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CompTIA A+ Cert Guide: Power Supplies and System Cooling

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This chapter delves into how power is conveyed to the computer, which power supply to select depending on your configuration and needs, how to install and troubleshoot power supplies, and how to cool the system. It covers CompTIA A+ 220-801 objectives 1.8 and 5.2 and CompTIA A+ 220-802 objective 4.2.

This chapter covers the following subjects:

  • Power Supplies—This section describes the device that transforms AC power from the wall outlet into DC power that your computer can use. It also describes the various form factors and voltage levels, and how to protect your power supply.
  • Troubleshooting Power Problems—This section demonstrates how to troubleshoot complete failure and intermittent power supply problems that you might encounter.
  • Avoiding Power Supply Hazards—This section has guidelines for avoiding shock and fire hazards when working with power supplies.
  • Power Protection Types—In this section you learn about devices that can protect your computer from over and under voltage issues. These include surge protectors, uninterruptible power supplies, and line conditioners.
  • System Cooling—This last section describes the various ways to cool your system, including fans and liquid cooling, and demonstrates how to monitor the system temperature.

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 troubleshoot power supplies, and how to cool the system.

Foundation Topics

Power Supplies

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 unrecognized 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.

Figure 4-1

Figure 4-1. A typical ATX 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.

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).

Figure 4-2

Figure 4-2. A typical power supply label.


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 motherboard, 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.
  • 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).

Table 4-1 provides calculations for typical compact desktop and performance desktop systems.

Table 4-1. Calculating Power Supply Requirements

MicroATX System with Integrated Video

Full-Size ATX System with SLI (Dual Graphics Cards)





AMD A8 3800 (4 core with in-core graphics and L2 cache)


Intel Core i7-3960X Extreme Edition (6 cores with L3 cache)


microATX motherboard


ATX motherboard






Rewritable DVD drive


Rewritable Blu-ray drive


SATA hard disk


SATA hard disk


Two case fans


Three case fans


CPU fan


CPU fan


Integrated graphics (in CPU)

High-end SLI video cards (2)

210 (105×2)

Estimated wattage


Estimated wattage


Minimum power supply size recommended (80% efficiency assumed)


Minimum power supply size recommended (80% efficiency assumed)


Multivoltage Power Supplies


Most power supplies are designed to handle two different voltage ranges:

  • 110–120V/60Hz
  • 220–240V/50Hz

Standard North American power is now 115–120V/60Hz-cycle AC (the previous standard was 110V). The power used in European and Asian countries is typically 230–240V/50Hz AC (previously 220V). Power supplies typically have a slider switch with two markings: 115 (for North American 110–120V/60HzAC) and 230 (for European and Asian 220–240V/50Hz AC). Figure 4-3 shows a slider switch set for correct North American voltage. If a power supply is set to the wrong input voltage, the system will not work. Setting a power supply for 230V with 110–120V current is harmless; however, feeding 220–240V into a power supply set for 115V will destroy the power supply, and possibly other onboard hardware.

Figure 4-3

Figure 4-3. A typical power supply’s sliding voltage switch set for correct North American voltage (115V). Slide it to 230V for use in Europe and Asia.

The on/off switch shown in Figure 4-3 controls the flow of current into the power supply. It is not the system power switch, which is located on the front of most recent systems and is connected to the motherboard. When you press the system power switch, the motherboard signals the power supply to provide power.

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 motherboards:

  • 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 motherboards that use either ATX12V or EPS12V power leads.
  • Some older motherboards use a six-wire AUX connector to provide additional power.

Figure 4-5 lists the pinouts for the 20-pin and 24-pin ATX power supply connectors shown in Figure 4-4.

Figure 4-4

Figure 4-4. 20-pin ATX and 24-pin ATX power connectors compared to four-pin ATX12V and six-wire AUX power connectors.

Figure 4-5

Figure 4-5. Pinout for standard ATX 20-pin and 24-pin power connectors.

The power supply also powers various peripherals, such as the following:

  • PATA hard disks, CD and DVD optical drives, and case fans that do not plug into the motherboard use a four-pin Molex power connector.
  • 3.5-inch floppy drives use a four-pin Berg power connector.
  • Serial ATA (SATA) hard disks use an L-shaped 15-pin thinline power connector.
  • High-performance PCI Express x16 video cards that require additional 12V power use a PCI Express six-pin or eight-pin power cable.

