What’s a Power Rail?

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Power rails supply voltage to electronic devices on circuit boards. Different types of power rails exist, with varying voltage and current limits. Multiple power supplies may be needed for high-current loads. Balancing circuits ensure equal current distribution among rectifiers in a power system.

A power rail is a voltage supply that supplies power to electrical or electronic devices or components on a circuit board. Power rails used for early desktop computers included +/–5 volts (V) as direct current (DC), +/– 12 volts as direct current (VDC), and a common or return line. Later, computers started using +5V and +3V DC for rails. The power rail used for most analog circuits still requires a split power rail. Operational amplifiers can use a +/–12 V power rail and operate with input and output signals that are typically dual-polar in nature similar to sine or sine waves, which have alternating polarity like alternating current (AC).

The voltage rail in a power supply will have a corresponding current limit. Power supplies with an absolute maximum rating of 10 amps (A) can operate at 90% full current, which is 9 A. If your load requires 18 A, you will need to use two power supplies. Manufacturers may provide special instructions when connecting two power supplies to drive the same power rail. There are also cases where the load has been split to provide the correct load for each power supply.

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

When multiple rectifiers are connected together, a balancing circuit ensures that they all deliver approximately the same current. In the example above it is possible to use six rectifier modules capable of supplying a maximum current of 25 A each. When the battery bank is fully charged, the rectifiers should each supply 100/6 or approximately 17A each.

This current is approximately 8A below the maximum output current for each rectifier. Assuming each rectifier can draw an absolute maximum of 24A each, there will be a total of 42 more Amp to recharge the battery bank. The charging current must be within the permitted limits to avoid overheating of the battery. To achieve this, the low voltage disconnect circuit for the battery bank must ensure that the battery does not discharge beyond acceptable limits.

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