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Resolving power supply issue

Question : After changing my power supply for ATX, my PC always shuts off by itself. This problem happens more than five times every day. I tried to change the "PC health status in the BIOS" by disabling Auto Shut-down if the central processing unit's temperature went above 70 degrees Celsius, but that did not help at all. What should I do?

Answer : It looks like a power supply issue. This seems to happen with many low-cost generic power supply units. When people talk about power supplies, they talk about peak load (that is, how many watts/W it can put out in total).

Many power supplies are compared based on total wattage alone. For in- stance, some people may say that a 300W power supply is inadequate, and a 350W power supply should be used instead. This over-emphasis on total wattage is, we think, not a very good idea at all.

As far as power supply units are concerned, there are many factors that are as important (if not more important) than total wattage. In fact, the total wattage figure is like the peak maximum power output (PMPO) figure that's used to sell "mini-component" stereo sets.

Every audiophile knows that the PMPO figure of a stereo set does not say much about how powerful it is. A much better figure for comparison, they say, is the root mean square (RMS) figure, which represents the "real world" output of the stereo and is a more accurate statistic.

The PMPO figure is used to sell stereo sets most probably because it's usually a larger figure than the RMS, and thus sounds more impressive. In the same way, the total wattage figure is used to list and sell power supplies.

There's a lot more to a power supply unit than total wattage. If one looks at the label (or the user manual) for the power supply unit, one will see several statistics (+5V, +12V, +3.3V, etc). These statistics are more relevant than total wattage when choosing an adequate power supply.

Before choosing a power supply, add up the power requirements for the PC. Many hardware manufacturers have technical documents available for download at their Web sites. These documents will provide information on the power drawn by that particular peripheral.

The "power supply unit" in a PC actually delivers power at several levels called rails. These rails are the ones labelled +5V, +12V, etc, on the power supply unit. Different peripherals will draw power from different rails, so in a power supply unit it's important to see if the individual rails will supply enough power to run the system than to look at the overall total wattage figure.

Also useful is the basic physics equation W = VA, which states that the wattage of a particular peripheral is equal to the voltage multiplied by the amperage. With this equation (and the technical documents available), one should be able to find out how much power one needs to run the PC. A good site to automatically calculate the size of the power supply needed is http://www.casemodgod.com/psu_calculator.htm -- we find the calculations here tend to be more accurate than other sites.

One specification that is frequently overlooked when considering power supplies is the operating temperature. The operating temperature is the temperature range at which it will work as advertised.

With lower-quality power supplies, the power output drops significantly after the temperature exceeds the stated operating temperature. Some lower-quality power supplies have an operating temperature of zero to 25 degrees centigrade. This basically means that the power supply will start the PC, but then will slowly lose power after the temperature rises above 25 degrees centigrade. At one point the power supplied will not be enough to power the PC and the PC will turn off.

This is one reason why lower-quality power supplies seem to work well initially and then shut off after a while.
Good-quality power supplies will have an operating temperature of at least zero to 50 degrees centigrade. Check to see that the power supply is certified by the standards bodies of various countries. If so, their logos will be present on the label. If you have a lower-quality, "generic" power supply, try one from a company with reasonable quality control measures.

Also ensure that all connectors are secure (the ATX power plug especially can be a bit tight). Ensure that the plug fits securely and the latch is positioned over the notch on the mainboard power connector. An automatic voltage regulator is also recommended. This helps regulate power delivered to the power supply unit.


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