Hardware

Power Supply Unit – What is a power supply unit?

What is a power supply Unit?

A computer power supply unit changes AC current to low voltage DC current to operate the processor and peripheral devices. Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances.

Power supply rail or Voltage rail – It refers to a single voltage provided by a power supply unit (PSU).

+3.3 V rails  –  supplies +3.3 V of current only

+5 V rails     –  supplies +5 V of current only

+12 V rails   –  supplies +12 V of current only

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OCP – Over Current Protection

An international safety regulation called IEC 60950-1 states that no single conductor can carry more than 240 W in computer equipments. Since computer power supplies deliver continuous current, this means that no output wire from the power supply can carry more than 240 W. This way the ATX12V specification includes a requirement for an over current protection circuit in order to shut down any rail that pulls more than 240 W.

There can be only one OCP for one rail.

Types of power supply unit based on number of rails

  1. Single rail PSU = Number of rail is 1
  2. Multi rail PSU  = Number of rails are more than 1

Which one is better out of these two?

Answer to this question is that both are good for different purposes

Single rail has a single output wire for all the components, therefore, a single OCP. So, overloading may interrupt all system components. But single rail works good for low wattage units.

While a multi rail has multiple output wires with different OCPs. So overloading in one circuit will not affect the other circuit, therefore multi rail is considered to be better for high wattage units.

With a modern power supply you are unlikely to overload any rail without using a large number of adapters and splitters to get more connectors, the manufacturer already spread the connectors across the rails so that it would be very unlikely for you to overload any single rail without trying hard.
In short, its the total capacity of the 12V source that matters, how the rails are laid out does not affect you much.

Where to find PSU inside your CPU?

The main power supply to the computer is directly attached to the Power Supply Unit. So you can easily locate the PSU in your computer with the help of main power supply wire.

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