Unformatted text preview: All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 10 C HAPTER Power Supplies
In this chapter, you will learn how to
• Explain the basics of electricity
• Describe the details about powering the PC
• Install, maintain, and troubleshoot power supplies Powering the PC requires a single box—the power supply—that takes electricity from
the wall socket and transforms it into electricity to run the motherboard and other
internal components. Figure 10-1 shows a typical power supply inside a case. All of the
wires dangling out of it connect to the motherboard and peripherals. Figure 10-1 Typical power supply mounted inside the PC system unit 359 ch10.indd 359 12/11/09 12:22:22 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 360 As simple as this appears on the surface, power supply issues are of critical importance for techs. Problems with power can create system instability, crashes, and data
loss—all things most computer users would rather avoid! Good techs, therefore, know
an awful lot about powering the PC, from understanding the basic principles of electricity to knowing the many variations of PC power supplies. Plus, you need to know
how to recognize power problems and implement the proper solutions. Too many
techs fall into the “just plug it in” camp and never learn how to deal with power, much
to their clients’ unhappiness.
EXAM TIP Some questions on the CompTIA A+ certification exams could
refer to a power supply as a PSU, for power supply unit. A power supply also
falls into the category of field replaceable unit (FRU), which refers to the typical
parts a tech should carry, such as RAM and a hard drive. Historical/Conceptual
Electricity is simply a flow of negatively charged particles, called electrons, through
matter. All matter enables the flow of electrons to some extent. This flow of electrons
is very similar to the flow of water through pipes; so similar that the best way to learn
about electricity is by comparing it to how water flows though pipes. So let’s talk about
water for a moment.
Water comes from the ground, through wells, aquifers, rivers, and so forth. In a typical city, water comes to you through pipes from the water supply company that took it
from the ground. What do you pay for when you pay your water bill each month? You
pay for the water you use, certainly, but built into the price of the water you use is the
surety that when you turn the spigot, water will flow at a more or less constant rate. The
water sits in the pipes under pressure from the water company, waiting for you to turn
Electricity works essentially the same way as water. Electric companies gather or generate electricity and then push it to your house under pressure through wires. Just like
water, the electricity sits in the wires, waiting for you to plug something into the wall
socket, at which time it’ll flow at a more or less constant rate. You plug a lamp into an
electrical outlet and flip the switch, electricity flows, and you have light. You pay for
reliability, electrical pressure, and electricity used.
The pressure of the electrons in the wire is called voltage and is measured in units
called volts (V). The amount of electrons moving past a certain point on a wire is called
the current or amperage, which is measured in units called amperes (amps or A). The
amount of amps and volts needed so that a particular device will function is expressed ch10.indd 360 12/11/09 12:22:23 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 361 as how much wattage (watts or W) that device needs. The correlation between the three
is very simple math: V × A = W. You’ll learn more about wattage a little later in this
Wires of all sorts—whether copper, tin, gold, or platinum—have a slight resistance to
the flow of electrons, just as water pipes have a slight amount of friction that resists the
flow of water. Resistance to the flow of electrons is measured in ohms (Ω).
• Pressure = voltage (V)
• Volume flowing = amperes (A)
• Work = wattage (W)
• Resistance = ohms (Ω)
A particular thickness of wire only handles so much electricity at a time. If you
push too much through, the wire will overheat and break, much as an overloaded water pipe will burst. To make sure you use the right wire for the right job, all electrical
wires have an amperage rating, such as 20 amps. If you try to push 30 amps through
a 20-amp wire, the wire will break and electrons will seek a way to return into the
ground. Not a good thing, especially if the path back to the ground is through you!
Circuit breakers and ground wires provide the basic protection from accidental
overflow. A circuit breaker is a heat-sensitive electrical switch rated at a certain amperage. If you push too much amperage through the circuit breaker, the wiring inside
detects the increase in heat and automatically opens, stopping the flow of electricity
before the wiring overheats and breaks. You reset the circuit breaker to reestablish the
circuit and electricity flows once more through the wires. A ground wire provides a
path of least resistance for electrons to flow back to ground in case of an accidental
Many years ago your electrical supply used fuses instead of circuit breakers. Fuses are
small devices with a tiny filament designed to break if subjected to too much current.
Unfortunately, fuses had to be replaced every time they blew, making circuit breakers
much more preferable. Even though you no longer see fuses in a building’s electrical
circuits, many electrical devicessuch as a PC’s power supplyoften still use fuses for
their own internal protection.
