PSU Terminology - Please Explain!

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A rail is a set of outputs from the power supply which have a common terminal voltage and are tied together to a single over-current protection (OCP) mechanism. Exceeding the maximum power allowed by the OCP on any combination of the outputs tied to that OCP will cause the PSU to shut off to prevent damage.

PSUs have 3 main supply voltage levels at 12 volts, 5 volts, and 3.3 volts. There's also a 5 volt standby rail, and a -12 volt rail but these are only used for specific purposes. There's also a common return rail, also called ground, or reference...

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Distinguished
PSU ratings:

Volts is the electrical rating that drives a component/device (it's the electrical pressure). Most components/devices have specific voltages necessary for them to operate correctly, thus a PSU will have various voltage levls (ie. 12V, 5V, 3.3V, etc).

When a component/device is connected to a voltage source it will draw a certain amount of current from that source (current is rated in amps).

Watts is the power rating for a component/device. Power is found by multiplying the voltage times the amps to get the watts.

There are several schools of thought concerning single and multiple 12V rails as to which is better. I prefer a single 12V rail, but either single or multiple can be used.

Regardless of which you choose, make sure your PSU is from a quality manufacturer, such as Seasonic, XFX, Corsair, or Antec just to name a few.
 


A rail is a set of outputs from the power supply which have a common terminal voltage and are tied together to a single over-current protection (OCP) mechanism. Exceeding the maximum power allowed by the OCP on any combination of the outputs tied to that OCP will cause the PSU to shut off to prevent damage.

PSUs have 3 main supply voltage levels at 12 volts, 5 volts, and 3.3 volts. There's also a 5 volt standby rail, and a -12 volt rail but these are only used for specific purposes. There's also a common return rail, also called ground, or reference.

Prior to the year 2,000 most PC components drew their power from the 5 volt rail. The 12 volt rail was used largely for driving fans and hard disk drive motors.

With the introduction of the Pentium 4, the industry switched to 12 volt main supply.

Originally, the ATX12v specification allowed a maximum of 240 voltamps of power on any one output wire. When multiple wires were tied together, this shifted the 240 voltamp limit to the common overcurrent protection mechanism. This is why most multi-rail PSUs have 12 volt rails with current limits of around 18 amperes.

With the introduction of increasingly power hungry CPUs and GPUs, it became obvious that a single 12 volt rail limited to 240 voltamps would not be sufficient. PSU manufacturers introduced "multi-rail" PSUs which isolated the 12 volt outputs into multiple groups, each with its own compliant OCP. Most multi-rail PSUs will have one 12 volt rail for the 24 pin EPS/ATX connector and auxiliary connectors, and one 12 volt rails for each of the 6+8 pin PCIe connector groupings (6 pin is rated to 75 watts, and 8 pin is rated to 150 watts for a total of 225 watts which is below the 240 VA allowed).

ATX12v version 2.3 removes the 240 VA limit (but it remains recommended) which allowed PSU manufacturers to use a single high current OCP or remove it all together. Thus, many high quality PSUs manufactured since 2007 have been of the single-rail design where all 12 volt wires are tied to a common supply. Prior to ATX12v 2.3, all PSUs capable of delivering more than 240 VA on the 12 volt rail had to be multi-rail to be considered compliant.

The only difference between two PSUs of equivalent quality with one being single rail and the other being multi-rail is that the multi-rail ones require a little bit of planning with respect to wire layout. This is only really important with GPUs that have dual 8 pin connectors, as these must come from separate rails on multi-rail systems with 18-20 ampere current limits.

Voltage and Wattage measure different things. Voltage is a measure of the difference in electrical potential between two points. Voltage, or electrical potential, is the amount of energy that a charge carrier gains or loses as it passes from one point to another. It's quite analogous to height. One coulomb of charge passing from a 12 volt point to a reference point (0 volts) would dissipate 12 joules of energy.

Amperage and Wattage are measures of current and power respectively. Current is the time rate of change of charge, and power is the time rate of change of energy. This makes them both the derivatives of charge (measured in coulombs) and energy (measured in joules) respectively.

Using the same expression above, a current of one ampere passing from a potential of 12 volts to 0 volts will dissipate 12 watts of power. If all values are sustained for a period of 1 second, one coulomb of charge will transfer between the terminals and 12 joules of energy will be dissipated.
 
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