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The Dummys Guide to Power Supplies
Written on August 27, 2007 by Daniel HermansonAlways look at the date when you read a hardware article. Some of the content in this article is most likely out of date, as it was written on August 27, 2007. For newer information, see our more recent articles.
Introduction
What does a power supply actually do?
The job of a power supply unit (PSU) is fairly simple: Take the 110 volt AC current from the wall and turn it into integrated circuit friendly DC, and at a fraction of the original voltage. A power supply, however, is much more than just a single transformer. Modern PSUs have at least five separate voltage outputs, and usually more. Each different power output is called a "rail": the -12 volt rail, the -5 volt rail, the 3.3 volt rail, the 5 volt rail, and one or more 12 volt rails. Each rail is an independent power source in its own right, monitoring and adjusting the voltage on its outputs.
The most important rail is, by far, the 12 volt rail. It has the lion's share of the wattage, usually accounting for 75% or more of the power output of the PSU. It's used by everything, from the processor (which has a dedicated 12 volt input on the motherboard) to video cards (many of which also have dedicated 12 volt inputs) to the case fans.
Most PSUs have multiple 12 volt rails, to share the work load that would otherwise have to be handled by a single rail. The entry level Seasonic 380 watt has two, while on the other end of the spectrum is the giant Enermax Galaxy 1000 watt, which has no less than four.
Picking the right PSU
So, how much power is enough? How much is too much? Is there even such a thing as too much? To answer these questions, you need to consider the power consumption of the two most power hungry components in your computer - the video card(s) and processor(s). Hard drives draw a fairly consistent 10 watts, CD drives about double that. Compared to the variations seen in processor (CPU) power usage (35 to over 150 watts) and video cards (15 to 215 watts), drives and fans are consistent enough that they only really make a difference in extreme cases.
While it is entirely possible to go through a system part-by-part and calculate the exact expected power usage, that process is long and boring. Instead, here are the most powerful systems I would trust with each PSU:
While it is entirely possible to go through a system part-by-part and calculate the exact expected power usage, that process is long and boring. Instead, here are the most powerful systems I would trust with each PSU:
380 watt |
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550 watt |
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650 watt |
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700 watt |
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1000 watt |
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Some other considerations
The next consideration is far more noticable in day-to-day operation. Noise. Some power supplies are designed to be dead silent, some are rather loud and others sound more like little jet engines. While we have tried to keep our options as quiet as possible, some of our offerings, such as the Enermax Galaxy, are not very quiet. It's not loud, per se, but it makes a noticable amount of noise compared to the very quiet Seasonics.
The last consideration is purely cosmetic. Some PSUs have modular cables: If you don't need four cables with SATA power plugs, they can be unplugged and left out of the computer. This makes my job of hiding all the extra cables during a build quite easy (we still send the cables to the customer just in case they want to have future upgrades). Especially in systems with windows and lights, a modular PSU can make the interior of a PC look much cleaner. However, there are who insist that modular PSUs are a bad idea, because the connectors offer slightly more resistance, and provide another point of failure.
Closing thoughts
Here at Puget Custom Computers, we try to make buying a very quiet computer as easy as possible, so we try to offer only the quietest and best quality of PSUs. Seasonic is a very high quality manufacturer who makes extremely quiet units, but there are more powerful PSUs available. PC Power and Cooling makes utterly amazing power supplies... as long as you don't mind your computer sounding like a 747 trying to take off. If you really want one, we'll happily order one in for you. As with every component of our systems, if you have any questions about your PSU selection, feel free to give us a call. We have people on staff dedicated to talking with customers to craft the most ideal computer systems for their needs.