PC use. An electrical outlet must have a ground wire to be suitable for Electricity comes in two flavors: direct current (DC), in which the electrons flow in
one direction around a continuous circuit, and alternating current (AC), in which the
flow of electrons alternates direction back and forth in a circuit (see Figure 10-2). Most
electronic devices use DC power, but all power companies supply AC power because AC
travels long distances much more efficiently than DC. ch10.indd 361 12/11/09 12:22:23 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 362 Figure 10-2
Diagrams showing DC and AC
flow of electrons Essentials
Powering the PC
Your PC uses DC voltage, so some conversion process must take place before the PC can
use AC power from the power company. The power supply in a computer converts highvoltage AC power from the wall socket to low-voltage DC. The first step in powering
the PC, therefore, is to get and maintain a good supply of AC power. Second, you need
a power supply to convert AC to the proper voltage and amperage of DC power for the
motherboard and peripherals. Finally, you need to control the byproduct of electricity
use, namely heat. Let’s look at the specifics of powering the PC. Supplying AC
Every PC power supply must have standard AC power from the power company, supplied steadily rather than in fits and spurts, and protection against accidental blurps
in the supply. The power supply connects to the power cord (and thus to an electrical
outlet) via a standard IEC-320 connector. In the United States, standard AC comes in
somewhere between 110 and 120 volts, often written as ∼115 VAC (volts of alternating
current). The rest of the world uses 220–240 VAC, so most power supplies have a little
switch in the back so you can use them anywhere. These power supplies with voltageselection switches are referred to as fixed-input. Power supplies that you do not have to
manually switch for different voltages are known as auto-switching. Figure 10-3 shows
the back of a power supply. Note the three components, from top to bottom: the hard
on/off switch, the 115/230 switch, and the IEC-320 connector.
CAUTION Flipping the AC switch on the back of a power supply can wreak
all kinds of havoc on a PC. Moving the switch to ∼230 V in the United States
makes for a great practical joke (as long as the PC is off when you do it)—the
PC might try to boot up but probably won’t get far. You don’t risk damaging
anything by running at half the AC the power supply is expecting. In countries that run
∼230 standard, on the other hand, firing up the PC with the AC switch set to ∼115 can
cause the power supply to die a horrid, smoking death. Watch that switch! ch10.indd 362 12/11/09 12:22:23 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 363 Figure 10-3
Back of fixedinput power
connection Before plugging anything into an AC outlet, take a moment to test the outlet first
by using a multimeter or a device designed exclusively to test outlets. Failure to test AC
outlets properly can result in inoperable or destroyed equipment, as well as possible
electrocution. The IEC-320 plug has three holes, called hot, neutral, and ground. These
names describe the function of the wires that connect to them behind the wall plate.
The hot wire carries electrical voltage, much like a pipe that delivers water. The neutral
wire carries no voltage, but instead acts like a water drain, completing the circuit by
returning electricity to the local source, normally a breaker panel. The ground wire
makes it possible for excess electricity to return safely to the ground. When testing AC
power, you want to check for three things: that the hot outputs approximately 115 V (or
whatever the proper voltage is for your part of the world), that the neutral connects to
ground (0 V output), and that the ground connects to ground (again, 0 V). Figure 10-4
shows the voltages at an outlet. Figure 10-4
Outlet voltages ch10.indd 363 12/11/09 12:22:24 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 364 You can use a multimeter—often also referred to as a volt-ohm meter (VOM) or digital multimeter (DMM)—to measure a number of aspects of electrical current. A multimeter consists
of two probes, an analog or digital meter, and a dial to set the type of test you want to perform. Refer to Figure 10-5 to become familiar with the components of the multimeter.
multimeter Note that some multimeters use symbols rather than letters to describe AC and DC
settings. The V with the solid line above a dashed line, for example, in Figure 10-6, refers to direct current. The V∼ stands for alternating current.
featuring DC and
AC symbols ch10.indd 364 12/11/09 12:22:24 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 365 Every multimeter offers at least four types of electrical tests: continuity, resistance,
AC voltage (VAC), and DC voltage (VDC). Continuity tests whether electrons can flow
from one end of a wire to the other end. If so, you have continuity; if not, you don’t.
You can use this setting to determine if a fuse is good or to check for breaks in wires. If
your multimeter doesn’t have a continuity tester (many cheaper multimeters do not),
you can use the resistance tester. A broken wire or fuse will show infinite resistance,
while a good wire or fuse will show no resistance. Testing AC and DC voltages is a matter of making sure the measured voltage is what it should be. Using a Multimeter to Test AC Outlets
Every competent technician knows how to use a multimeter. To set up the meter for
measuring AC, follow these steps:
1. Move the selector switch to the AC V (usually red). If multiple settings are
available, put it into the first scale higher than 120 V (usually 200 V). Auto-range
meters set their own range; they don’t need any selection except AC V.
