Direct current

Last updated
Direct current (DC) (red line). The vertical axis shows current or voltage and the horizontal 't' axis measures time and shows the zero value. Types of current.svg
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 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]

Contents

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 converted 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 via an 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 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

    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. Brush central power station dynamos New York 1881.jpg
    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.

    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 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

    Types of direct current Current rectification diagram.svg
    Types of direct current

    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. [9] For example, the voltage across a DC voltage source is constant as is the current through a DC 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 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 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

    This symbol which can be represented with Unicode character U+2393 ([?]) is found on many electronic devices that either require or produce direct current. Direct current symbol.svg
    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 applications, especially where these are powered by batteries or solar power systems (since both can produce only DC).

    Most electronic circuits 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 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, 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  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. 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

    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

    Related Research Articles

    <span class="mw-page-title-main">Voltmeter</span> Instrument used for measuring voltage

    A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit.

    <span class="mw-page-title-main">Three-phase electric power</span> Common electrical power generation, transmission and distribution method for alternating currents

    Three-phase electric power is a common type of alternating current used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires and is the most common method used by electrical grids worldwide to transfer power.

    <span class="mw-page-title-main">Alternating current</span> Electric current that periodically reverses direction

    Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.

    <span class="mw-page-title-main">High-voltage direct current</span> Electric power transmission system

    A high-voltage, direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) systems.

    <span class="mw-page-title-main">Rectifier</span> Electrical device that converts AC to DC

    A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation is performed by the inverter.

    <span class="mw-page-title-main">Electric generator</span> Device that converts other energy to electrical energy

    In electricity generation, a generator is a device that converts motive power into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines, wind turbines and even hand cranks. The first electromagnetic generator, the Faraday disk, was invented in 1831 by British scientist Michael Faraday. Generators provide nearly all of the power for electric power grids.

    <span class="mw-page-title-main">Diode bridge</span> Circuit arrangement of four diodes

    A diode bridge is a bridge rectifier circuit of four diodes that is used in the process of converting alternating current (AC) from the input terminals to the direct current on the output terminals. Its function is to convert the negative-going AC pulses into positive going pulses, after which a low-pass filter can be used to smooth the result into DC.

    <span class="mw-page-title-main">Electric power distribution</span> Final stage of electricity delivery to individual consumers in a power grid

    Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment and household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.

    <span class="mw-page-title-main">Alternator</span> Device converting mechanical into electrical energy

    An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines.

    <span class="mw-page-title-main">Power inverter</span> Device that changes direct current (DC) to alternating current (AC)

    A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.

    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 converter. Power levels range from very low to very high.

    <span class="mw-page-title-main">Power electronics</span> Technology of power electronics

    Power electronics is the application of electronics to the control and conversion of electric power.

    <span class="mw-page-title-main">Traction motor</span> Electric motor for vehicle propulsion

    A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric or hydrogen vehicles, elevators or electric multiple unit.

    <span class="mw-page-title-main">Motor–generator</span> Device for converting electrical power to another form

    A motor–generator is a device for converting electrical power to another form. Motor–generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line. Large motor–generators were widely used to convert industrial amounts of power while smaller motor–generators were used to convert battery power to higher DC voltages.

    <span class="mw-page-title-main">Mercury-arc valve</span>

    A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result, mercury-arc valves were much more rugged and long-lasting, and could carry much higher currents than most other types of gas discharge tube.

    <span class="mw-page-title-main">Vibrator (electronic)</span>

    A vibrator is an electromechanical device that takes a DC electrical supply and converts it into pulses that can be fed into a transformer. It is similar in purpose to the solid-state power inverter.

    <span class="mw-page-title-main">Dynamo</span> Electrical generator that produces direct current with the use of a commutator

    A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter.

    <span class="mw-page-title-main">Electric power system</span> Network of electrical component deployed to generate, transmit & distribute electricity

    An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The electrical grid can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centers to the load centers, and the distribution system that feeds the power to nearby homes and industries.

    <span class="mw-page-title-main">Alternator (automotive)</span> Devices in automobiles to charge the battery and power the electrical system

    An alternator is a type of electric generator used in modern automobiles to charge the battery and to power the electrical system when its engine is running.

    This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.

    References

    1. Andrew J. Robinson, Lynn Snyder-Mackler (2007). Clinical Electrophysiology: Electrotherapy and Electrophysiologic Testing (3rd ed.). Lippincott Williams & Wilkins. p. 10. ISBN   978-0-7817-4484-3.
    2. N. N. Bhargava and D. C. Kulshrishtha (1984). Basic Electronics & Linear Circuits. Tata McGraw-Hill Education. p. 90. ISBN   978-0-07-451965-3.
    3. National Electric Light Association (1915). Electrical meterman's handbook. Trow Press. p. 81.
    4. Mel Gorman. "Charles F. Brush and the First Public Electric Street Lighting System in America". Ohio History . Kent State University Press. Ohio Historical Society. 70: 142.[ permanent dead link ]
    5. "Alessandro Giuseppe Antonio Anastasio Volta – grants.hhp.coe.uh.edu". Archived from the original on 2017-08-28. Retrieved 2017-05-29.
    6. Jim Breithaupt, Physics, Palgrave Macmillan – 2010, p. 175
    7. "Pixii Machine invented by Hippolyte Pixii, National High Magnetic Field Laboratory". Archived from the original on 2008-09-07. Retrieved 2008-06-12.
    8. "The First Form of Electric Light History of the Carbon Arc Lamp (1800–1980s)".
    9. Roger S. Amos, Geoffrey William Arnold Dummer (1999). Newnes Dictionary of Electronic (4th ed.). Newnes. p. 83. ISBN   0-7506-4331-5.
    10. Arcus, Christopher (8 July 2018). "Tesla Model 3 & Chevy Bolt Battery Packs Examined". CleanTechnica. Retrieved 6 June 2022.