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Computer Component
Computer Case Modding
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...... />Considerations in use of LED lights in computer cases
Unlike incandescent light bulbs, which can operate with either AC or DC, LEDs require a DC supply of the correct electrical polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. Most LEDs have low reverse breakdown voltage ratings and will be damaged by an applied reverse voltage of more than a few volts.
Because the voltage versus current characteristics of an LED are much like any diode, they can be destroyed by connecting them to a voltage source higher than their turn on voltage. A good LED driver circuit is either a constant current source or an approximation to a current source made by connecting the LED in series with a current limiting resistor to a voltage source. The voltage drop across a forward biased LED can change significantly based on current flow.
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Parallel operation is generally problematic. The LEDs have to be of the same type in order to have a similiar forward voltage. Even then variations in the manufacturing process can make the odds of satisfactory operation low. For more information see Nichia Application Note (http://www.nichia.co.jp/specification/appli/electrical.pdf).
Some LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity. ---Paul LoIacono is the president and owner of ATX Cases. You can see a wild array of computer case shapes, sizes and designs by visiting http://atxcases.com.
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