2. Place the black lead in the common (–) hole. If the black lead is permanently
attached, ignore this step.
3. Place the red lead in the V-Ohm-A (+) hole. If the red lead is permanently
attached, ignore this step.
Once you have the meter set up for AC, go through the process of testing the various
wires on an AC socket. Just don’t put your fingers on the metal parts of the leads when
you stick them into the socket! Follow these steps:
1. Put either lead in hot, the other in neutral. You should read 110 to 120 V AC.
2. Put either lead in hot, the other in ground. You should read 110 to 120 V AC.
3. Put either lead in neutral, the other in ground. You should read 0 V AC.
If any of these readings is different from what is described here, it’s time to call an
NOTE Many devices in the computing world use an AC adapter rather than
an internal power supply. Even though it sits outside a device, an AC adapter
converts AC current to DC, just like a power supply. Unlike power supplies,
AC adapters are rarely interchangeable. Although manufacturers of different
devices often use the same kind of plug on the end of the AC adapter cable, these adapters
are not necessarily interchangeable. In other words, just because you can plug an AC
adapter from your friend’s laptop into your laptop does not mean it’s going to work. You
need to make sure that three things match before you plug an AC adapter into a device:
voltage, amperage, and polarity. If they don’t match, don’t plug it in! ch10.indd 365 12/11/09 12:22:24 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 366 Using Special Equipment to Test AC Voltage
A number of good AC-only testing devices are available. With these devices, you can test
all voltages for an AC outlet by simply inserting them into the outlet. Be sure to test all
of the outlets the computer system uses: power supply, external devices, and monitor.
Although convenient, these devices aren’t as accurate as a multimeter. My favorite tester
is made by Radio Shack, a simple-seeming tool (see Figure 10-7). This handy device
provides three light-emitting diodes (LEDs) that describe everything that can go wrong
with a plug. Figure 10-7
Circuit tester Protecting the PC from Spikes and Sags in AC Power
If all power companies could supply electricity in smooth, continuous flows with no
dips or spikes in pressure, the next two sections of this chapter would be irrelevant.
Unfortunately, no matter how clean the AC supply appears to a multimeter, the truth
is that voltage from the power company tends to drop well below (sag) and shoot far
above (surge or spike) the standard 115 V (in the United States). These sags and spikes
usually don’t affect lamps and refrigerators, but they can keep your PC from running or
can even destroy a PC or peripheral device. Two essential devices handle spikes and sags
in the supply of AC: surge suppressors and uninterruptible power supplies.
Surge Suppressors Surges or spikes are far more dangerous than sags. Even a
strong sag only shuts off or reboots your PC; any surge can harm your computer, and
a strong surge destroys components. Given the seriousness of surges, every PC should ch10.indd 366 12/11/09 12:22:25 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 367 use a surge suppressor device that absorbs the extra voltage from a surge to protect the
PC. The power supply does a good job of surge suppression and can handle many of
the smaller surges that take place fairly often. But the power supply takes a lot of damage from this and will eventually fail. To protect your power supply, a dedicated surge
suppressor works between the power supply and the outlet to protect the system from
power surges (see Figure 10-8). Figure 10-8
Surge suppressor Most people tend to spend a lot of money on their PC and for some reason suddenly
get cheap on the surge suppressor. Don’t do that! Make sure your surge suppressor has
the Underwriters Laboratories UL 1449 for 330 V rating to ensure substantial protection for your system. Underwriters Laboratories (www.ul.com) is a U.S.-based, not-forprofit, widely recognized industry testing laboratory whose testing standards are very
important to the consumer electronics industry. Additionally, check the joules rating
before buying a new surge suppressor. A joule is a unit of electrical energy. How much
energy a surge suppressor can handle before it fails is described in joules. Most authorities agree that your surge suppressor should rate at a minimum of 800 joules—and the
more joules, the better the protection. My surge suppressor rates out at 1,750 joules.
CAUTION No surge suppressor in the world can handle the ultimate surge,
the ESD of a lightning strike. If your electrical system takes such a hit, you
can kiss your PC goodbye if it was plugged in at the time. Always unplug
electronics during electrical storms!
