Thevenin's Theorem
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).
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.
Thevenin's Theorem: Detial
In principle, all currents and voltages of an arbitrary network of linear components and voltage/current sources can be found by any of the three methods discussed previous, namely, the branch current method, the loop current method and the node voltage method.
However, if only the current and/or voltage associated with one component are of interest, it is unnecessary to find voltages and currents elsewhere in the circuit. The methods considered below can be used in such situations.
Any one-port (two-terminal) network of resistance elements and energy sources is equivalent to an ideal voltage source 
in series with a resistor 
, where - is the open-circuit voltage of the network, and
- is the equivalent resistance when all energy sources are turned off (short-circuit for voltage sources, open-circuit for current sources).
If we are only interested in finding the voltage
across and current
through one particular resistor in a
complex circuit containing a large number of resistors, voltage and current sources, we can ``pull'' the resistor out and treat the rest of the circuit as a Thevenin voltage source
, and use Thevenin's theorem to find
and
.
Proof: 
Assume with the load the network's terminal voltage and current are
and
respectively.
- Replace the load by an ideal current source while keeping the terminal voltage the same (b), the voltage or current anywhere in the system should not be affected.
- Find the terminal voltage in terms of the internal energy sources inside the network and the external current source by superposition principle:
- When the external current source is off (open circuit), the terminal voltage
is due only to the sources internal to the network (c). - When all internal sources are turned off (short-circuit for voltage sources, open-circuit for current sources), the terminal voltage
where 
is the equivalent resistance of the network with all energy sources off (d).
The overall terminal voltage is 
.
As far as the port voltage
and current
are concerned, a one-port network is equivalent to an ideal voltage source
equal to the open-circuit voltage across the port, in series with an
internal resistance 
, which can be obtained as the ratio of the open-circuit voltage and the short-circuit current.