Power supply rail

A power supply rail or voltage rail refers to a single voltage provided by a power supply unit (PSU) relative to some understood ground. Although the term is generally used in electrical engineering, most people encounter it in the context of personal computer power supplies.

Original IBM PC standard

The original IBM PC power supply unit (PSU) supplied two main voltage rails: +5V and +12V. It supplied two other voltages, −5V and −12V, but with limited amounts of power.

Most of the standard silicon microchips of the time operated on 5V power. Of the 63.5 watts these PSUs could deliver, most of it was on this +5V rail.

The +12V supply was used primarily to operate motors. Fan motors, floppy disk drives and later, hard disk drives. As more peripherals were added, more power was delivered on the 12V rail. However, since most of the power is consumed by chips, the 5V rail still delivered most of the power.

The −12V rail was used primarily to provide the negative supply voltage to the RS-232 serial ports.

ATX standard

When Intel developed the ATX standard power supply connector (published in 1995), microchips operating on 3.3V were becoming more popular, beginning with the Intel 80486DX4 microprocessor in 1994, and the ATX standard supplies three positive rails: +3.3V, +5V, and +12V. Earlier computers which wished to operate on 3.3V typically used a simple but inefficient linear regulator to generate it from the +5V rail.

The ATX connector provides multiple wires and power connections for the 3.3V supply, because it is most sensitive to voltage drop in the supply connections.

Another ATX addition was the +5sb rail for providing a small amount of standby power, even when the computer was nominally "off".

Increase in +12V demand

As transistors become smaller on chips, it becomes preferable to operate them on lower supply voltages, and the lowest supply voltage is often desired by the densest chip, the central processing unit. In order to supply large amounts of low-voltage power to the Pentium and subsequent microprocessors, a special power supply, the voltage regulator module began to be included on motherboards.

Initially, this was supplied by the main +5V supply, but as power demands increased, the high currents required to supply sufficient power became problematic. To reduce the power losses in the 5V supply, with the introduction of the Pentium 4 microprocessor, Intel changed the processor power supply to operate on +12V, and added the separate P4 connector to supply that power.

Modern high-powered graphics processing units do the same thing, resulting in the vast majority of the power requirements of a modern personal computer being on the +12V rail.

When high-powered GPUs were first introduced, typical ATX power supplies were "5V-heavy", and could only supply 50–60% of their output in the form of 12V power. Thus, GPU manufacturers, to ensure 200–250 watts of 12V power (peak load, CPU+GPU), recommended power supplies of 5–600 W or higher.

More modern ATX power supplies can deliver almost all (typically 80–90%) of their total rated capacity in the form of +12V power. When using such a supply, a grossly overspecified power supply is no longer necessary; it is very difficult to construct a personal computer that requires more than 250 W of power. (300–350 W from the mains electricity supply, given typical power supply efficiencies.) The main reason for preferring a power supply of 400–500 W is that most power supplies operate most efficiently, and generate less cooling noise, when operating at approximately 50% of capacity.

Because of this change, it is important to consider the +12V supply capacity, rather than the overall power capacity, when using an older ATX power supply with a more recent computer.

Low-quality power supply manufacturers sometimes take advantage of this overspecification by assigning unrealistically optimistic power supply ratings, knowing that very few customers will notice the deception. [cite web |last=Rutter |first=Daniel |title=Lemon-fresh power supplies |url=http://www.dansdata.com/gz086.htm |publisher=dansdata.com |date=2008-09-27 |accessdate=2008-09-28 |quote=The lemon-market in PC power supplies has now officially become bad enough that no-name generic "500W" PSUs may actually barely even be able to deliver 250 watts. A realistic constant rating for these units is more like 200 watts. So the capacity inflation factor's hit 2.5, and it's still rising.]

+3.3V and +5V rails

As mentioned above, these supplies are rarely a limiting factor when selecting a power supply for a modern personal computer; generally any supply with a sufficient +12V rating will have adequate capacity at lower voltages.

It is worth noting that most PSUs create their 3.3V output by regulating down their 5V rail. As such, 3.3V and 5V typically have a combined limit as well. For example, a 3.3V rail may have a 10 amp rating by itself (33 watts), and the 5V rail may have a 20 A rating (100 W) by itself, but the two together may only be able to output 110 W. In this case, loading the 3.3V rail to maximum (33 W), would leave the 5V rail only be able to output 77 W.

As all of the rails come from one transformer and primary-side switching components, there is also an overall maximum power limit.

Multiple +12V Rails

As power supply capacity increased, the ATX power supply standard was amended to include:quote|3.2.4. Power Limit / Hazardous Energy Levels

Under normal or overload conditions, no output shall continuously provide more than 240 VA under any conditions of load including output short circuit, per the requirement of UL 1950/wbrCSA 950/wbrEN 60950/wbrIEC 950.|ATX12V Power Supply Design Guide, version 2.2This is a safety limit on the amount of power that may pass, in case of a fault, through any one wire. That much power can significantly overheat a wire, and more would be likely to melt the insulation and possibly start a fire.

Because implementing one current limit per wire is prohibitively expensive, and the limit is far larger than the reasonable current draw through a single wire, manufacturers typically group several wires together and apply the current limit to thewire as a group. Obviously, if the group is limited to nobreak|240 VA, so is each wire in it. Typically, a power supply will guarantee at least nobreak|17 A at nobreak|12 V by having a current limit of nobreak|18.5 A, plus or minus 8%. Thus, it is guaranteed to supply at least nobreak|17 A, and guaranteed to cut off before nobreak|20 A.

These are the so-called "multiple power supply rails". They are not fully independent; they are all connected to a single high-current 12V source inside the power supply, but have separate current limit circuitry. The current limit groups are documented so the user can avoid placing too many high-current loads in the same group.

This works in the same way, and for the same reason, as the many small circuit breakers in a circuit breaker panel as well as the main supply breaker. And just like typical domestic wiring, multiple outlets are connected to each circuit breaker for reasons of cost.

Originally, a power supply featuring "multiple +12V rails" implied one able to deliver more than 20 A of +12V power, and was seen as a good thing. However, people found the need to balance loads across many +12V rails inconvenient. This problem was exacerbated by the fact that the assignment of connectors to rails is done at manufacturing time, and it is not always possible to move a given load to a different rail.

Rather than add more current limit circuits, many manufacturers are ignoring the requirement and providing "single-rail" power supplies that omit the current limit circuitry. Although capable of starting a fire under the appropriate circumstances, there have not been a noticeable increase in accidental fires, and as of 2008, product safety testers like Underwriters Laboratories continue to approve the supplies.

For a time, power supplies were marked and sold as having multiple +12V rails, although no current limit circuitry was included. As of 2008, having only an overall +12V current limit is seen as a desirable feature, and "single-rail" power supplies are advertised and sold as such, although it is still common to find power supplies for sale that falsely claim to have multiple +12V rails.

Operation of overcurrent protection

When a power supply has multiple-rail overcurrent protection, if any rail reaches that limit, the entire power supply will shut down. This is not associated with any overheating or increase in ripple voltage by the power supply as a whole, as might be caused by an overall overload. The only reliability penalty from operating a rail close to its current limit comes from the risk of triggering the shutdown.

References

* [http://www.playtool.com/pages/psurailhistory/rails.html A short history of PC power supply voltage rails]
* [http://compreviews.about.com/od/cases/a/PSUWattage.htm PC Power Supply Wattage]
* [http://www.dansdata.com/psus.htm DansData - Inside your PSU]
* [http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf ATX12V Power Supply Design Guide v2.2]