Investigations into Socket 939 Athlon 64 Overclocking

Power Supply

Despite what manufacturers might want you to believe, power supplies are less about wattage and more about the amount and quality of current that they can supply. In theory, the Watts rating of a PSU can be determined with the current and voltage ratings. Using the equation P = I x V (Power = Current x Voltage), you can come up with a Wattage for each voltage that the PSU provides, add them all together, and you have the rating. Simple enough, right? Unfortunately, there are problems with this method of rating a power supply.

The biggest problem is that PCs don't require equal amounts of power from each voltage, and the wattage rating simply serves to obfuscate the real power levels. The +12V rating is generally the most important rating, and modern ATX2.0 PSUs actually require two +12V rails (i.e. outputs form the PSU). Two 500W PSUs from different manufacturers could actually have wildly different characteristics in the type of power that they provide. In a really bad PSU, reality can be further distorted by providing high output ratings on the -5V and -12V lines. Computers draw very little power from the negative lines, so if a PSU were to rate the -12V line at 3A instead of a more common 1A (or less), they can inflate their wattage by 25W or more. As if that isn't bad enough, there are even more ways to "cheat" the rating.

Temperature plays a role in determining the output capacity of a power supply. You can read about it elsewhere, but the main concept is the following: "The thermal capacity of materials changes slightly with temperature primarily due to changes in density." Part of what allows a power supply to provide current at a specific voltage is the ability to transform the 115V input from the wall (or 230V in other areas of the world) to a different value. Such a change creates heat, and the heat has to be dissipated. Inside a power supply, you will find heat sinks much like what you see on a motherboard, along with a cooling fan or fans. Depending on how the power supply is rated, it might actually provide 450W at 10 degrees C and only 375 W at 30 degrees C. (You'd have to know the specific heat values for the various materials inside a PSU to really be able to calculate how temperature affects the output capacity for a specific PSU.) Nearly all modern computers will have a case temperature in the 30 degrees C or higher range, so a PSU rated using 10-25 degrees C values is far from a realistic representation of the PSU's output capacity.

Lastly, just because a power supply can provide a specific output doesn't mean it can do so well. In the US, power from the wall outlets comes at 115V, but variance is allowed. In fact, the output voltage can fluctuate between 110V and 121V (5%) while still being within spec. That may be fine for some household items like lamps and coffee makers, but computers tend to be a little more demanding in their requirements. A power supply that outputs 3.2V, 4.8V, and 11.5V is still technically within the required range, and there's a good chance that it will work with a typical PC. What really causes problems are fluctuations, which are usually influenced by the use of lower quality components as well as temperature changes. Even though a PSU might work in a regular PC, though, overclocking really pushes things to the limit, and it's far better to have a PSU that can output voltages exactly at spec than a few percent high or low.

One of the easiest ways to determine the quality of a power supply is to simply pick it up. A 500W power supply should weigh quite a bit more than a 350W power supply; if it doesn't, be suspicious. Reading the label on a power supply can be helpful, but that doesn't usually tell you the temperature at which it was tested, and of course, it could always be inaccurate. The saying "you get what you pay for" also applies, so if a PSU costs far less than the rating would suggest, it's likely that the unit isn't really as good as the sticker claims. A better idea is to just go with a respected name, as we suggested with motherboards. Our top picks for PSU manufacturers are Antec, Enermax, Fotron Source, OCZ, and Seasonic. Enermax, OCZ and Seasonic are probably the safest bets, as they don't really have "value" and "performance" parts right now, though the more expensive Antec and Fotron Source units are just as good. If you want a high quality power supply and you're shopping online, here's the fastest test: does it cost less than $75? If so, it's probably a moderate unit, and under $50 is an inexpensive unit. The good power supplies almost always cost $80 or more. If you're not sure, though, ask around! Some times, there are good deals to be had on high quality power supplies.

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We're using an OCZ PowerStream 600W power supply for our system. There are bigger, better power supplies out there for extreme overclocking, but they cost a lot more. We're not going to be playing with liquid nitrogen or even phase change cooling, so the 600W OCZ is more than sufficient. With adjustable voltages and a dual 20A +12V rails, we have everything that we need from a quality power supply.

With all the above talk about getting a quality power supply, we also ran some tests using a cheap PSU that came with an even cheaper case. The case was the MGE and 400W PSU that we recommended in our last Budget Buyer's Guide. The case is flimsy, made of thin aluminum, and the cables for the front USB and Firewire ports were very difficult to work with - they were separated into single-pin connectors rather than a block of pins. It's impossible to say what the long-term reliability of such a case is, but it's been running nearly 24/7 for a couple of months now without any problems. The highest overclocks seemed a bit less stable with the 20-pin power connection, but we did manage to match the overclock of the OCZ PowerStream 600W. Maximum power draw for the test configuration was measured at around 220W, so we never came close to the 400W power rating.