What is a Motherboard Power Supply?

R. Kayne

A motherboard power supply is the replaceable unit inside a computer that delivers power to the system. Also known as a power supply unit (PSU), it provides cabling that runs from the unit to the case’s power button, to the motherboard, drives, graphics card and fan(s). The PSU also incorporates a fan to cool the unit, which can exhaust out the rear, bottom or top, depending on the model.

A computer motherboard.
A computer motherboard.

As motherboards have evolved over the years, so has the PSU. In 1995 the Advanced Technology eXtended (ATX) form factor debuted, replacing previous AT motherboard and power supply standards. ATX remains the most common form factor, encompassing various versions representing evolving standards. While it was simple at one point many years back to buy an ATX power supply for an ATX motherboard, things have become a little more complicated.

A power supply provides energy to the motherboard and other computer components.
A power supply provides energy to the motherboard and other computer components.

Aside from the changing connectors used for the more recent Serial ATA (SATA) drives versus legacy parallel ATA (PATA) drives, and the ability to run two graphics card for more gaming bang for your buck, the way motherboards utilize power has also changed. Newer motherboards use a 24-pin main power cable while older motherboards use a 20-pin cable. Some PSUs come with a 20+4 connector that splits apart to accommodate either type of motherboard. An adapter might also be purchased to convert one type of connector to another.

In the past, the main power cable was used to power the computer processing unit (CPU), but today virtually all current motherboards use a dedicated 12-volt CPU power cable. Here again, there are two standards or versions: the 4-pin (P4) and 8-pin (ESP12V) connector. Just like the main power cable, some motherboard power supply models come with a 4+4 12V CPU cable to accommodate either type of motherboard, or an adapter may be used.

If working with an older AMD® dual CPU motherboard, a 6-pin auxiliary connector might be required. Most PSUs do not supply this cable, so if the motherboard needs it, a compatible PSU can be purchased.

Older computers derive power from the 3.3 and 5-volt rails provided by the motherboard power supply unit, but with the introduction of AMD®’s Athlon 64 and Intel®’s Pentium 4, a new strategy emerged utilizing the 12-volt rail. As a result, PSUs following the ATX12V 2.0 standard (or newer) direct most of their wattage here. If buying a PSU for an older motherboard that relies on the 3.3/5-volt rail, consider a motherboard supply unit made to the ATX12V 1.3 (or earlier) standard, which delivers the bulk of its power to the 3.3/5-volt rail. Alternately, some newer PSUs deliver enough wattage on both the 12-volt and 3.3/5-volt rails to be compatible with any motherboard.

The number of connectors or cables on the PSU is an important consideration. Some less expensive models have fewer connectors, which might be fine for some systems, but not for others. Also, modular PSUs are generally a little more expensive but allow the user to plug in only those cables required, avoiding the clutter of extra cables inside the case. Some enthusiasts, however, shy away from modular designs believing plug-in connections are a potential source for unstable power delivery, unlike hard-wired models.

Another consideration when searching for a motherboard power supply is wattage. There are several online calculators available for getting a general idea of what range to consider. The average system with a single graphics card is generally well-served by a PSU in the 550 to 650-watt range, but its mileage may vary. Also be sure to check motherboard specifications or the manufacturer’s website for power recommendations, as a particular number of amps might be required as a minimum on a particular rail.

Not all motherboard power supply units are created equal. Some deliver cleaner power while others might be made with second-rate components. When checking specs, compare supported or tested hardware, efficiency ratings, certifications and warranties as a few indicators of quality. Customer reviews can also be helpful.

A motherboard's power supply runs from the unit to the computer's power button.
A motherboard's power supply runs from the unit to the computer's power button.

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