Datasheet

48
Chapter 1
Hardware
Working with Power Supplies
Power-supply problems are usually among the ones that are easy to troubleshoot: If the
system doesn’t respond in any way when the power is turned on or you are experiencing
randomly frying parts, or an incredibly annoying sound, open the case, remove the power
supply, and replace it with a new one.
Critical Information
The device in the computer that provides the power is the power supply. A power supply
converts 110-volt AC current into the voltages a computer needs to operate. On an AT
motherboard, these are +5 volts DC, –5 volts DC, +12 volts DC, and –12 volts DC. Com-
ponents in modern PCs don’t use the negative voltages; they’re provided for backward com-
patibility only. On an ATX motherboard, an additional voltage is provided: +3.3 volts DC.
Be aware that different cases have different types of on/off switches. The process of replac-
ing a power supply is a lot easier if you purchase a replacement with the same mechanism.
Even so, remember to document exactly how the power supply was connected to the on/off
switch before you remove it.
Power supplies contain transformers and capacitors that carry lethal amounts of current.
They aren’t meant to be serviced. Do not attempt to open them or do any work on them.
Figure 1.18 shows a generic power supply.
FIGURE 1.18 A power supply
A power supply has a rated output capacity in watts, and when you fill a system with
power-hungry devices, you must make sure that maximum capacity isn’t exceeded. Other-
wise, problems with power can occur, creating lockups or spontaneous reboots.
To determine the wattage a device draws, multiply voltage by current. For example, if
a device uses 5 amps of +3.3V and 0.7 amps of +12V, a total of 25 watts is consumed. Do
this calculation for every device installed. Most devices have labels that state their power
requirements. Some devices don’t have power labels; for such devices, use the numbers in
Table 1.6 for estimations.
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