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Direct current (DC) is a flow of electrical charge carriers that always takes place in the same direction. The current need not always have the same magnitude, but if it is to be defined as dc, the direction of the charge carrier flow must never reverse. This contrasts with alternating current which varies the direction of flow.
Sources of direct current include power supplies, electrochemical cells and batteries, and photovoltaic cells and panels. The intensity, or amplitude, of a direct current might fluctuate with time, and this fluctuation might be periodic. In some such cases the dc has an ac component superimposed on it. An example of this is the output of a photovoltaic cell that receives a modulated light communications signal. A source of dc is sometimes called a dc generator. 
Batteries and various other sources of dc produce a constant voltage. This is called pure dc and can be represented by a straight, horizontal line on a graph of voltage versus time. The peak and effective values are the same. The peak to peak value is zero because the instantaneous amplitude never changes. In some instances the value of a dc voltage pulsates or oscillates rapidly with time, in a manner similar to the changes in an ac wave. The unfiltered output of a half wave or a full wave rectifier, for example, is pulsating dc.
In 1820, Hans Christian Orsted discovered that electrical current creates a magnetic field. This discovery made scientists relate magnetism to the electric phenomena.
In 1879, Thomas Edison invented the electric light bulb. He improved a 50-year-old idea using lower current electricity, an improved vacuum inside the globe and a small carbonized filament, and produced a reliable and long-lasting source of light. At that time, the idea of electric lightning was not new, but nothing had been developed that was practical enough for home use. Edison not only invented an incandescent electric light, but an electric lighting system that contained all the necessary elements to make the incandescent light safe, economical, and practical. Prior to 1879, direct current (DC) electricity had been used in lighting for the outdoors.
It was in the 1880's when the modern electric utility industry began. It was an evolution from street lighting systems and from gas and electric carbon-arc commercial systems. On September 4th, 1882, Edison switched on the world's first electrical power distribution system, providing 110 volts of direct current (DC) to fifty-nine customers, and the first commercial power station began working. It was located in Lower Manhattan, on Pearl Street. This station provided light and electricity to customers in a one square mile range. The electric age had begun. The station was called "Thomas Edison's Pearl Street Electricity Generating Station." This station introduced four elements of a modern electric utility system: Efficient distribution, competitive price, reliable central generation and successful end use.
The War of the Currents-
Edison developed direct current. During the early years of electricity, direct current was the standard in the U.S.
Tesla believed that alternating current (or AC) was the solution to this problem. Alternating current reverses direction a certain number of times per second -- 60 in the U.S. -- and can be converted to different voltages relatively easily using a transformer.
The Chicago World’s Fair -- also known as the World’s Columbian Exposition -- took place in 1893, at the height of the Current War.
General Electric bid to electrify the fair using Edison’s direct current for $554,000, but lost to George Westinghouse, who said he could power the fair for only $399,000 using Tesla’s alternating current.
That same year, the Niagara Falls Power Company decided to award Westinghouse -- who had licensed Tesla’s polyphase AC induction motor patent -- the contract to generate power from Niagara Falls. Although some doubted that the falls could power all of Buffalo, New York, Tesla was convinced it could power not only Buffalo, but also the entire Eastern United States.
On Nov. 16, 1896, Buffalo was lit up by the alternating current from Niagara Falls. By this time General Electric had decided to jump on the alternating current train, too.
It would appear that alternating current had all but obliterated direct current, but in recent years direct current has seen a bit of a renaissance.
Today our electricity is still predominantly powered by alternating current, but computers, LEDs, solar cells and electric vehicles all run on DC power. And methods are now available for converting direct current to higher and lower voltages. Since direct current is more stable, companies are finding ways of using high voltage direct current (HVDC) to transport electricity long distances with less electricity loss.
So it appears the War of the Currents may not be over yet. But instead of continuing in a heated AC vs. DC battle, it looks like the two currents will end up working parallel to each other in a sort of hybrid armistice.
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. 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.
In regard to radio frequency applications, DC commonly denotes a frequency of 0 Hz. An example of this is the phrase "DC to daylight", meaning a radio system which can receive or transmit in a frequency range from 0 Hz to some higher frequency. Most radio systems do not provide DC coverage.
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.
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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.
Most automotive applications use DC. The alternator is an AC device which uses a rectifier to produce DC. Usually 12 V DC are used, but a few have a 6 V (e.g. classic VW Beetle) or a 42 V electrical system.
Through the use of a DC-DC converter, higher DC voltages such as 48 V to 72 V DC can be stepped down to 36 V, 24 V, 18 V, 12 V, or 5 V to supply different loads. In a telecommunications system operating at 48 V DC, it is generally more efficient to step voltage down to 12 V to 24 V DC with a DC-DC converter and power equipment loads directly at their native DC input voltages, versus operating a 48 V DC to 120 V AC inverter to provide power to equipment.
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).
Telephone exchange communication equipment, such as DSLAMs, 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.
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.
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 20 V DC.
- Electric current
- High-voltage direct current power transmission.
- Alternating current
- DC offset
- Neutral direct-current telegraph system
- Gibilisco, Stan (2001). Electronics Portable Handbook. New York, United States: Mc-Graw Hill. pp. Page 1. ISBN 9780071378109 – via Gibilisco, S. (2000). Electronics Portable Handbook. New York: McGraw-Hill Professional.
Direct current (dc) is a flow of electrical charge carriers that always takes place in the same direction. This is what distinguishes direct current from alternating current (ac). The current need not always have the same magnitude, but if it is to be defined as dc, the direction of the charge carrier flow must never reverse.
- Gibilisco, Stan (2000). Electronics Portable Manual. New York : McGraw-Hill Professional. Ebook: New York : McGraw-Hill Professional. pp. page 4. ISBN 9780071378109 – via ebook.
Typical sources of dc include power supplies, electrochemical cells and batteries, and photovoltaic cells and panels. The intensity, or amplitude, of a direct current might fluctuate with time, and this fluctuation might be periodic. In some such cases the dc has an ac component superimposed on it (as in Fig. 1.1B). An example of this is the output of a photovoltaic cell that receives a modulated light communications signal. A source of dc is sometimes called a dc generator Batteries and various other sources of dc produce a constant voltage. This is called pure dc and can be represented by a straight, horizontal line on a graph of voltage versus time (as in Fig. 1.1A). The peak and effective values are the same. The peak to peak value is zero because the instantaneous amplitude never changes. In some instances the value of a dc voltage pulsates or oscillates rapidly with time, in a manner similar to the changes in an ac wave. The unfiltered output of a half wave or a full wave rectifier, for example, is pulsating dc. APA (American Psychological Assoc.) Gibilisco, S. (2000). Electronics Portable Handbook. New York: McGraw-Hill Professional. MLA (Modern Language Assoc.) Gibilisco, Stan. Electronics Portable Handbook. New York: McGraw-Hill Professional, 2000. eBook Collection (EBSCOhost). Web. 21 Feb. 2017.
- "The War of the Currents". U.S. Department of Energy.
- Roger S. Amos, Geoffrey William Arnold Dummer (1999). Newnes Dictionary of Electronic (4th ed.). Newnes. p. 83. ISBN 0-7506-4331-5.