Figure 4-6 illustrates these power connectors.

Figure 4-6

Figure 4-6. Power supply connectors for peripherals and modern motherboards.

If your power supply doesn’t have enough connectors, you can add Y-splitters to divide one power lead into two, but these can short out and can also reduce your power supply’s efficiency. You can also convert a standard Molex connector into an SATA or floppy drive power connector with the appropriate adapter.

Some power supplies (see Figure 4-7) use modular connections so that you can customize the power supply connections needed for your hardware.

Figure 4-7

Figure 4-7. A modular power supply includes cables you can attach to customize support for your system’s needs.

If your wattage calculations or your tests (covered later in this chapter) agree that it’s time to replace the power supply, make sure the replacement meets the following criteria:

  • Have the same power supply connectors and the same pinout as the original.
  • Have the same form factor (shape, size, and switch location)
  • Have the same or higher wattage rating; a higher wattage rating is highly desirable
  • Support any special features required by your CPU, video card, and motherboard, such as SLI support (support for PCI Express connectors to power dual high-performance PCI Express x16 video cards), high levels of +12V power (ATX12V v2.2 4-pin or EPS12V 8-pin power connectors), and so on

Removing and Replacing the Power Supply

Installing a new power supply is one of the easier repairs to make. You don’t need to fiddle with driver CDs or Windows Update to get the new one working. But, you do need to be fairly handy with a screwdriver or nut driver.

Typical power supplies are held in place by several screws that attach the power supply to the rear panel of the computer. The power supply also is supported by a shelf inside the case, and screws can secure the power supply to that shelf. To remove a power supply, follow these steps:


Step 1. Power down the computer. If the power supply has an on/off switch, turn it off as well.

Step 2. Disconnect the AC power cord from the computer.

Step 3. Open the case to expose the power supply, which might be as simple as removing the cover on a desktop unit or as involved as removing both side panels, front bezel, and case lid on a tower PC. Consult the documentation that came with your computer to determine how to expose the power supply for removal.

Step 4. Disconnect the existing power supply from the motherboard (see Figure 4-8). The catch securing the power supply connector must be released to permit the connector to be removed.

Figure 4-8

Figure 4-8. Disconnecting the power supply from the motherboard.

Step 5. Disconnect all other power supply leads to the motherboard (fan monitors, ATX12V, EPS12V, AUX).

Step 6. Disconnect the power supply from all drives and add-on cards.

Step 7. Disconnect the power supply from all fans.

Step 8. Remove the power supply screws from the rear of the computer case (see Figure 4-9).

Figure 4-9

Figure 4-9. Removing the mounting screws from a typical power supply.

Step 9. Remove any screws holding the power supply in place inside the case. (Your PC might not use these additional screws.)

Step 10. Lift or slide the power supply out of the case.

Before installing the replacement power supply, compare it to the original, making sure the form factor, motherboard power connectors, and switch position match the original. If the new power supply has a fan on top (as well as the typical rear-mounted fan), make sure the fan faces the inside of the case.

To install the replacement power supply, follow these steps:

Step 1. Lift or slide the power supply into the case.

Step 2. Attach the power supply to the shelf with screws (if required).

Step 3. Slide the power supply to the rear of the computer case; line up the holes in the unit carefully with the holes in the outside of the case.

Step 4. Connect the power supply to all fans, drives, add-on cards, and motherboard.

Step 5. Check the voltage setting on the power supply. Change it to the correct voltage for your location if necessary.

Step 6. Connect the AC power cord to the new power supply.

Step 7. Turn on the computer.

Step 8. Start the system normally to verify correct operation, and then run the normal shutdown procedure for the operating system. If necessary, turn off the system with the front power switch only.

Step 9. Close the case and secure it.

Troubleshooting Power Supplies

Problems with power supplies can cause a variety of symptoms, including

  • Overheating
  • Spontaneous rebooting
  • Intermittent device failure (particularly of bus-powered USB devices)
  • Loud noises

What can cause these symptoms, and how can you solve the problems behind the symptoms?

Overloaded Power Supplies—Symptoms and Solutions


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
  • 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.

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.

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.

