A voltage source is a two-
terminal device which can maintain a fixed
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
. An ideal voltage source can maintain the fixed voltage independent of the
load resistance or the output
current. However, a real-world voltage source cannot supply unlimited current.
A voltage source is the
dual of a
current source
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources fed ...
. Real-world sources of electrical energy, such as
batteries and
generators, can be modeled for analysis purposes as a combination of an ideal voltage source and additional combinations of
impedance elements.
Ideal voltage sources
An ideal voltage source is a two-terminal device that maintains a fixed
voltage drop
In electronics, voltage drop is the decrease of electric potential along the path of a current flowing in a circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are unde ...
across its terminals. It is often used as a mathematical abstraction that simplifies the analysis of real electric circuits. If the voltage across an ideal voltage source can be specified independently of any other variable in a circuit, it is called an independent voltage source. Conversely, if the voltage across an ideal voltage source is determined by some other voltage or current in a circuit, it is called a dependent or controlled voltage source. A mathematical model of an amplifier will include dependent voltage sources whose magnitude is governed by some fixed relation to an input signal, for example.
[K. C. A. Smith, R. E. Alley , ''Electrical circuits: an introduction'', Cambridge University Press, 1992
, pp. 11-13 ] In the analysis of
faults on electrical power systems, the whole network of interconnected sources and transmission lines can be usefully replaced by an ideal (AC) voltage source and a single equivalent impedance.
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, Ideal Voltage Source
,
Ideal Current Source
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, Controlled Voltage Source
, Controlled Current Source
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,
Battery of cells
, Single cell
The
internal resistance
In electrical engineering, a practical electric power source which is a linear circuit may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance. This impedance is termed the internal resis ...
of an ideal voltage source is zero; it is able to supply or absorb any amount of current. The current through an ideal voltage source is completely determined by the external circuit. When connected to an open circuit, there is zero current and thus zero power. When connected to a
load resistance, the current through the source approaches infinity as the load resistance approaches zero (a short circuit). Thus, an ideal voltage source can supply unlimited power.
If two ideal independent voltage source are directly connected in
parallel, they must have exactly the same voltage; Otherwise, it creates a
fallacy
A fallacy is the use of invalid or otherwise faulty reasoning in the construction of an argument that may appear to be well-reasoned if unnoticed. The term was introduced in the Western intellectual tradition by the Aristotelian '' De Sophisti ...
in logic, similar to writing down the equation
.
If two ideal independent voltage sources are connected in parallel via a resistor, the source with the lower voltage becomes a consumer. If the source voltages are equal, there is zero current and thus zero power.
Voltage sources in parallel shares the burden of current: If an exact duplicate of voltage is connected in parallel to the original one, either one of them will provide half of the
electric current
An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...
that the original voltage source would provide. For the remainder of the circuit, nothing has changed: These two voltage sources together provide the same voltage, and the same current as the original one alone.
No real voltage source is ideal; all have a non-zero effective internal resistance, and none can supply unlimited current. However, the internal resistance of a real voltage source is effectively modeled in linear circuit analysis by combining a non-zero resistance in series with an ideal voltage source (a
Thévenin equivalent circuit).
Comparison between voltage and current sources
Most sources of electrical energy (the
mains, a
battery) are modeled as voltage sources.
An ''ideal'' voltage source provides no energy when it is loaded by an
open circuit Open circuit may refer to:
* Open circuit breathing apparatus, any type of breathing apparatus where the exhaled gas is discharged to the surroundings without recycling any of it
** Open-circuit scuba, a type of Scuba-diving equipment where the user ...
(i.e. an infinite
impedance), but approaches infinite energy and current when the
load resistance approaches zero (a
short circuit
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...
). Such a theoretical device would have a zero
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (1 ...
output impedance
In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
in series with the source. A real-world voltage source has a very low, but non-zero
internal resistance
In electrical engineering, a practical electric power source which is a linear circuit may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance. This impedance is termed the internal resis ...
and
output impedance
In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
, often much less than 1 ohm.
Conversely, a
current source
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources fed ...
provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the
load resistance approaches infinity (an open circuit). An ''ideal'' current source has an
infinite output impedance
In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
in parallel with the source. A ''real-world'' current source has a very high, but finite
output impedance
In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
. In the case of
transistor
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...
current sources, impedance of a few
megohms (at low frequencies) is typical.
Since no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the ''same''
source impedance
In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
and vice versa. Voltage sources and current sources are sometimes said to be
duals of each other and any non ideal source can be converted from one to the other by applying
Norton's theorem
In Direct current, direct-current circuit theory, Norton's theorem, also called the Mayer–Norton theorem, is a simplification that can be applied to Electrical network, networks made of Linear time-invariant system, linear time-invariant Resi ...
or
Thévenin's theorem
As originally stated in terms of direct-current resistance (electricity), resistive circuits only, Thévenin's theorem states that ''"Any linear circuit, linear electrical network containing only voltage source, voltage sources, current source, c ...
.
References and notes
See also
*
Dependent source
*
Bandgap voltage reference
*
Voltage divider
In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the i ...
*
Voltage reference
*
Voltage regulator
A voltage regulator is a system designed to automatically maintain a constant voltage. It may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism or electronic components. Depending on the ...
{{DEFAULTSORT:Voltage Source
Analog circuits