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Direct current ( DC) is one-directional electric current 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 , insulators, or even through a vacuum as in electron beam. The electric current flows in a constant direction, distinguishing it from alternating current (AC). A archaism for this type of current was galvanic current.Andrew J. Robinson, Lynn Snyder-Mackler (2025). 9780781744843, Lippincott Williams & Wilkins. . ISBN 9780781744843
The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify Electric current or voltage.
N. N. Bhargava and D. C. Kulshrishtha (1984). 9780074519653, Tata McGraw-Hill Education. . ISBN 9780074519653
Direct current may be converted from an alternating current supply by use of a rectifier, which contains Electronics 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 electrochemistry processes. It is also used for some railways, especially in . High-voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.
The late 1870s and early 1880s saw electricity starting to be generated at power stations. These were initially set up to power arc lamp (a popular type of street lighting) running on very high voltage (usually higher than 3,000 volts) direct current or alternating current. 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" in 1882. Because of the significant advantages of alternating current over direct current in using 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.
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 and current.
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 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.
Most electronics circuits or devices require a DC power supply.
Domestic DC installations usually have different types of sockets, DC connector, , and Light fixture 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.
It is usually important with a DC appliance to observe polarity, unless the device has a diode bridge to correct for this.
Other devices may be powered from the telecommunications DC system using a DC-DC converter to provide any convenient voltage.
Many 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).
Light aircraft electrical systems are typically 12 V or 24 V DC similar to automobiles.
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