This blog post is article six in an educational series designed to give our readers an opportunity to learn more about how computers function without getting burdened with highly technical details. In our last installment, we discussed motherboards. Having now taken two months away from this series to write about exciting developments at GeekABC Computer Repair, we will now turn our attention to another essential computer part, the power supply unit, or PSU.

At this point in the series, you may have begun to wonder how all the various components in the modern computer receive power. This is the responsibility of the PSU, which connects to each active component of the machine to distribute electricity in watt capacities at appropriate voltages throughout the system. At its most basic definition, the power supply converts standard AC wall power into DC electricity for the computer to use.

The modern consumer power supply complies with Intel’s ATX standard, which brought peace to the power supply scene, which was chaotic and frankly out of control as companies mass produced lower quality proprietary PSUs until the introduction of ATX, which was adopted around the turn of the millennium. ATX not only stipulated a physical form factor for PSUs but also standardized connection interfaces that we now take for granted. On a typical ATX PSU, you will find a 24-pin motherboard connector, an 8-pin EPS connector (borrowed from the server world!), 6/8 pin PCI-E power connectors, and some combination of Molex and SATA power connections for drives and peripherals.

You may note from this list that all components of the computer do not seem to have a direct connection to the power supply. There is a very intuitive reason for why this should be the case. PSUs typically split power into 12v, 5v, and 3v, and if you recall our discussion of CPUs, even 3 volts would be enough to fry the chip in its socket! Because most computer parts require tailored, low voltage high current power, motherboards will use something called a Voltage Regulator Module, or VRM, to adjust power to the components they are associated with (this is the reason that more expensive motherboards, typically featuring higher quality VRMs, are capable of achieving higher overclocks on CPUs). Still, having standardized connectors helps with universal compatibility not only within product tiers but even between component categories entirely. For example, not only will every ATX motherboard have a 20-24 pin connector, but standards like Molex can power a wide variety of devices from case fans to optical drives!

In terms of numerical evaluations, there are a few things to keep in mind when purchasing a power supply. First is the wattage rating. It is critical to ensure the power supply is capable of outputting enough total power (which is measured in watts) for the system. In custom PCs with dedicated graphics cards, 550-750W PSUs are often a good fit, but in low end, SI computers, smaller PSUs may be used. In the case of PC enthusiasts sporting multiple GPUs and a large number of fans, there are power supplies even in the range of 1500W! While anything in that upper end territory is absurd for most of our readers, it is worth noting that efficiency curves do play a part in some power supply purchasing decisions. For example, a 1000W PSU will convert electricity at 500W more efficiently than a 750W PSU will, so it will require less cooling (and generate less noise) as a result. Information on efficiency is encoded in 80 Plus ratings, which indicate higher efficiency as the badge increases in “prestige” (an 80 Plus Bronze rating is good, but an 80 Plus Platinum rating is fantastic). Another metric to understand with PSUs, besides there wattage and connection support, is their modularity. Some cheaper power supplies have all connections wired internally, whereas others are modular (their connector cables can be plugged in or removed at will). Modular power supplies help reduce the amount of excess cables in a system, but they are generally more expensive than their non-modular counterparts.

Finally, for those aspiring custom PC builders out there, it is important to note that the PSU is NOT a place to cheap out. Get a power supply from a reputable brand with good reviews, not some cheap overseas PSU you found online for $10.

To summarize, the PSU converts wall power into rails of electricity that a computer can use. The ATX standard is by far the most common form factor for consumers, and it is important to check that the PSU supports the number of connectors you need for your computer, supports the system’s combined power draw, and is manufactured by a reputable brand. Failures in a power supply are so severe that they will prevent a system from booting entirely or, even worse, cause damage to other components of the computer.

If you are interested in purchasing a power supply for a custom build, or if you suspect that your current power supply is not working properly because your computer will not boot up, feel free to bring your computer to one of GeekABC’s drop off locations, give us a call, or email us at! Our technicians are more than happy to address whatever issues your computer might be facing. Thanks for reading, and be sure to check out our other blog posts! We hope to see you back for next month’s installment in “How Does My Computer Work” and Saturday’s from 6-7PM with our writer, Andy, the host of GeekABC News on Facebook Live!

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