Finding Solutions to a “Dead” System


A dead system that gives no signs of life when turned on can be caused by the following:

  • Defects in AC power to the system
  • Power supply failure or misconfiguration
  • Temporary short circuits in internal or external components
  • Power supply or other component failure

With four suspects, it’s time to play detective. Use the procedure outlined next to find the actual cause of a dead system. If one of the test procedures in the following list corrects the problem, the item that was changed is the cause of the problem. Power supplies have a built-in safety feature that shuts down the unit immediately in case of short circuit.


The following steps are designed to determine whether the power problem is caused by a short circuit or another problem:

Step 1. Smell the power supply’s outside vent. If you can detect a burnt odor, the power supply has failed (see previous section).

Step 2. Check the AC power to the system; a loose or disconnected power cord, a disconnected surge protector, a surge protector that has been turned off, or a dead AC wall socket will prevent a system from receiving power. If the wall socket has no power, reset the circuit breaker in the electrical service box for the location.

Step 3. Check the AC voltage switch on the power supply; it should be set to 115V for North America. Turn off the power, reset the switch, and restart the system if the switch was set to 230V. Note that many desktop computer power supplies no longer require a switch selection because they are autoranging.

Step 4. If the system uses a PS/2 mouse or keyboard, check the connectors; a loose keyboard connector could cause a short circuit.

Step 5. Turn off the system, disconnect power, and open the system. Verify that the power leads are properly connected to the motherboard. Connect loose power leads, reconnect power, and restart the computer.

Step 6. Check for loose screws or other components such as loose slot covers, modem speakers, or other metal items that can cause a short circuit. Correct them and retest.

Step 7. Remove all expansion cards and disconnect power to all drives; restart the system and use a multimeter to test power to the motherboard per Table 4-3.

Step 8. If the power tests within accepted limits with all peripherals disconnected, reinstall one card at a time and check the power. If the power tests within accepted limits, reattach one drive at a time and check the power.

Step 9. If a defective card or drive has a dead short, reattaching the defective card or drive should stop the system immediately upon power-up. Replace the card or drive and retest.

Step 10. Check the Power Good line at the power supply motherboard connector with a multimeter.

It’s a long list, but chances are you will track down the offending component before you reach the end of it.



Got an overheated power supply? Not sure? If you touch the power supply case and it’s too hot to touch, it’s overheated. Overheated power supplies can cause system failure and possible component damage, due to any of the following causes:

  • Overloading
  • Fan failure
  • Inadequate airflow outside the system
  • Inadequate airflow inside the system
  • Dirt and dust

Use the following sections to figure out the possible effects of these problems in any given situation.


An overloaded power supply is caused by connecting devices that draw more power (in watts) than the power supply is designed to handle. As you add more card-based devices to expansion slots, use more bus-powered USB and IEEE-1394 drives and devices, and install more internal drives in a system, the odds of having an overloaded power supply increase.

If a power supply fails or overheats, check the causes listed in the following sections before determining whether you should replace the power supply. If you determine that you should replace the power supply, purchase a unit that has a higher wattage rating.

Fan Failure

The fan(s) inside the power supply cool it and are partly responsible for cooling the rest of the computer. If they fail, the power supply and the entire computer are at risk of damage. Fans also might stop turning as a symptom of other power problems.

A fan that stops immediately after the power comes on usually indicates incorrect input voltage or a short circuit. If you turn off the system and turn it back on again under these conditions, the fan will stop each time.

To determine whether a fan has failed, listen to the unit; it should make less noise if the fan has failed. You can also see the fan blades spinning rapidly on a power supply fan that is working correctly. If the blades aren’t turning or are turning very slowly, the fan has failed or is too clogged with dust to operate correctly.

To determine whether case fans have failed, look at them through the front or rear of the system, or, if they are connected to the motherboard, use the system monitoring feature in the system BIOS to check fan speed. Figure 4-10 illustrates a typical example.

Figure 4-10

Figure 4-10. The system fan (case fan) has either failed or was never connected to the motherboard power/monitor header.

If the system starts normally but the fan stops turning later, this indicates a true fan failure instead of a power problem.

Inadequate Airflow Outside the System

The power supply’s capability to cool the system depends in part on free airflow space outside the system. If the computer is kept in a confined area (such as a closet or security cabinet) without adequate ventilation, power supply failures due to overheating are likely.

