circuit. Similarly, a zero valued current source and an open circuit both pass zero current.

Example

In the example, calculating the equivalent voltage:

$${\displaystyle {\begin{aligned}V_{\mathrm {Th} }&={R_{2}+R_{3} \over (R_{2}+R_{3})+R_{4}}\cdot V_{\mathrm {1} }\\&={1\,\mathrm {k} \Omega +1\,\mathrm {k} \Omega \over (1\,\mathrm {k} \Omega +1\,\mathrm {k} \Omega )+2\,\mathrm {k} \Omega }\cdot 15\,\mathrm {V} \\&={1 \over 2}\cdot 15\,\mathrm {V} =7.5\,\mathrm {V} \end{aligned}}}$$

(Notice that R₁ is not taken into consideration, as above calculations are done in an open-circuit condition between A and B, therefore no current flows through this part, which means there is no current through R₁ and therefore no voltage drop along this part.)

Calculating equivalent resistance (R_x || R_y is the total resistance of two parallel resistors):

$${\displaystyle {\begin{aligned}R_{\mathrm {Th} }&=R_{1}+\left[\left(R_{2}+R_{3}\right)\|R_{4}\right]\\&=1\,\mathrm {k} \Omega +\left[\left(1\,\mathrm {k} \Omega +1\,\mathrm {k} \Omega \right)\|2\,\mathrm {k} \Omega \right]\\&=1\,\mathrm {k} \Omega +\left({1 \over (1\,\mathrm {k} \Omega +1\,\mathrm {k} \Omega )}+{1 \over (2\,\mathrm {k} \Omega )}\right)^{-1}=2\,\mathrm {k} \Omega .\end{aligned}}}$$

A Norton equivalent circuit is related to the Thévenin equivalent by

$${\displaystyle {\begin{aligned}R_{\mathrm {Th} }&=R_{\mathrm {No} }\!\\V_{\mathrm {Th} }&=I_{\mathrm {No} }R_{\mathrm {No} }\!\\I_{\mathrm {No} }&={\frac {V_{\mathrm {Th} }}{R_{\mathrm {Th} }}}\!\end{aligned}}}$$

• Many circuits are only linear over a certain range of values, thus the Thévenin equivalent is valid only within this linear range.
• The Thévenin equivalent has an equivalent I–V characteristic only from the point of view of the load.
• The power dissipation of the Thévenin equivalent is not necessarily identical to the power dissipation of the real system. However, the power dissipated by an external resistor between the two output terminals is the same regardless of how the internal circuit is implemented.

In 1933, A. T. Starr published a generalization of Thévenin's theorem in an article of the magazine Institute of Electrical Engineers Journal, titled A New Theorem for Active Networks,^[11] which states that any three-terminal active linear network can be substituted by three voltage sources with corresponding impedances, connected in wye or in delta.

• Extra element theorem
• Maximum power transfer theorem
• Millman's theorem
• Source transformation