“Direct current”的意思、由来-开放百科全书

The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.^[2]^[3]

Direct current may be obtained from an alternating current supply by use of a rectifier, which contains electronic elements (usually) or electromechanical elements (historically) that allow current to flow only in one direction. Direct current may be converted into alternating current with an inverter or a motor-generator set.

Direct current is used to charge batteries and as a power supply for electronic systems. Very large quantities of direct-current power are used in production of aluminum and other electrochemical processes. It is also used for some railways, especially in urban areas. High-voltage direct current 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's battery, his Voltaic pile.^[5] The nature of how current flowed was not understood. French physicist André-Marie Ampère conjectured that current travelled in one direction from positive to negative.^[6] When French instrument maker Hippolyte Pixii built the first 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.^[7] At Ampère's suggestion, Pixii later added a 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 lighting (a popular type of street lighting) running on very high voltage (usually higher than 3000 volt) direct current or alternating current.^[8] 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 launched his incandescent bulb based electric "utility" in 1882. Because of the significant advantages of alternating current over direct current in using transformers 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 underseas 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

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 power and current.

Circuits

A direct current circuit is an electrical circuit that consists of any combination of constant voltage sources, constant current sources, and resistors. 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 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 applications and some low-voltage applications, especially where these are powered by batteries or solar power systems (since both can produce only DC).

Domestic DC installations usually have different types of sockets, connectors, switches, 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.

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 houses and commercial buildings.

Automotive

Most automotive applications use DC. An automotive battery provides power for engine starting, lighting, and ignition system. The alternator is an AC device which uses a rectifier to produce DC for battery charging. Most highway passenger vehicles use nominally 12 V systems. Many heavy trucks, farm equipment, or earth moving equipment with Diesel engines use 24 volt systems. In some older vehicles, 6 V was used, such as in the original 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 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 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 telephones 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

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 V or 24 V DC similar to automobiles.

See also

References

1. ^{{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&pg=PA10&dq=%22galvanic+current%22+%22direct+current%22#v=onepage&q=%22galvanic%20current%22%20%22direct%20current%22&f=false}}
2. ^{{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}}
3. ^{{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}}
4. ^{{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 }}
5. ^Alessandro Giuseppe Antonio Anastasio Volta – grants.hhp.coe.uh.edu
6. ^Jim Breithaupt, Physics, Palgrave Macmillan – 2010, page 175
7. ^{{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 |dead-url=yes |df= }}
8. ^The First Form of Electric Light History of the Carbon Arc Lamp (1800 – 1980s)
9. ^{{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&pg=PA83&dq=dc+zero-frequency#PPA83,M1 }}