Even systems in ordinary office environments can have airflow problems; make sure that several inches of free air space exist behind the fan outputs for any computer.

Inadequate Airflow Inside the System

As you have seen in previous chapters, the interior of the typical computer is a messy place. Wide ribbon cables used for some types of drives, drive power cables, and expansion cards create small air dams that block airflow between the heat sources—such as the motherboard, CPU, drives, and memory modules—and the fans in the power supply. Figure 4-11 illustrates a typical system with a lot of cable clutter that can interfere with airflow.

Figure 4-11

Figure 4-11. A cluttered system with plenty of unsecured cables to block airflow.

You can do the following to improve airflow inside the computer:

  • Use cable ties to secure excess ribbon cable and power connectors out of the way of the fans and the power supply.
  • Replace any missing slot covers.
  • Make sure that auxiliary case fans, chipset fans, and CPU fans are working correctly.
  • Use SATA drives in place of PATA drives. SATA drives use narrow data cables.

Figure 4-12 illustrates a different system that uses cable management (cable ties, bundling cables between the drive bays and outer case wall, and routing behind the motherboard) to improve airflow.

Figure 4-12

Figure 4-12. A system with good airflow due to intelligent cable management.

For more information about cooling issues, see the section “System Cooling,” later in this chapter for details.

Dirt and Dust

Most power supplies, except for a few of the early ATX power supplies, use a cooling technique called negative pressure; in other words, the power supply fan works like a weak vacuum cleaner, pulling air through vents in the case, past the components, and out through the fan. Vacuum cleaners are used to remove dust, dirt, cat hairs, and so on from living rooms and offices, and even the power supply’s weak impression of a vacuum cleaner works the same way.

When you open a system for any kind of maintenance, look for the following:

  • Dirt, dust, hair, and gunk clogging the case vents
  • A thin layer of dust on the motherboard and expansion slots
  • Dirt and dust on the power supply vent and fans

Yuck! You never know what you’ll find inside a PC that hasn’t been cleaned out for a year or two. So how can you get rid of the dust and gunk? You can use either a vacuum cleaner specially designed for computer use or compressed air to remove dirt and dust from inside the system. If you use compressed air, be sure to spread newspapers around the system to catch the dirt and dust. If possible, remove the computer from the computer room so the dust is not spread to other equipment.

Fans Turn But System Doesn’t Start

Fans connected directly to the power supply will run as soon as the system is turned on, but if the system doesn’t start up, this could indicate a variety of problems. Check the following:

  • Make sure the main ATX and 12V ATX or EPS power leads are securely connected to the appropriate sockets.
  • Make sure the CPU and memory modules are securely installed in the appropriate sockets.

Testing Power Supplies and Other Devices with a Multimeter

How can you find out that a defective power supply is really defective? How can you make sure that a cable has the right pinouts? Use a multimeter. A multimeter is one of the most flexible diagnostic tools around. It is covered in this chapter because of its usefulness in testing power supplies, but it also can be used to test coaxial, serial, and parallel cables, as well as fuses, resistors, and batteries.

Multimeters are designed to perform many different types of electrical tests, including the following:

  • DC voltage and polarity
  • AC voltage and polarity
  • Resistance (Ohms)
  • Diodes
  • Continuity
  • Amperage

All multimeters are equipped with red and black test leads. When used for voltage tests, the red is attached to the power source to be measured and the black is attached to ground.

Multimeters use two different readout styles: digital and analog. Digital multimeters are usually autoranging, which means they automatically adjust to the correct range for the test selected and the voltage present. Analog multimeters, or non–autoranging digital meters, must be set manually to the correct range and can be damaged more easily by overvoltage. Figure 4-13 compares typical analog and digital multimeters.

Figure 4-13

Figure 4-13. Typical analog (left) and digital (right) multimeters. Photos courtesy of Colacino Electric Supply, Newark, NJ.


Multimeters are designed to perform tests in two ways: in series and in parallel. Most tests are performed in parallel mode, in which the multimeter is not part of the circuit but runs parallel to it. On the other hand, amperage tests require that the multimeter be part of the circuit, so these tests are performed in series mode. Many low-cost multimeters do not include the ammeter feature for testing amperage (current), but you might be able to add it as an option.

