Power Supplies and System
Cooling
Clean, well-planned power is imperative, from the AC outlet to the electrical protection equipment to the power supply. Many of the issues that you see concerning power are due to lack of protection or improper planning, and as such you will see several questions on the A+ exams regarding this subject.
In this chapter we delve into how power is conveyed to the computer, which power supply to select depending on your configuration and needs, how to install and trou- bleshoot power supplies, and how to cool the system.
Clean, well-planned power is imperative, from the AC outlet to the electrical protection equipment to the power supply. Many of the issues that you see concerning power are due to lack of protection or improper planning, and as such you will see several questions on the A+ exams regarding this subject.
In this chapter we delve into how power is conveyed to the computer, which power supply to select depending on your configuration and needs, how to install and trou- bleshoot power supplies, and how to cool the system.
Power issues are largely ignored by most computer users, but a properly working
power supply is the foundation to correct operation of the system. When the power
supply stops working, the computer stops working, and when a power supply stops
functioning properly—even slightly—all sorts of computer problems can take
place. From unexpected system reboots to data corruption, from unrecognised bus-
powered USB devices to system overheating, a bad power supply is bad news. The
power supply is vital to the health of the computer. So, if your computer is acting
“sick,” you should test the power supply to see if it’s the cause. To keep the power
supply working properly, use surge suppression and battery backup (UPS) units.
The power supply is really misnamed: It is actually a power converter that changes
high-voltage alternating current (AC) to low-voltage direct current (DC). There are
lots of wire coils, capacitors, and other components inside the power supply that do
the work, and during the conversion process, a great deal of heat is produced. Most
power supplies include one or two fans to dissipate the heat created by the operation
of the power supply; however, a few power supplies designed for silent operation use
passive heat sink technology instead of fans. On power supplies that include fans,
fans also help to cool the rest of the computer. Figure 4-1 shows a typical desktop
computer’s power supply.
Power Supply Ratings
Power supply capacity is rated in watts, and the more watts a power supply provides, the more devices it can safely power.
You can use the label attached to the power supply, shown in Figure 4-2, to determine its wattage rating and see important safety reminders.
Power Supply Ratings
Power supply capacity is rated in watts, and the more watts a power supply provides, the more devices it can safely power.
You can use the label attached to the power supply, shown in Figure 4-2, to determine its wattage rating and see important safety reminders.
Typically, power supplies in recent tower-case (upright case) machines use 400-watt
or larger power supplies, reflecting the greater number of drives and cards that can
be installed in these computers. Power supplies used in slimline desktop computers
have typical ratings of around 220–300 watts,. The power supply rating is found
on the top or side of the power supply, along with safety rating information and amperage levels produced by the power supply’s different DC outputs.
How can you tell whether a power supply meets minimum safety standards? Look for the appropriate safety certification mark for your country or locale. For example, in the U.S. and Canada, the backward UR logo is used to indicate the power supply has the UL and UL Canada safety certifications as a component (the familiar circled UL logo is used for finished products only).
Use an interactive power supply sizing tool such as the calculators provided
by eXtreme Outervision (www.extreme.outervision.com) or PC Power and
Cooling (www.pcpower.com).
on the top or side of the power supply, along with safety rating information and amperage levels produced by the power supply’s different DC outputs.
How can you tell whether a power supply meets minimum safety standards? Look for the appropriate safety certification mark for your country or locale. For example, in the U.S. and Canada, the backward UR logo is used to indicate the power supply has the UL and UL Canada safety certifications as a component (the familiar circled UL logo is used for finished products only).
Use the following methods to determine the wattage rating needed for a
replacement power supply:
Whip out your calculator and add up the wattage ratings for everything
connected to your computer that uses the power supply, including the moth-
erboard, processor, memory, cards, drives, and bus-powered USB devices. If
the total wattage used exceeds 70% of the wattage rating of your power supply,
you should upgrade to a larger power supply. Check the vendor spec sheets for
wattage ratings.
If you have amperage ratings instead of wattage ratings, multiply the amperage by the volts to determine wattage and then start adding. If a device uses two
or three different voltage levels, be sure to carry out this calculation for each voltage level, and add up the figures to determine the wattage requirement for the device.
If you have amperage ratings instead of wattage ratings, multiply the amperage by the volts to determine wattage and then start adding. If a device uses two
or three different voltage levels, be sure to carry out this calculation for each voltage level, and add up the figures to determine the wattage requirement for the device.
