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Direct current (DC) is one-directional
flow Flow may refer to: Science and technology * Flow (fluid) or fluid dynamics, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathematics), a group action of the real numbers on ...
of
electric charge Electric charge is the physical property A physical property is any property Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of th ...
. An
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. When used loosely, ''electrical energy'' refers to energy that has been c ...
is a prime example of DC power. Direct current may flow through a
conductor Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble like, for example, an orchestra. * Conductor (album), ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of ...
such as a wire, but can also flow through
semiconductor A semiconductor material has an value falling between that of a , such as metallic copper, and an , such as glass. Its falls as its temperature rises; metals behave in the opposite way. Its conducting properties may be altered in useful ways ...
s,
insulators Insulator may refer to: * Insulator (electricity) An electrical insulator is a material in which the electron does not flow freely or the atom of the insulator have tightly bound electrons whose internal electric charge Electric charge is th ...
, or even through a
vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Gree ...

vacuum
as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through ...
(AC). A term formerly used for this type of current was galvanic current. The abbreviations ''AC'' and ''DC'' are often used to mean simply ''alternating'' and ''direct'', as when they modify ''
current Currents or The Current may refer to: Science and technology * Current (fluid) A current in a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...
'' or ''
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

voltage
''. Direct current may be converted from an alternating current supply by use of a
rectifier A rectifier is an electrical device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a ...

rectifier
, which contains
electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow b ...
elements (usually) or electromechanical elements (historically) that allow current to flow only in one direction. Direct current may be converted into alternating current via an
inverter A power inverter, or inverter, is a power electronic device or circuitry that changes direct current Direct current (DC) is the one directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct curren ...
. Direct current has many uses, from the charging of batteries to large power supplies for electronic systems, motors, and more. Very large quantities of electrical energy provided via direct-current are used in smelting of
aluminum Aluminium (aluminum in American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly known as the Uni ...

aluminum
and other
electrochemical Electrochemistry is the branch of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying ...

electrochemical
processes. It is also used for some
railways Rail transport (also known as train transport) is a means of transferring passengers and goods on wheeled vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor veh ...
, especially in
urban area An urban area, or built-up area, is a human settlement with a high population density and infrastructure of built environment. Urban areas are created through urbanization and are categorized by urban morphology as city, cities, towns, conurbat ...
s.
High-voltage direct current A high-voltage, direct current (HVDC) electric power transmission Electric power transmission is the bulk movement of electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. When used loosely, ...
is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.


History

Direct current was produced in 1800 by Italian physicist
Alessandro Volta Alessandro Giuseppe Antonio Anastasio Volta (, ; 18 February 1745 – 5 March 1827) was an Italian physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empiric ...

Alessandro Volta
's battery, his
Voltaic pile –zinc Zinc is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the sa ...

Voltaic pile
. The nature of how current flowed was not yet understood. French physicist
André-Marie Ampère André-Marie Ampère (, ; ; 20 January 177510 June 1836) was a French physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of ...
conjectured that current travelled in one direction from positive to negative. When French instrument maker
Hippolyte Pixii An early form of an alternating current electrical generator built by Pixii Hippolyte Pixii (1808–1835) was an instrument maker from Paris, France. In 1832 he built an early form of alternating current Alternating current (AC) is an electric ...
built the first
dynamo electric generator
dynamo electric generator
in 1832, he found that as the magnet used passed the loops of wire each half turn, it caused the flow of electricity to reverse, generating an
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through ...
. At Ampère's suggestion, Pixii later added a
commutator In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gene ...
, a type of "switch" where contacts on the shaft work with "brush" contacts to produce direct current. The late 1870s and early 1880s saw electricity starting to be generated at
power stations A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation A generation is "all of the people born and living Living or The Living may refer to: ...
. These were initially set up to power arc lighting (a popular type of street lighting) running on very high voltage (usually higher than 3000 volt) direct current or alternating current. This was followed by the wide spread use of low voltage direct current for indoor electric lighting in business and homes after inventor
Thomas Edison Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation Electricity generation is the process of generating electric power from s ...

Thomas Edison
launched his incandescent bulb based electric "
utility As a topic of economics Economics () is a social science Social science is the Branches of science, branch of science devoted to the study of society, societies and the Social relation, relationships among individuals within thos ...
" in 1882. Because of the significant advantages of alternating current over direct current in using
transformer A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple Electrical network, circuits. A varying current in any one coil of the transformer produces a varying magnetic flux ...

transformer
s to raise and lower voltages to allow much longer transmission distances, direct current was replaced over the next few decades by alternating current in power delivery. In the mid-1950s,
high-voltage direct current A high-voltage, direct current (HVDC) electric power transmission Electric power transmission is the bulk movement of electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. When used loosely, ...
transmission was developed, and is now an option instead of long-distance high voltage alternating current systems. For long distance underseas cables (e.g. between countries, such as
NorNed NorNed is a long high-voltage direct current submarine power cable between Feda Feda is a village in Kvinesdal municipality in Agder county, Norway. The village is located on the north side of the Fedafjorden, about southwest of the village ...
), this DC option is the only technically feasible option. For applications requiring direct current, such as
third rail Shoe of NYC Subway car making contact with third rail. In the foreground is the third rail for the adjacent track. A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power Electric p ...
power systems, alternating current is distributed to a substation, which utilizes a
rectifier A rectifier is an electrical device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a ...

rectifier
to convert the power to direct current.


