Power Supply Ratings

In previous chapters, you've learned about bits, nibbles , bytes, base 2, base 10, and base 16. Here's another measurement for you: watts. There are all types of complex calculations for watts, but they aren't important in this case. What is important to realize is that power supplies are rated in watts, and the more watts a power supply provides, the more devices it can safely power.

Typically, power supplies in tower-case (upright case) machines use 300-watt or larger power supplies, reflecting the greater number of drives and cards that can be installed in these computers. Power supplies used in smaller desktop computers have typical ratings of 145 watts or less. The power supply rating is found on the top of the power supply, along with safety rating information and amperage levels produced by the power supply's different DC outputs.

caution

In the replacement market, unlabeled power supplies are often bad news. If you're shopping for a replacement power supply, and you're not buying from one of the major vendors such as Antec or PC Power and Cooling, avoid unlabeled models because you literally don't know what you're getting.

However, don't assume that all unlabeled power supplies are questionable. Some computer vendors prefer to use unlabeled power supplies in new equipment. If you find an unlabeled power supply in a system you bought new, contact the system vendor for information about it.

If you have a processor that runs faster than 1GHz, a mid-range or high-end gaming 3D card, or multiple hard or optical drives and your power supply is under 300 watts, you're just asking for trouble. Fast processors and 3D graphics cards along with multiple drives are notorious power munchers.

What happens if you connect devices that require more wattage than a power supply can provide? A big problem we call an overload .

An overloaded power supply has two major symptoms:

• Overheating .

• Spontaneous rebooting (cold boot with memory test) due to incorrect voltage on the Power Good line running from the power supply to the motherboard. See Figure 5.7, later in this chapter, for details.

  Figure 5.7. Pinouts for an ATX power connector (left) and Baby-AT (LPX) power connector (right). The color coding in the figure represents the standard wire colors used.

  

caution

Power supplies that do not bear the UL or CE marks should not be used, as their safety is unknown.

Here's a good rule of thumb: If your system starts spontaneously rebooting, replace the power supply as soon as possible. However, power supply overheating can have multiple causes; follow the steps listed in the section "Causes and Cures of Power Supply Overheating," later in this chapter, before replacing an overheated power supply.

To determine whether Power Good or other motherboard voltage levels are within limits, perform the measurements listed in the section "Determining Power Supply DC Voltage Levels" later in this chapter.