Computer Power Supply

Everything you need to know about your computer's PSU

The power supply unit is the piece of hardware that converts the power provided from the outlet into usable power for the many parts inside the computer case.

It converts the alternating current from your wall outlet into a continuous form of power called direct current that the computer components require. It also regulates overheating by controlling voltage, which might change automatically or manually depending on the power supply.

The power supply is a crucial piece because, without it, the rest of the internal hardware can't function. Motherboards, cases, and power supplies all come in different sizes called form factors. All three must be compatible to work properly together.

CoolMax, CORSAIR, and Ultra are the most popular PSU makers, but most are included with a computer purchase, so you only deal with manufacturers when you replace the PSU.

A PSU isn't usually user serviceable. For your safety, never open a power supply unit. See Important Computer Repair Safety Tips for more help staying safe while working on a computer.

Power Supply Unit Description

Photo of a Corsair Enthusiast TX650 V2 ATX12V EPS12V Power Supply
Corsair Enthusiast TX650 V2 ATX12V EPS12V Power Supply. © Corsair

The power supply unit is mounted just inside the back of the case. If you follow the computer's power cable from the wall or battery backup device, you'll find that it attaches to the back of the power supply. It's the backside that's usually the only portion of the unit that most people will ever see.

There's also a fan opening at the back that sends air out the back of the computer case.

The side of the PSU facing outside the case has a male, three-pronged port that a power cable, connected to a power source, plugs into. There's also often a power switch and a power supply voltage switch.

Large bundles of colored wires extend from the opposite side of the power supply unit into the computer. Connectors at the opposite ends of the wires connect to various components inside the computer to supply them with power. Some are specifically designed to plug into the motherboard while others have connectors that fit into fans, floppy drives, hard drives, optical drives, and even some high-powered video cards.

Power supply units are rated by wattage to show how much power they can provide to the computer. Since each computer part requires a certain amount of power to function properly, it's important to have a PSU that can provide the right amount. The very handy Cooler Master Supply Calculator tool can help you determine how much you need.

ATX vs ATX12V Power Supplies

ATX and ATX12V are configuration specifications that are important to differentiate when dealing with power supplies. For most people, the noticeable differences just speak to the physical connection plug on the motherboard. Choosing one over the other depends on the type of motherboard that's being used.

The newest standard, ATX12V v2.4, has been in use since 2013. Motherboards using ATX12V 2.x use a 24-pin connector. ATX motherboards use a 20-pin connector.

One situation where the pin count comes into play is when deciding if a particular power supply works with your system. ATX12V-compliant power supplies, although they have 24 pins, can actually be used on an ATX motherboard that has a 20-pin connector. The remaining, unused four pins will just sit off of the connector. If your computer case has the room, this is a completely doable setup.

However, this doesn't work the other way around. If you have an ATX power supply that therefore has a 20-pin connector, it won't work with a newer motherboard that requires all 24 pins to be connected. The extra four pins were added with this specification to supply extra power through 12V rails, so a 20-pin PSU can't provide enough power to run this kind of motherboard.

ATX is also a term used to describe the size of a motherboard.

Something else that sets ATX12V and ATX power supplies apart is the power connectors they provide. The ATX12V standard (as of version 2.0) requires a 15-pin SATA power connector. If you need to use a SATA device but the PSU doesn't have a SATA power connector, you'll need a Molex 4-pin to SATA 15-pin adapter (such as this one).

Another difference between ATX and ATX12V is the power efficiency rating, which determines how much power is pulled from the wall compared to the output of the computer. Some older ATX PSUs have an efficiency rating below 70 percent, while the ATX12V standard requires a minimum rating of 80 percent.

Other Kinds of Power Supplies

The power supply units described above are the ones that are inside a desktop computer. The other type is an external power supply.

For example, some gaming consoles and mini PCs have a power supply attached to the power cable that must sit between the device and the wall. Here's an example of an Xbox One power supply that serves the same function as a desktop power supply, but is external and therefore completely movable and far easier to replace than a desktop PSU:

Xbox One power supply
Xbox One Power Supply.

Others are similar, like the power supply unit built-in to some ​external hard drives, which are required if the device can't draw enough power from the computer over USB.

External power supplies are beneficial because it allows the device to be smaller and more attractive. However, some of these types of power supply units are attached to the power cable and, since they're generally pretty large, sometimes make it difficult to position the device against the wall.

An uninterruptible power supply (UPS) is another type of power supply. They're like backup power supplies that provide power when the primary PSU is disconnected from its regular power source. Since power supply units are often victims of power surges and power spikes because it's where the device receives electrical power, you can plug the device into a UPS (or a surge protector).

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