Various definitions

The term ''DC'' is used to refer to power systems that use only one polarity of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current. For example, the voltage across a DC
voltage source A voltage source is a two- terminal device which can maintain a fixed voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ' ...

voltage source
is constant as is the current through a DC
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 duality (electrical circuits), dual of a voltage source. The term ''current sink'' is so ...

current source
. The DC solution of an
electric circuit An electrical network is an interconnection of electrical component An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic componen ...

electric circuit
is the solution where all voltages and currents are constant. It can be shown that any
stationary In addition to its common meaning, stationary may have the following specialized scientific meanings: Mathematics * Stationary point In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity ...
voltage or current waveform can be decomposed into a sum of a DC component and a zero-mean time-varying component; the DC component is defined to be the expected value, or the average value of the voltage or current over all time. Although DC stands for "direct current", DC often refers to "constant polarity". Under this definition, DC voltages can vary in time, as seen in the raw output of a rectifier or the fluctuating voice signal on a telephone line. Some forms of DC (such as that produced by a
voltage regulator A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward Feedforward is the provision of context of what one wants to communicate prior to that communication. In purp ...

voltage regulator
) have almost no variations in
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

voltage
, but may still have variations in output
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
and current.


Circuits

A direct current circuit is an
electrical circuit An electrical network is an interconnection of electrical component An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic component ...

electrical circuit
that consists of any combination of constant
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

voltage
sources, constant
current Currents or The Current may refer to: Science and technology * Current (fluid) A current in a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...
sources, and
resistor A resistor is a that implements as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to , active elements, and terminate s, among other uses. High-power resistors that can dissipat ...

resistor
s. In this case, the circuit voltages and currents are independent of time. A particular circuit voltage or current does not depend on the past value of any circuit voltage or current. This implies that the system of equations that represent a DC circuit do not involve integrals or derivatives with respect to time. If a
capacitor A capacitor is a device that stores electric charge in an electric field. It is a passivity (engineering), passive electronic component with two terminal (electronics), terminals. The effect of a capacitor is known as capacitance. While som ...

capacitor
or
inductor An inductor, also called a coil, choke, or reactor, is a passive Passive may refer to: * Passive voice, a grammatical voice common in many languages, see also Pseudopassive (disambiguation), Pseudopassive * Passive language, a language from whi ...

inductor
is added to a DC circuit, the resulting circuit is not, strictly speaking, a DC circuit. However, most such circuits have a DC solution. This solution gives the circuit voltages and currents when the circuit is in DC steady state. Such a circuit is represented by a system of
differential equations In mathematics, a differential equation is an equation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), ...
. The solution to these equations usually contain a time varying or
transient Transience or Transient may refer to: Music * Transient (album), ''Transient'' (album), a 2004 album by Gaelle * Transience (album), ''Transience'' (album), a 2015 album by Steven Wilson Science and engineering * Transient state, when a process v ...
part as well as constant or steady state part. It is this steady state part that is the DC solution. There are some circuits that do not have a DC solution. Two simple examples are a constant current source connected to a capacitor and a constant voltage source connected to an inductor. In electronics, it is common to refer to a circuit that is powered by a DC voltage source such as a battery or the output of a DC power supply as a DC circuit even though what is meant is that the circuit is DC powered.


Applications


Domestic and commercial buildings

DC is commonly found in many
extra-low voltage Extra-low voltage (ELV) is an electricity supply voltage Voltage, electric potential difference, electromotive force emf, electric pressure or electric tension is the difference in electric potential between two points, which (in a static ...
applications and some low-voltage applications, especially where these are powered by
batteries Battery may refer to: Energy source * Electric battery, an electrochemical device to provide electrical power ** Automotive battery, a device to provide power to certain functions of an automobile ** List of battery types * Energy storage, inclu ...
or
solar power Solar power is the conversion of renewable energy Renewable energy is energy that is collected from renewable resource File:Global Vegetation.jpg, Global vegetation A renewable resource, also known as a flow resource, is a natural re ...

solar power
systems (since both can produce only DC). Most
electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow b ...
circuits require a DC
power supply #REDIRECT Power supply#REDIRECT Power supply A power supply is an electrical device that supplies electric power to an electrical load. The primary function of a power supply is to convert electric current from a source to the correct voltage, el ...

