View source for Direct current

{{short description|Unidirectional flow of electric charge}}
{{More citations needed|date=June 2009}}
[[File:Types of current.svg|thumb|250px|Direct current (DC) (red line). The vertical axis shows current or voltage and the horizontal 't' axis measures time and shows the zero value.]]
'''Direct current''' ('''DC''') is one-directional [[electric current|flow]] of [[electric charge]]. An [[electrochemical cell]] is a prime example of  DC power. Direct current may flow through a [[conductor (material)|conductor]] such as a wire, but can also flow through [[semiconductor]]s, [[electrical insulation|insulators]], or even through a [[vacuum]] as in [[electron beam|electron or ion beams]]. The electric current flows in a constant direction, distinguishing it from [[alternating current]] (AC). A [[archaism|term formerly used]] for this type of current was '''galvanic current'''.<ref>{{cite book |title=Clinical Electrophysiology: Electrotherapy and Electrophysiologic Testing |author=Andrew J. Robinson, Lynn Snyder-Mackler|edition=3rd|year=2007 |publisher= [[Lippincott Williams & Wilkins]]|isbn= 978-0-7817-4484-3|page=10|url=https://books.google.com/books?id=C2-9bcIjPBsC&q=%22galvanic+current%22+%22direct+current%22&pg=PA10}}</ref>

The abbreviations ''AC'' and ''DC'' are often used to mean simply ''alternating'' and ''direct'', as when they modify ''[[Electric current|current]]'' or ''[[voltage]]''.<ref>
{{cite book
| title = Basic Electronics & Linear Circuits
| author = N. N. Bhargava and D. C. Kulshrishtha
| publisher = [[Tata McGraw-Hill Education]]
| year = 1984
| isbn = 978-0-07-451965-3
| page = 90
| url = https://books.google.com/books?id=C5bt-oRuUzwC&pg=PA90
}}</ref><ref>
{{cite book
| title = Electrical meterman's handbook
| author = National Electric Light Association
| publisher = Trow Press
| year = 1915
| page = 81
| url = https://books.google.com/books?id=ZEpWAAAAMAAJ&pg=PA81
}}</ref>

Direct current may be converted from an alternating current supply by use of a [[rectifier]], which contains [[Electronics|electronic]] 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 (electrical)|inverter]].

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]] and other [[electrochemistry|electrochemical]] processes. It is also used for some [[Railway electrification system#Direct current|railways]], especially in [[urban area]]s. [[High-voltage direct current]] is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.

{{notelist}}

{{Electromagnetism |Network}}

==History==
{{further|History of electric power transmission}}
[[File:Brush central power station dynamos New York 1881.jpg|400px|thumb|left|Brush Electric Company's central power plant with dynamos generating direct current to power arc lamps for public lighting in New York. Beginning operation in December 1880 at 133 West Twenty-Fifth Street, the high voltages it operated at allowed it to power a {{convert|2|mi|km|adj=on}} long circuit.<ref>{{cite journal|url=http://publications.ohiohistory.org/ohstemplate.cfm?action=detail&Page=0070142.html&StartPage=128&EndPage=144&volume=70&newtitle=Volume%2070%20Page%20128 |volume=70 |page=142 |title=Charles F. Brush and the First Public Electric Street Lighting System in America |author=Mel Gorman |journal=[[Ohio History]] |agency=[[Ohio Historical Society]] |publisher=[[Kent State University Press]] }}{{dead link|date=April 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>]]