While you’re protecting your system, don’t forget that surges also come from telephone and cable connections. If you use a modem, DSL, or cable modem, make sure to
get a surge suppressor that includes support for these types of connections. Many manufacturers make surge suppressors with telephone line protection (see Figure 10-9).
line protection ch10.indd 367 12/11/09 12:22:25 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 368 No surge suppressor works forever. Make sure your surge suppressor has a test/reset
button so you’ll know when the device hasas we say in the businessturned into an
extension cord. If your system takes a hit and you have a surge suppressor, call the company! Many companies provide cash guarantees against system failure due to surges,
but only if you follow their guidelines.
If you want really great surge suppression, you need to move up to power conditioning. Your power lines take in all kinds of strange signals that have no business being
in there, such as electromagnetic interference (EMI) and radio frequency interference
(RFI). Most of the time, this line noise is so minimal it’s not worth addressing, but occasionally events (such as lightning) generate enough line noise to cause weird things
to happen to your PC (keyboard lockups, messed-up data). All better surge suppressors
add power conditioning to filter out EMI and RFI.
UPS An uninterruptible power supply (UPS) protects your computer (and, more importantly, your data) in the event of a power sag or power outage. Figure 10-10 shows
a typical UPS. A UPS essentially contains a big battery that provides AC power to your
computer regardless of the power coming from the AC outlet.
power supply NOTE There are two main types of UPS: online, where devices are constantly
powered through the UPS’s battery, and standby, where devices connected
to the UPS only receive battery power when the AC sags below ∼80–90V.
Another type of UPS is called line-interactive, which is similar to a standby
UPS but has special circuitry to handle moderate AC sags and surges without the need to
switch to battery power.
All uninterruptible power supplies are measured in both watts (the true amount of
power they supply in the event of a power outage) and in volt-amps (VA). Volt-amps is
the amount of power the UPS could supply if the devices took power from the UPS in
a perfect way. Your UPS provides perfect AC power, moving current smoothly back and
forth 60 times a second. Power supplies, monitors, and other devices, however, may not
take all of the power the UPS has to offer at every point as the AC power moves back
and forth, resulting in inefficiencies. If your devices took all of the power the UPS offered at every point as the power moved back and forth, VA would equal watts. ch10.indd 368 12/11/09 12:22:25 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 369 If the UPS makers knew ahead of time exactly what devices you planned to plug
into their UPS, they could tell you the exact watts, but different devices have different
efficiencies, forcing the UPS makers to go by what they can offer (VAs), not what your
devices will take (watts). The watts value they give is a guess, and it’s never as high as
the VAs. The VA rating is always higher than the watt rating.
Because you have no way to calculate the exact efficiency of every device you’ll plug
into the UPS, go with the wattage rating. You add up the total wattage of every component in your PC and buy a UPS with a higher wattage. You’ll spend a lot of time and
mental energy figuring precisely how much wattage your computer, monitor, drives,
and so on require to get the proper UPS for your system. But you’re still not finished!
Remember that the UPS is a battery with a limited amount of power, so you then need
to figure out how long you want the UPS to run when you lose power.
The quicker and far better method to use for determining the UPS you need is to go
to any of the major surge suppressor/UPS makers’ Web sites and use their handy power
calculators. My personal favorite is on the American Power Conversion Web site: www
.apc.com. APC makes great surge suppressors and UPSs, and the company’s online calculator will show you the true wattage you needand teach you about whatever new
thing is happening in power at the same time.
Every UPS also has surge suppression and power conditioning, so look for the joule
and UL 1449 ratings. Also look for replacement battery costssome UPS replacement
batteries are very expensive. Last, look for a UPS with a USB or serial port connection.
These handy UPSs come with monitoring and maintenance software (Figure 10-11) Figure 10-11 APC PowerChute software ch10.indd 369 12/11/09 12:22:26 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 370 that tells you the status of your system and the amount of battery power available, logs
power events, and provides other handy options.
Table 10-1 gives you a quick look at the low end and the very high end of UPS products (as of late 2009). Brand Model Outlets
Protected APC BE350G 3 @ 120 V 3 min @ 200W, $49.99
10 min @ 100W Standby APC BP500UC 4 @ 120 V 4 min @ 315W, $129.99
14 min @ 157W Standby CyberPower CPS1500AVR 3 @ 120 V 18 min @ 950W, $299.99
6 min @ 475W Line-interactive APC SYA4K8RMP 6 @ 120,208V 6 min @ 3200 W, $ 6,925.00 Double17 min @
1600W Backup Time Price Type Table 10-1 Typical UPS Devices Supplying DC
After you’ve assured the supply of good AC electricity for the PC, the power supply unit
(PSU) takes over, converting high-voltage AC into several DC voltages (notably, 5.0,
12.0, and 3.3 volts) usable by the delicate interior components. Power supplies come
in a large number of shapes and sizes, but the most common size by far is the standard
150 mm × 140 mm × 86 mm desktop PSU shown in Figure 10-12.
