Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current.[1]
The abbreviations AC and DC are often used đồ sộ mean simply alternating and direct, as when they modify current or voltage.[2][3]
Direct current may be converted from an alternating current supply by use of a rectifier, which contains electronic elements (usually) or electromechanical elements (historically) that allow current đồ sộ flow only in one direction. Direct current may be converted into alternating current via an inverter.
Direct current has many uses, from the charging of batteries đồ sộ 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 electrochemical processes. It is also used for some railways, especially in urban areas. High-voltage direct current is used đồ sộ transmit large amounts of power from remote generation sites or đồ sộ interconnect alternating current power grids.
History[edit]
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 yet understood. French physicist André-Marie Ampère conjectured that current travelled in one direction from positive đồ sộ 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 đồ sộ 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 đồ sộ produce direct current.
The late 1870s and early 1880s saw electricity starting đồ sộ be generated at power stations. These were initially phối up đồ sộ power arc lighting (a popular type of street lighting) running on very high voltage (usually higher than vãn 3000 volt) direct current or alternating current.[8] 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 "utility" in 1882. Because of the significant advantages of alternating current over direct current in using transformers đồ sộ raise and lower voltages đồ sộ 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 đồ sộ a substation, which utilizes a rectifier đồ sộ convert the power đồ sộ direct current.
Various definitions[edit]
The term DC is used đồ sộ refer đồ sộ power systems that use only one electrical polarity of voltage or current, and đồ sộ refer đồ sộ the constant, zero-frequency, or slowly varying local mean value of a voltage or current.[9] 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 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 đồ sộ 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 đồ sộ "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[edit]
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 vì thế not involve integrals or derivatives with respect đồ sộ time.
If a capacitor or inductor is added đồ sộ 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 đồ sộ 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 vì thế not have a DC solution. Two simple examples are a constant current source connected đồ sộ a capacitor and a constant voltage source connected đồ sộ an inductor.
In electronics, it is common đồ sộ refer đồ sộ 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 đồ sộ flow from the power source đồ sộ the load. The charges will then return đồ sộ the negative terminal of the load, which will then flow back đồ sộ 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[edit]
Domestic and commercial buildings[edit]
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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).
Most electronic circuits or devices require a DC power supply.
Domestic DC installations usually have different types of sockets, connectors, switches, and fixtures from those suitable for alternating current. This is mostly due đồ sộ the lower voltages used, resulting in higher currents đồ sộ produce the same amount of power.
It is usually important with a DC appliance đồ sộ observe polarity, unless the device has a diode bridge đồ sộ correct for this.
EMerge Alliance is the open industry association developing standards of DC power distribution in hybrid houses and commercial buildings.[citation needed]
Automotive[edit]
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 đồ sộ 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 đồ sộ 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 motors.[10] Increasing the voltage for the traction motors reduces the current flowing through them, increasing efficiency.
Telecommunication[edit]
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 ngân hàng. This is done đồ sộ prevent electrolysis depositions. Telephone installations have a battery system đồ sộ 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 đồ sộ provide any convenient voltage.
Many telephones connect đồ sộ a twisted pair of wires, and use a bias tee đồ sộ 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 đồ sộ power the phone).
High-voltage power transmission[edit]
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.
Low voltage power transmission[edit]
Low voltage direct current (LVDC) transmission was developed by Thomas Edison back in the 1900's for Westinghouse Company. Back then, the only thing the public wanted was light using incandescent lights, which don't care about AC or DC. Thomas Edison's idea of passing low voltage over long distances ultimately failed because of resistance on the power lines. Edison tried đồ sộ combat this by making the power lines thicker đồ sộ reduce resistance, but there is a limitation of how thick he could make the wires before they either became too expensive or they broke under their own weight. Since resistance was such a big problem, Edison's idea of LVDC power transmission could only transmit power from a mile or two from the power source, and ví Edison's LVDC idea ultimately failed.
Other[edit]
Applications using fuel cells (mixing hydrogen and oxygen together with a catalyst đồ sộ produce electricity and water as byproducts) also produce only DC.
Light aircraft electrical systems are typically 12 V or 24 V DC similar đồ sộ automobiles.
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See also[edit]
- CCS
- DC bias
- Electric current
- High-voltage direct current power transmission.
- Neutral direct-current telegraph system
- Polarity symbols
- Solar panel
References[edit]
External links[edit]
Media related đồ sộ Direct current at Wikimedia Commons- AC/DC: What's the Difference? Archived 2017-08-26 at the Wayback Machine – PBS Learning Media
- DC And AC Supplies Archived 2016-12-28 at the Wayback Machine – ITACA
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