In circuit theory, Thévenin's theorem for linear electrical networks states that any combination of voltage sources, current sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem can also be applied to general impedances, not just resistors. The theorem was first discovered by German scientist Hermann von Helmholtz in 1853^{[1]}, but was then rediscovered in 1883 by French telegraph engineer Léon Charles Thévenin (1857–1926).^{[2]}^{[3]}
This theorem states that a circuit of voltage sources and resistors can be converted into a Thévenin equivalent, which is a simplification technique used in circuit analysis. The Thévenin equivalent can be used as a good model for a power supply or battery (with the resistor representing the internal impedance and the source representing the electromotive force). The circuit consists of an ideal voltage source in series with an ideal resistor.
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To calculate the equivalent circuit, one needs a resistance and some voltage  two unknowns. And so, one needs two equations. These two equations are usually obtained by using the following steps, but any conditions one places on the terminals of the circuit should also work:
Step 2 could also be thought of like this:
The Théveninequivalent voltage is the voltage at the output terminals of the original circuit. When calculating a Théveninequivalent 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 Théveninequivalent resistance is the resistance measured across points A and B "looking back" into the circuit. It is important to first replace all voltage and currentsources with their internal resistances. For an ideal voltage source, this means replace the voltage source with a short circuit. For an ideal current source, this means replace the current source 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.



In the example, calculating the equivalent voltage:
(notice that R_{1} 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 R_{1} and therefore no voltage drop along this part)
Calculating equivalent resistance:
A Norton equivalent circuit is related to the Thévenin equivalent by the following equations:
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Thévenin`s theorem states that every network can be reduced to voltage viz (Thévenin's voltage), Series with resistance viz. (Thévenin's resistance)between which is measured between open terminals of the network 59.93.69.206 15:14, 5 February 2008 (UTC)