Direct current was produced in 1800 by Italian physicist [[Alessandro Volta]]'s battery, his [[Voltaic pile]].<ref>{{Cite web |url=http://grants.hhp.coe.uh.edu/clayne/HistoryofMC/HistoryMC/VoltaII.htm |title=Alessandro Giuseppe Antonio Anastasio Volta – grants.hhp.coe.uh.edu |access-date=2017-05-29 |archive-url=https://web.archive.org/web/20170828022138/http://grants.hhp.coe.uh.edu/clayne/HistoryofMC/HistoryMC/VoltaII.htm |archive-date=2017-08-28 |url-status=dead }}</ref> The nature of how current flowed was not yet understood. French physicist [[André-Marie Ampère]] conjectured that current travelled in one direction from positive to negative.<ref>{{cite book|first=Jim|last=Breithaupt|title=Physics|publisher=Palgrave Macmillan|year=2010|page=175|isbn=9780230231924}}</ref> When French instrument maker [[Hippolyte Pixii]] built the first [[Dynamo|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]].<ref>{{Cite web |url=http://www.magnet.fsu.edu/education/tutorials/java/pixiimachine/index.html |title=Pixii Machine invented by Hippolyte Pixii, National High Magnetic Field Laboratory |access-date=2008-06-12 |archive-url=https://web.archive.org/web/20080907092008/http://www.magnet.fsu.edu/education/tutorials/java/pixiimachine/index.html |archive-date=2008-09-07 |url-status=dead }}</ref> At Ampère's suggestion, Pixii later added a [[Commutator (electric)|commutator]], 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]]. These were initially set up to power [[arc lamp|arc lighting]] (a popular type of street lighting) running on very high voltage (usually higher than 3000 volt) direct current or alternating current.<ref>{{cite web| url = http://www.edisontechcenter.org/ArcLamps.html| title = The First Form of Electric Light History of the Carbon Arc Lamp (1800–1980s)}}</ref> This was followed by the widespread use of low voltage direct current for indoor electric lighting in business and homes after inventor [[Thomas Edison]] launched his incandescent bulb based electric "[[Public utility|utility]]" in 1882. Because of the significant advantages of alternating current over direct current in using [[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]] transmission was developed, and is now an option instead of long-distance high voltage alternating current systems. For long distance undersea cables (e.g. between countries, such as [[NorNed]]), this DC option is the only technically feasible option. For applications requiring direct current, such as [[third rail]] power systems, alternating current is distributed to a substation, which utilizes a [[rectifier]] to convert the power to direct current.

==Various definitions==
[[File:Current rectification diagram.svg|right|250px|thumb|Types of direct current]] 

The term ''DC'' is used to refer to power systems that use only one [[electrical polarity]] of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current.<ref>{{cite book | title = Newnes Dictionary of Electronic | author = Roger S. Amos, Geoffrey William Arnold Dummer | publisher = Newnes |edition=4th| year = 1999 | isbn = 0-7506-4331-5 |page=83| url = https://books.google.com/books?id=c4qHqtC9JkgC&q=dc+zero-frequency&pg=PA83 }}</ref> For example, the voltage across a DC [[voltage source]] is constant as is the current through a direct [[current source]]. The DC solution of an [[electric circuit]] is the solution where all voltages and currents are constant. It can be shown that any [[stationary process|stationary]] 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]]) have almost no variations in [[voltage]], but may still have variations in output [[electric power|power]] and current.

==Circuits==
A direct current circuit is an [[electrical circuit]] that consists of any combination of constant [[voltage]] sources, constant [[current (electricity)|current]] sources, and [[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]] or [[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]]. The solution to these equations usually contain a time varying or [[Transient state|transient]] 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.

In a DC circuit, a power source (e.g. a battery, capacitor, etc.) has a positive and negative terminal, and likewise, the load also has a positive and negative terminal. To complete the circuit, positive charges need to flow from the power source to the load. The charges will then return to the negative terminal of the load, which will then flow back to the negative terminal of the battery, completing the circuit. If either the positive or negative terminal is disconnected, the circuit will not be complete and the charges will not flow.

In some DC circuit applications, polarity does not matter, which means you can connect positive and negative backwards and the circuit will still be complete and the load will still function normally. However, in most DC applications, polarity does matter, and connecting the circuit backwards will result in the load not working properly.