Figure 4-14 shows a typical parallel mode test (DC voltage for a motherboard CMOS battery) and the current (amperage) test, which is a serial-mode test.

Figure 4-14

Figure 4-14. A parallel-mode (DC current) test setup (left) and an amperage (current) serial-mode test setup (right).

Table 4-2 summarizes the tests you can perform with a multimeter.

Table 4-2. Using a Multimeter

Test to Perform

Multimeter Setting

Probe Positions


AC voltage (wall outlet)


Red to hot, black to ground.

Read voltage from meter; should be near 115V in North America.

DC voltage (power supply outputs to motherboard, drives, batteries)


Red to hot, black to ground.

Read voltage from meter; compare to default values.

Continuity (cables, fuses)


Red to lead at one end of cable; black to corresponding lead at other end.

No CONT signal indicates bad cable or bad fuse.

For a straight-through cable, check the same pin at each end. For other types of cables, consult a cable pinout to select the correct leads.

Double-check leads and retest to be sure.

Resistance (Ohms)


Connect one lead to each end of resistor.

Check reading; compare to rating for resistor.

A fuse should have no resistance.

Amperage (Ammeter)


Red probe to positive lead of circuit (power disconnected!); black lead to negative lead running through component to be tested.

Check reading; compare to rating for component tested.

You can use a multimeter to find out whether a power supply is properly converting AC power to DC power. Here’s how: Measure the DC power going from the power supply to the motherboard. A power supply that does not meet the measurement standards listed in Table 4-3 should be replaced.


Table 4-3. Acceptable Voltage Levels

Rated DC Volts

Acceptable Range











Power Good


If the system monitor functions in the system BIOS do not display voltage levels (refer to Figure 4-10 for an example of a system that does display voltage levels in the BIOS), you can take the voltage measurements directly from the power supply connection to the motherboard. Both 20-pin and 24-pin P1 (ATX) power connectors are designed to be back-probed as shown in Figure 4-15; you can run the red probe through the top of the power connector to take a reading (the black probe uses the power supply enclosure or metal case frame for ground). Some motherboards bring these same voltage levels to a more convenient location on the motherboard for testing.

Figure 4-15

Figure 4-15. Testing the +12V line on an ATX power supply. The voltage level indicated (+11.92V) is well within limits.

If a power supply fails any of these measurements, replace it and retest the new unit.

Avoiding Power Supply Hazards

To avoid shock and fire hazards when working with power supplies, follow these important guidelines:

  • Never disassemble a power supply or push metal tools through the openings in the case—Long after you shut off the system, the capacitors inside the power supply retain potentially fatal voltage levels. If you want to see the interior of a power supply safely, check the websites of leading power supply vendors such as PC Power and Cooling.
  • If you are replacing the power supply in a Dell desktop computer, determine whether the computer uses a standard ATX or Dell proprietary ATX power supply—Many Dell computers built from September 1998 to the present use a nonstandard version of the ATX power supply with a different pinout for the power connector. Install a standard power supply on a system built to use a Dell proprietary model, or upgrade from a Dell motherboard that uses the Dell proprietary ATX design to a standard motherboard, and you can literally cause a power supply and system fire!
  • Always use a properly wired and grounded outlet for your computer and its peripherals—You can use a plug-in wiring tester to quickly determine whether a three-prong outlet is properly wired; signal lights on the tester indicate the outlet’s status (see Figure 4-16).
Figure 4-16

Figure 4-16. An outlet tester like this one can find wiring problems quickly. This outlet is wired correctly.

Power Protection Types

Question. How well can a power supply work if it has poor-quality AC power to work with?

Answer. Not very well. Because computers and many popular computer peripherals run on DC power that has been converted from AC power, it’s essential to make sure that proper levels of AC power flow to the computer and its peripherals. There are four problems you might run into:

  • Overvoltages (spikes and surges)
  • Undervoltages (brownouts)
  • Power failure (blackouts)
  • Noisy power (interference)

Extremely high levels of transient or sustained overvoltages can damage the power supply of the computer and peripherals, and voltage that is significantly lower than required will cause the computer and peripherals to shut down. Shutdowns happen immediately when all power fails. A fourth problem with power is interference; “noisy” electrical power can cause subtle damage, and all four types of problems put the most valuable property of any computer, the data stored on the computer, at risk. Protect your computer’s power supply and other components with appropriate devices:

  • Surge suppressors, which are also referred to as surge protectors
  • Battery backup systems, which are also referred to as uninterruptible power supply (UPS) or standby power supply (SPS) systems
  • Power conditioning devices

Surge Suppressors


Stop that surge! While properly designed surge suppressors can prevent power surges (chronic overvoltage) and spikes (brief extremely high voltage) from damaging your computer, low-cost ones are often useless because they lack sufficient components to absorb dangerous surges. Surge suppressors range in price from under $10 to close to $100 per unit.