power supply
. Domestic DC installations usually have different types of sockets, connectors,
switch In , a switch is an that can disconnect or connect the conducting path in an , interrupting the or diverting it from one conductor to another. The most common type of switch is an electromechanical device consisting of one or more sets of movab ...

switch
es, and fixtures from those suitable for alternating current. This is mostly due to the lower voltages used, resulting in higher currents to produce the same amount of
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...

power
. It is usually important with a DC appliance to observe polarity, unless the device has a
diode bridge A diode bridge is an arrangement of four diode A diode is a two- that conducts primarily in one direction (asymmetric ); it has low (ideally zero) in one direction, and high (ideally infinite) in the other. A diode or thermionic diode ...

diode bridge
to correct for this. EMerge Alliance is the open industry association developing standards of DC power distribution in hybrid houses and
commercial building Commercial may refer to: * a dose of advertising Advertising is a marketing communication that employs an openly sponsored, non-personal message to promote or sell a product, service or idea.William J. Stanton. ''Fundamentals of Marketing'' ...
s.


Automotive

Most automotive applications use DC. An
automotive battery An automotive battery or car battery is a rechargeable battery that is used to start a motor vehicle Electric bicycles parked in Yangzhou's main street, Wenchang Lu. They are a very common way of transport in this city, in some areas almost ou ...
provides power for engine starting, lighting, and ignition system. The
alternator An alternator is an electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the s ...

alternator
is an AC device which uses a
rectifier A rectifier is an electrical device that converts Religious conversion is the adoption of a set of beliefs identified with one particular religious denomination A religious denomination is a subgroup within a religion Religion is a ...

rectifier
to produce DC for battery charging. Most highway passenger vehicles use nominally 12 
V
V
systems. Many heavy trucks, farm equipment, or earth moving equipment with
Diesel engine The diesel engine, named after Rudolf Diesel Rudolf Christian Karl Diesel ( (); 18 March 1858 – 29 September 1913) was a German inventor and mechanical engineer, famous for the invention of the Diesel engine The diesel engine, name ...

Diesel engine
s use 24 volt systems. In some older vehicles, 6 V was used, such as in the original
classic Volkswagen Beetle
classic Volkswagen Beetle
. At one point a 42 V electrical system was considered for automobiles, but this found little use. To save weight and wire, often the metal frame of the vehicle is connected to one pole of the battery and used as the return conductor in a circuit. Often the negative pole is the chassis "ground" connection, but positive ground may be used in some wheeled or marine vehicles.


Telecommunication

Telephone exchange A telephone exchange, telephone switch, or central office is a system used in the (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish s between subscribers. In histori ...

Telephone exchange
communication equipment uses standard −48 V DC power supply. The negative polarity is achieved by grounding the positive terminal of power supply system and the
battery Battery may refer to: Energy source * Electric battery, an electrochemical device to provide electrical power ** Automotive battery, a device to provide power to certain functions of an automobile ** List of battery types * Energy storage, inclu ...
bank. This is done to prevent
electrolysis In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...

electrolysis
depositions. Telephone installations have a battery system to ensure power is maintained for subscriber lines during power interruptions. Other devices may be powered from the telecommunications DC system using a
DC-DC converter A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of Electric power conversion, electric power converter. Power levels range from v ...

DC-DC converter
to provide any convenient voltage. Many
telephone A telephone is a telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anyt ...

telephone
s connect to a
twisted pair Twisted pair cabling is a type of wiring in which two conductors of a single Electronic circuit, circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a Single-ended signaling, single conductor o ...

twisted pair
of wires, and use a bias tee to internally separate the AC component of the voltage between the two wires (the audio signal) from the DC component of the voltage between the two wires (used to power the phone).


High-voltage power transmission

High-voltage direct current (HVDC) electric power transmission systems use DC for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses.


Other

Applications using
fuel cells A fuel cell is an electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of ...
(mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts) also produce only DC. Light aircraft electrical systems are typically 12 V or 24 V DC similar to automobiles.


See also

* CCS *
DC bias When describing a periodic function A periodic function is a function that repeats its values at regular intervals, for example, the trigonometric functions In mathematics Mathematics (from Ancient Greek, Greek: ) includes the stud ...
*
Electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, ...
*
High-voltage direct current A high-voltage, direct current (HVDC) electric power transmission Electric power transmission is the bulk movement of electrical energy Electrical energy is energy derived from electric potential energy or kinetic energy. When used loosely, ...
power transmission. * Neutral direct-current telegraph system *
Solar panel A solar cell panel, solar electric panel, photo-voltaic (PV) module or just solar panel is an assembly of photo-voltaic cells mounted in a framework for installation. Solar panels use sunlight Sunlight is a portion of the given off by ...

Solar panel


References


External links

*
AC/DC: What's the Difference?
– PBS Learning Media
DC And AC Supplies
– ITACA {{DEFAULTSORT:Direct Current Electrical engineering Electric current Electric power