Direct current
Various definitions
Within electrical engineering, 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 sometimes refers to “constant polarity.” With this definition, DC voltages can vary in time, such as 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.
Applications
Direct-current installations usually have different types of sockets, switches, and fixtures, mostly due to the low voltages used, from those suitable for alternating current. It is usually important with a direct-current appliance not to reverse polarity unless the device has a diode bridge to correct for this (most battery-powered devices do not).
This symbol is found on many electronic devices that either require or produce direct current.
DC is commonly found in many low-voltage applications, especially where these are powered by batteries, which can produce only DC, or solar power systems, since solar cells can produce only DC. Most automotive applications use DC, although the alternator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Applications using fuel cells (mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts) also produce only DC.
Many telephones connect to a twisted pair of wires, and 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 DSLAM, uses standard -48V 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.
An electrified third rail can be used to power both underground (subway) and overground trains.
See also
Electronics portal
Alternating current
DC offset
Neutral direct-current telegraph system
Notes
^ Roger S. Amos, Geoffrey William Arnold Dummer (1999). Newnes Dictionary of Electronic. Newnes. ISBN 0750643315. http://books.google.com/books?id=c4qHqtC9JkgC&pg=PA83&dq=dc+zero-frequency&lr=&as_brr=3&ei=wYWfSKqSE5CKtAPH_q2eBQ&sig=ACfU3U3FOU9Uvk4wRAj2tBTa-zlD86YyHg#PPA83,M1.
External links
“AC/DC: What’s the Difference?”.
Categories: Electrical engineering | Electricity | Electric powerHidden categories: Articles needing additional references from June 2009 | All articles needing additional references
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