Both spikes and surges are overvoltages: voltage levels higher than the normal voltage levels that come out of the wall socket. Spikes are momentary overvoltages, whereas surges last longer. Both can damage or destroy equipment and can come through data lines (such as RJ-11 phone or RJ-45 network cables) as well as through power lines. In other words, if you think of your PC as a house, spikes and surges can come in through the back door or the garage as well as through the front door. Better “lock” (protect) all the doors. Many vendors sell data-line surge suppressors.

How can you tell the real surge suppressors from the phonies? Check for a TVSS (transient voltage surge suppressor) rating on the unit. Multi-outlet power strips do not have a TVSS rating.

Beyond the TVSS rating, look for the following features to be useful in preventing power problems:

  • A low TVSS let-through voltage level (400V AC or less). This might seem high compared to the 115V standard, but power supplies have been tested to handle up to 800V AC themselves without damage.
  • A covered-equipment warranty that includes lightning strikes (one of the biggest causes of surges and spikes).
  • A high Joule rating. Joules measure electrical energy, and surge suppressors with higher Joule ratings can dissipate greater levels of surges or spikes.
  • Fusing that prevents fatal surges from getting through.
  • Protection for data cables such as telephone/fax (RJ-11), network (RJ-45), or coaxial (RG6).
  • EMI/RFI noise filtration (a form of line conditioning).
  • Site fault wiring indicator (no ground, reversed polarity warnings).
  • Fast response time to surges. If the surge suppressor doesn’t clamp fast enough, the surge can get through.
  • Protection against surges on hot, neutral, and ground lines.

If you use surge protectors with these features, you will minimize power problems. The site-fault wiring indicator will alert you to wiring problems that can negate grounding and can cause serious damage in ordinary use.

A surge suppressor that meets the UL 1449 or ANSI/IEEE C62.41 Category A (formerly IEEE 587 Category A) standards provides protection for your equipment. You might need to check with the vendor to determine whether a particular unit meets one of these standards.

Battery Backup Units (UPS and SPS)

A UPS is another name for a battery backup unit. A UPS provides emergency power when a power failure strikes (a blackout) or when power falls below minimum levels (a brownout).


There are two different types of UPS systems: true UPS and SPS systems. A true UPS runs your computer from its battery at all times, isolating the computer and monitor from AC power. There is no switchover time with a true UPS when AC power fails because the battery is already running the computer. A true UPS inherently provides power conditioning (preventing spikes, surges, and brownouts from reaching the computer) because the computer receives only battery power, not the AC power coming from the wall outlet. True UPS units are sometimes referred to as line-interactive battery backup units because the battery backup unit interacts with the AC line, rather than the AC line going directly to the computer and other components.

An SPS is also referred to as a UPS, but its design is quite different. Its battery is used only when AC power fails. A momentary gap in power (about 1ms or less) occurs between the loss of AC power and the start of standby battery power; however, this switchover time is far faster than is required to avoid system shutdown because computers can coast for several milliseconds before shutting down. SPS-type battery backup units are far less expensive than true UPSs but work just as well as true UPSs when properly equipped with power-conditioning features.

Battery backup units can be distinguished from each other by differences in the following:

  • Runtimes—The amount of time a computer will keep running on power from the UPS. A longer runtime unit uses a bigger battery and usually costs more than a unit with a shorter runtime. Fifteen minutes is a minimum recommendation for a UPS for an individual workstation; much larger systems are recommended for servers that might need to complete a lengthy shutdown procedure.
  • Network support—Battery backup units made for use on networks are shipped with software that broadcasts a message to users about a server shutdown so that users can save open files and close open applications and then shuts down the server automatically before the battery runs down.
  • Automatic shutdown—Some low-cost UPS units lack this feature, but it is essential for servers or other unattended units. The automatic shutdown feature requires an available USB (or RS-232 serial) port and appropriate software from the UPS maker. If you change operating systems, you need to update the software for your UPS to be supported by the new operating system.
  • Surge suppression features—Virtually all UPS units today have integrated surge suppression, but the efficiency of integrated surge suppression can vary as much as separate units. Check for UL-1449 and ANSI/IEEE C62.41 Category A ratings to find reliable surge suppression in UPS units.