==Applications==
===Domestic and commercial buildings===
[[File:Direct current symbol.svg|right|200px|thumb|This symbol which can be represented with [[Unicode]] character {{U+|2393}} (⎓) is found on many electronic devices that either require or produce direct current.]]
DC is commonly found in many [[extra-low voltage]] applications and some [[low voltage|low-voltage]] applications, especially where these are powered by [[battery (electricity)|batteries]] or [[solar power]] systems (since both can produce only DC).

Most [[electronics|electronic]] circuits or devices require a DC [[power supply]].

Domestic DC installations usually have different types of [[Jack (connector)|sockets]], [[DC connector|connectors]], [[switch]]es, and [[Light fixture|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 [[Watt|power]].

It is usually important with a DC appliance to observe polarity, unless the device has a [[diode bridge]] to correct for this.

[[EMerge Alliance]] is the open industry association developing standards of DC power distribution in [[hybrid house]]s and  [[commercial building]]s.{{cn|date=June 2023}}

===Automotive===
Most automotive applications use DC. An [[automotive battery]] provides power for engine starting, lighting, the ignition system, the climate controls, and the infotainment system among others.  The [[alternator]] is an AC device which uses a [[rectifier]] to produce DC for battery charging.  Most highway passenger vehicles use nominally  12&nbsp;[[volt|V]] systems.  Many heavy trucks, farm equipment,  or earth moving equipment with [[Diesel engine]]s use 24 volt systems. In some older vehicles, 6&nbsp;V was used, such as in the original [[Volkswagen Beetle|classic Volkswagen Beetle]]. At one point a [[42&nbsp;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.
In a [[battery electric vehicle]], there are usually two separate DC systems. The "low voltage" DC system typically operates at 12V, and serves the same purpose as in an internal combustion engine vehicle. The "high voltage" system operates at 300-400V (depending on the vehicle), and provides the power for the [[traction motor|traction motors]].<ref>{{cite web |last1=Arcus |first1=Christopher |title=Tesla Model 3 & Chevy Bolt Battery Packs Examined |url=https://cleantechnica.com/2018/07/08/tesla-model-3-chevy-bolt-battery-packs-examined/ |website=CleanTechnica |date=8 July 2018 |access-date=6 June 2022}}</ref> Increasing the voltage for the traction motors reduces the current flowing through them, increasing efficiency.

===Telecommunication===
[[Telephone exchange]] communication equipment uses standard −48&nbsp;V DC power supply. The negative polarity is achieved by [[ground (electricity)|grounding]] the positive terminal of power supply system and the [[battery (electricity)|battery]] bank. This is done to prevent [[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]] to provide any convenient voltage.

Many [[telephone]]s connect to a [[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===
{{main|High-voltage direct current}}
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]] (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&nbsp;V or 24&nbsp;V DC similar to automobiles.

==See also==
{{Portal|Electronics|Energy}}
* [[Combined Charging System|CCS]]
* [[DC bias]]
* [[Electric current]]
* [[High-voltage direct current]] power transmission.
* [[Neutral direct-current telegraph system]]
* [[Polarity symbols]]
* [[Solar panel]]

==References==
{{reflist|30em}}

==External links==
{{Wiktionary}}
*{{Commons category-inline}}
* [https://www.pbslearningmedia.org/resource/phy03.sci.phys.mfw.acdc/ac-dc-whats-the-difference/#.WaGYTdGQyHs AC/DC: What's the Difference?] {{Webarchive|url=https://web.archive.org/web/20170826194725/https://www.pbslearningmedia.org/resource/phy03.sci.phys.mfw.acdc/ac-dc-whats-the-difference/#.WaGYTdGQyHs |date=2017-08-26 }} – PBS Learning Media
* [http://www.itacanet.org/eng/elec/edu/pt13.pdf DC And AC Supplies] {{Webarchive|url=https://web.archive.org/web/20161228002928/http://www.itacanet.org/eng/elec/edu/pt13.pdf |date=2016-12-28 }} – ITACA
{{Electric motor}}
{{DC power delivery standards}}
{{Authority control}}

{{DEFAULTSORT:Direct Current}}
[[Category:Electrical engineering]]
[[Category:Electric current]]
[[Category:Electric power]]