The abbreviationsAC andDC are often used to mean simplyalternating anddirect, as when they modifycurrent orvoltage.[2][3]
Direct current may be converted from an alternating current supply by use of arectifier, which containselectronic elements (usually) or electromechanical elements (historically) that allow current to flow only in one direction. Direct current may be converted into alternating current via aninverter.
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 ofaluminum and otherelectrochemical processes. It is also used for somerailways, especially inurban areas.High-voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.
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 2-mile (3.2 km) long circuit.[4]
Direct current was produced in 1800 by Italian physicistAlessandro Volta's battery, hisVoltaic pile.[5] The nature of how current flowed was not yet understood. French physicistAndré-Marie Ampère conjectured that current travelled in one direction from positive to negative.[6] When French instrument makerHippolyte Pixii built the firstdynamo 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 analternating current.[7] At Ampère's suggestion, Pixii later added acommutator, 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 atpower stations. These were initially set up to powerarc lighting (a popular type of street lighting) running on very high voltage (usually higher than 3,000 volts) 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 inventorThomas Edison launched his incandescent bulb based electric "utility" in 1882. Because of the significant advantages of alternating current over direct current in usingtransformers 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 asNorNed), this DC option is the only technically feasible option. For applications requiring direct current, such asthird rail power systems, alternating current is distributed to a substation, which utilizes arectifier to convert the power to direct current.
The termDC is used to refer to power systems that use only oneelectrical polarity of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current.[9] For example, the voltage across a DCvoltage source is constant as is the current through a directcurrent source. The DC solution of anelectric circuit is the solution where all voltages and currents are constant. Anystationary 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 avoltage regulator) have almost no variations involtage, but may still have variations in outputpower and current.
A direct current circuit is anelectrical circuit that consists of any combination of constantvoltage sources, constantcurrent sources, andresistors. 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 acapacitor orinductor 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 inDC steady state. Such a circuit is represented by a system ofdifferential equations. The solution to these equations usually contain a time varying ortransient 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.
This symbol which can be represented withUnicode characterU+2393 (⎓) is found on many electronic devices that either require or produce direct current.
DC is commonly found in manyextra-low voltage applications and somelow-voltage applications, especially where these are powered bybatteries orsolar power systems (since both can produce only DC).
Domestic DC installations usually have different types ofsockets,connectors,switches, andfixtures from those suitable for alternating current. This is mostly due to the lower voltages used, resulting in higher currents to produce the same amount ofpower.
It is usually important with a DC appliance to observe polarity, unless the device has adiode bridge to correct for this.
Most automotive applications use DC. Anautomotive battery provides power for engine starting, lighting, the ignition system, the climate controls, and the infotainment system among others. Thealternator is an AC device which uses arectifier to produce DC for battery charging. Most highway passenger vehicles use nominally 12 V systems. Many heavy trucks, farm equipment, or earth moving equipment withDiesel engines use 24 volt systems. In some older vehicles, 6 V was used, such as in the originalclassic Volkswagen Beetle. At one point a42 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 abattery 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.[10] The "high voltage" system operates at 300-400V (depending on the vehicle), and provides the power for thetraction motors.[11] Increasing the voltage for the traction motors reduces the current flowing through them, increasing efficiency.
Telephone exchange communication equipment uses standard −48 V DC power supply. The negative polarity is achieved bygrounding the positive terminal of power supply system and thebattery bank. This is done to preventelectrolysis 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 aDC-DC converter to provide any convenient voltage.
Manytelephones connect to atwisted pair of wires, and use abias 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 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.
Applications usingfuel 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.