Figure 4-17 illustrates the rear of a typical UPS unit.

Figure 4-17

Figure 4-17. A typical UPS with integrated surge suppression for printers and other AC powered devices, 10/100/1000 Ethernet (including VoIP), and conventional telephony devices.

Buying the Correct-Sized Battery Backup System

Battery backups can’t run forever. But then, they’re not supposed to. This section describes how you can make sure you get enough time to save your files and shut down your computer. UPS units are rated in VA (volt-amps), and their manufacturers have interactive buying guides you can use online or download to help you select a model with adequate capacity. If you use a UPS with an inadequate VA rating for your equipment, your runtime will be substantially shorter than it should be.

Here’s how to do the math: You can calculate the correct VA rating for your equipment by adding up the wattage ratings of your computer and monitor and multiplying the result by 1.4. If your equipment is rated in amperage (amps), multiply the amp rating by 120 (volts) to get the VA rating.

For example, my computer has a 450W power supply, which would require a 630VA-rated UPS (450×1.4) and a 17-inch monitor that is rated in amps, not watts. The monitor draws 0.9A, which would require a 108VA-rated UPS (0.9×120). Add the VA ratings together, and my computer needs a 750VA-rated battery backup unit or larger. Specifying a UPS with a VA rating at least twice what is required by the equipment attached to the UPS (for example, a 1500VA or higher rating, based on a minimum requirement of 750VA) will greatly improve the runtime of the battery.

In this example, a typical 750VA battery backup unit would provide about 5 minutes of runtime when used with my equipment. However, if I used a 1500VA battery backup, I could increase my runtime to more than 15 minutes because my equipment would use only about half the rated capacity of the UPS unit.

If you need a more precise calculation, for example, if you will also power an additional monitor or other external device, use the interactive sizing guides provided by battery backup vendors, such as American Power Conversion (www.apc.com).

Power-Conditioning Devices

Although power supplies are designed to work with voltages that do not exactly meet the 115V or 230V standards, power that is substantially higher or lower than what the computer is designed for can damage the system. Electrical noise on the power line, even with power at the correct voltage, also causes problems because it disrupts the correct sinewave alternating-current pattern the computer, monitor, and other devices are designed to use.

Better-quality surge protectors often provide power filtration to handle electromagnetic interference (EMI)/radio frequency interference (RFI) noise problems from laser printers and other devices that generate a lot of electrical interference. However, to deal with voltage that is too high or too low, you need a true power conditioner.


Power-conditioning units take substandard or overstandard power levels and adjust them to the correct range needed by your equipment. Some units also include high-quality surge protection features.

To determine whether you need a power-conditioning unit, you can contact your local electric utility company to see whether it loans or rents power-monitoring devices. Alternatively, you can rent them from power consultants. These units track power level and quality over a set period of time (such as overnight or longer) and provide reports to help you see the overall quality of power on a given line.

Moving surge- and interference-causing devices such as microwaves, vacuum cleaners, refrigerators, freezers, and furnaces to circuits away from the computer circuits helps minimize power problems. However, in older buildings, or during times of peak demand, power conditioning might still be necessary. A true (line-interactive) UPS provides built-in power conditioning by its very nature (see the previous discussion).

System Cooling

Today’s computers often run much hotter than systems of a few years ago, so it’s important to understand how to keep the hottest-running components running cooler. The following sections discuss the components that are most in need of cooling and how to cool them (processor cooling is discussed in Chapter 2, “Motherboards and Processors”).

Northbridge and Southbridge Chips and Voltage Regulators

Motherboards use a one-chip or two-chip chipset (also referred to as northbridge and southbridge chips) to route data to and from the processor. The northbridge or Memory Controller Hub (MCH) chip, because it carries high-speed data such as memory and video to and from the processor, becomes hot during operation, and, if the component overheats and is damaged, the entire motherboard must be replaced. For this reason, most motherboards feature some type of cooler for the northbridge chip.