Power Supply Form Factors and Connectors
When you shop for a power supply, you also need to make sure it can connect to
your motherboard. There are two major types of power connectors on mother-
boards:
20-pin, used by older motherboards in the ATX family
24-pin, used by recent ATX/BTX motherboards requiring the ATX12V 2.2
power supply standard
Some high-wattage power supplies with 20-pin connectors might also include a 20-pin to 24-pin adapter. Some 24-pin power supplies include a 24-pin to 20-pin connector.
Some motherboards use power supplies that feature several additional connectors to supply added power, as follows (see Figure 4-4):
The four-wire square ATX12V connector provides additional 12V power to the motherboard; this connector is sometimes referred to as a “P4” or “Pentium 4” connector.
Many recent high-end power supplies use the eight-wire EPS12V connector (see Figure 4-6) instead of the ATX12V power connector. Often, the EPS12V lead is split into two four-wire square connectors to be compatible with moth- erboards that use either ATX12V or EPS12V power leads.
Some older motherboards use a six-wire AUX connector to provide additional power.
20-pin, used by older motherboards in the ATX family
24-pin, used by recent ATX/BTX motherboards requiring the ATX12V 2.2
power supply standard
Some high-wattage power supplies with 20-pin connectors might also include a 20-pin to 24-pin adapter. Some 24-pin power supplies include a 24-pin to 20-pin connector.
Some motherboards use power supplies that feature several additional connectors to supply added power, as follows (see Figure 4-4):
The four-wire square ATX12V connector provides additional 12V power to the motherboard; this connector is sometimes referred to as a “P4” or “Pentium 4” connector.
Many recent high-end power supplies use the eight-wire EPS12V connector (see Figure 4-6) instead of the ATX12V power connector. Often, the EPS12V lead is split into two four-wire square connectors to be compatible with moth- erboards that use either ATX12V or EPS12V power leads.
Some older motherboards use a six-wire AUX connector to provide additional power.
Overloaded Power Supplies—Symptoms and Solutions
Troubleshooting Power Supplies 141
What happens if you connect devices that require more wattage than a power supply
can provide? This is a big problem called an overload. An overloaded power supply
has three 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
A power supply that makes a loud bang, followed by a system crash, has had an onboard capacitor blow up. The easiest way to diagnose this is to smell the power supply after turning it off and disconnecting it from AC power. If you can smell a burnt odor with a chemical overtone to it coming from the power supply’s outside vent, your power supply has died. This odor can linger for weeks. Sadly, when a power supply blows up like this, it can also destroy the motherboard, bus-powered USB devices connected to the computer, and other components.
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
Intermittent failures of USB bus-powered devices (mice, keyboard, USB flash
drives, portable USB hard disks) because these devices draw power from the
system’s power supply via the USB port
Here’s a good rule of thumb: If your system starts spontaneously rebooting and you don’t see a blue screen (STOP) error, replace the power supply as soon as possible. However, power supply overheating can have multiple causes; follow the steps listed in the section “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 “Testing Power Supplies and Other Devices with a Multimeter,” later in this chapter.
Here’s a good rule of thumb: If your system starts spontaneously rebooting and you don’t see a blue screen (STOP) error, replace the power supply as soon as possible. However, power supply overheating can have multiple causes; follow the steps listed in the section “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 “Testing Power Supplies and Other Devices with a Multimeter,” later in this chapter.
Loud Noises from the Power Supply
Computers usually run quietly, but if you hear loud noises coming from the power supply, it’s a sure sign of problems. A whirring, rattling, or thumping noise while the system is on usually indicates a fan failure. If a fan built in to a component such as a heat sink or power supply is failing, replace the component immediately.
Computers usually run quietly, but if you hear loud noises coming from the power supply, it’s a sure sign of problems. A whirring, rattling, or thumping noise while the system is on usually indicates a fan failure. If a fan built in to a component such as a heat sink or power supply is failing, replace the component immediately.
CAUTION Should you try to replace a standard power supply fan? No. Because the
power supply is a sealed unit, you would need to remove the cover from most power
supplies to gain access to the fan. The capacitors inside a power supply retainpoten-
tially lethal electrical charges. Instead, scrap the power supply and replace it with a
higher-rated unit. Refer to the section “Removing and Replacing the Power Supply,”
earlier in the chapter.
A power supply that makes a loud bang, followed by a system crash, has had an onboard capacitor blow up. The easiest way to diagnose this is to smell the power supply after turning it off and disconnecting it from AC power. If you can smell a burnt odor with a chemical overtone to it coming from the power supply’s outside vent, your power supply has died. This odor can linger for weeks. Sadly, when a power supply blows up like this, it can also destroy the motherboard, bus-powered USB devices connected to the computer, and other components.
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