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
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.