Although the southbridge or I/O Controller Hub (ICH) chip carries lower-speed traffic, such as hard disk, audio, and network traffic, it can also become overheated. As a result, most recent motherboards also feature cooling for the southbridge chip. Some chipsets combine both functions into a single chip, which also requires cooling.


Three methods have been used for cooling the motherboard chipset. Passive heat sinks attached directly to the chipset chips are inexpensive but do not provide sufficient cooling for high-performance systems. Active heat sinks provide better cooling than passive heat sinks, but low-quality sleeve-bearing fans often used in these coolers can cause premature fan failure and lead to overheating. The latest trend in chipset and motherboard cooling uses heat pipes, which draw heat away from the chipset or other high-temperature components, such as the voltage regulator for CPU power, and dissipates it through high-performance, very large passive heat sinks located away from the chipset itself. While you can add other types of coolers to chipset chips, heat pipes are factory-installed.

Figure 4-18 illustrates passive and active heat sinks for northbridge and southbridge chips.

Figure 4-18

Figure 4-18. Passive and active heat sinks for chipsets.

Figure 4-19 illustrates a motherboard that uses heat pipes for component cooling.

Figure 4-19

Figure 4-19. Motherboard with heat pipe cooling. Heat is transferred from components under heat sink (A) via heat pipe (B) to be dissipated by radiator at rear of motherboard (C).

Video Card Cooling

Another major heat source in modern systems is the video card’s graphics processing unit (GPU) chip, which renders the desktop, graphics, and everything else you see on your computer screen. With the exception of a few low-end video cards, almost all video cards use active heat sinks to blow hot air away from the GPU.

However, the memory chips on a video card can also become very hot. To cool both the GPU and video memory, most recent midrange and high-end video card designs use a fan shroud to cool both components. Fan shrouds often require enough space to prevent the expansion slot next to the video card from being used.

Figure 4-20 illustrates a typical video card with a two-slot fan shroud.

Figure 4-20

Figure 4-20. The EVGA GeForce GTX 580 is a high-performance PCI Express x16 video card that requires a two-slot fan shroud. Image courtesy of EVGA Corporation.

Case Fans

Most ATX chassis have provisions for at least two case fans: one at the front of the system and one at the rear of the system. Case fans can be powered by the motherboard or by using a Y-splitter connected to a four-pin Molex power connector. Case fans at the front of the system should draw air into the system, while case fans at the rear of the system should draw air out of the system.

Figure 4-21 shows a typical rear case fan. You can plug fans like this into the three-prong chassis fan connection found on many recent motherboards or into the 4-pin Molex drive power connector used by hard drives. If the motherboard power connector is used, the PC Health or hardware monitor function found in many recent system BIOS setup programs can monitor fan speed (refer to Figure 4-10).

Figure 4-21

Figure 4-21. A case fan that can be plugged into the motherboard or into a Molex power connector.

Case fans are available in various sizes up to 200mm (80, 92, and 120mm are the most common sizes). Measure the opening at the rear of the case to determine which fan size to purchase. Some systems, such as the one shown earlier in Figure 4-11, might feature two rear fans or a rear fan and a top fan.

Thermal Compound

When passive or active heat sinks are installed on a processor, northbridge or southbridge chip, GPU or other component, thermal compound (also known as thermal transfer material, thermal grease, or phase change material) must be used to provide the best possible thermal transfer between the component and the heat sink.

Heat sinks supplied with boxed processors might use a preapplied phase-change material on the heat sink, whereas OEM processors with third-party heat sinks usually require the installer to use a paste or thick liquid thermal grease or silver-based compound. Coolers for northbridge or southbridge chips might use thermal grease or a phase-change pad.

If the thermal material is preapplied to the heat sink, make sure you remove the protective tape before you install the heat sink. If a third-party heat sink is used, or if the original heat sink is removed and reinstalled, carefully remove any existing thermal transfer material from the heat sink and processor die surface. Then, apply new thermal transfer material to the processor die before you reinstall the heat sink on the processor. Figure 4-22 illustrates the application of thermal compound to a northbridge chip before attaching a heat sink.

Figure 4-22

Figure 4-22. Applying thermal grease to the northbridge chip.

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