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What Is a Power Supply Rail?

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  • Written By: Geisha A. Legazpi
  • Edited By: Shereen Skola
  • Last Modified Date: 14 November 2016
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    Conjecture Corporation
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A power supply rail is a supply voltage that is provides power for electrical or electronic devices or components on a circuit board. Power supply rails used for early desktop computers included +/–5 volts (V) as direct current (DC), +/– 12 volts direct current (VDC), and a common or return line. Later, computers began using +5 V and +3 VDC for the rails. The power supply rail used for most analog circuits still requires a split power rail. Operational amplifiers may use a +/–12 V power supply rail, and they operate with input and output signals that are typically dual polarity in nature similar to the sine waves or sinusoidal waves, which have alternating polarity like alternating current (AC).

The voltage rail in a power supply unit will have a corresponding current limit. Power supplies with an absolute maximum rating of 10 amperes (A) may be operated at 90% full current, which is 9 A. If the load requires 18 A, two power supplies will need to be used. Manufacturers may provide special instructions when connecting two power supplies to drive the same power supply rail. There are also cases where the load has been split to provide the proper loading to each power supply.

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A power supply could be a single rail power supply (SRPS), a dual rail power supply (DRPS), or a multiple-rail power supply (MRPS), and as digital circuits evolved, the DPRS became more common as the +5 V and +3.3 V power supply rail. The negative voltage that previously appeared as –5 and –12 V supply rails are usually no longer used by the year 2000. The SRPS may require more than just a single power supply unit. In –48 VDC power supply systems that can provide 100 A, it is common to provide a battery bank that can provide 100 A to the load when the AC mains is interrupted. In this arrangement, there is a parallel connection of multiple rectifiers, each of which can provide a fraction of the required total current.

When several rectifiers are connected together, a balancing circuit ensures they are all providing roughly the same current. In the above example, it is possible to use six rectifier modules that can provide a maximum current of 25 A each. When the battery bank is fully charged, the rectifiers will each have to provide 100/6 or about 17 A each.

This current is about 8 A below the maximum output current for each rectifier. Assuming each rectifier will be allowed to draw an absolute maximum of 24 A each, there will be a total of extra 42 A to recharge the battery bank. The recharge current has to fall within the allowed limits to prevent overheating of the battery. To achieve this the low-voltage disconnection circuit for the battery bank has to ensure that the battery is not discharged beyond tolerable limits.

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