Desktop PSU The PC uses the 12.0-volt current to power motors on devices such as hard drives
and CD-ROM drives, and it uses the 5.0-volt and 3.3-volt current for support of onboard electronics. Manufacturers may use these voltages any way they wish, however, ch10.indd 370 12/11/09 12:22:26 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 371 and may deviate from these assumptions. Power supplies also come with standard connectors for the motherboard and interior devices. Power to the Motherboard
Modern motherboards use a 20- or 24-pin P1 power connector. Some motherboards may
require special 4-, 6-, or 8-pin connectors to supply extra power (Figure 10-13). We’ll
talk about each of these connectors in the form factor standards discussion later in this
connectors Power to Peripherals: Molex, Mini, and SATA
Many devices inside the PC require power. These include hard drives, floppy drives,
optical-media drives, zip drives (for techs who enjoy retro computing), and fans. The
typical PC power supply has up to three types of connectors that plug into peripherals:
Molex, mini, and SATA.
Molex Connectors The most common type of power connection for devices that
need 5 or 12 volts of power is the Molex connector (Figure 10-14). The Molex connector
has notches, called chamfers, that guide its installation. The tricky part is that Molex
connectors require a firm push to plug in properly, and a strong person can defeat the
chamfers, plugging a Molex in upside down. Not a good thing. Always check for proper
orientation before you push it in!
Molex connector Mini Connectors All power supplies have a second type of connector, called a
mini connector (Figure 10-15), that supplies 5 and 12 volts to peripherals, although
only floppy disk drives in modern systems use this connector. Drive manufacturers ch10.indd 371 12/11/09 12:22:27 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 372 adopted the mini as the standard connector
on 3.5-inch floppy disk drives. Often these
mini connectors are referred to as floppy
Be extra careful when plugging in a mini
connector! Whereas Molex connectors are
difficult to plug in backward, you can insert a mini connector incorrectly with very
little effort. As with a Molex connector, doing so will almost certainly destroy the floppy
drive. Figure 10-16 depicts a correctly oriented mini connection, with the small ridge on
the connector away from the body of the data socket.
Mini connector Figure 10-16
a mini connector CAUTION As with any power connector, plugging a mini connector into a
device the wrong way will almost certainly destroy the device. Check twice
before you plug one in!
SATA Power Connectors Serial ATA (SATA) drives need a special 15-pin SATA
power connector (Figure 10-17). The larger pin count supports the SATA hot-swappable
feature and 3.3 V, 5.0 V, and 12.0 V devices. SATA power connectors are L shaped, making
it almost impossible to insert one incorrectly into a SATA drive. No other device on your
computer uses the SATA power connector. For more information about SATA drives, see
Chapter 11, “Hard Drive Technologies.” Figure 10-17
connector ch10.indd 372 12/11/09 12:22:27 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 373 NOTE SATA also supports a slimline connector that has a 6-pin power
segment and a micro connector that has a 9-pin power segment. Splitters and Adapters You may occasionally find yourself without enough
connectors to power all of the devices inside your PC. In this case, you can purchase
splitters to create more connections (see Figure 10-18). You might also run into
the phenomenon of needing a SATA connector, but having only a spare Molex. Because the voltages on the wires are the same, a simple adapter will take care of the
problem nicely. Figure 10-18
Molex splitter NOTE It’s normal and common to have unused power connectors inside
your PC case. Testing DC
A common practice for techs troubleshooting a system is to test the DC voltages coming
out of the power supply. Even with good AC, a bad power supply can fail to transform
AC to DC at voltages needed by the motherboard and peripherals. Grab your trusty
multimeter and try this on a powered-up PC with the side cover removed. Note that ch10.indd 373 12/11/09 12:22:28 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 374 you must have P1 connected to the motherboard and the system must be running (you
don’t have to be in Windows, of course).
1. Switch your multimeter to DC, somewhere around 20 V DC if you need
to make that choice. Make sure your leads are plugged into the multimeter
properly: red to hot, black to ground. The key to testing DC is that, which lead
you touch to which wire matters. Red goes to hot wires of all colors; black
always goes to ground.
2. Plug the red lead into the red wire socket of a free Molex connector and plug
the black lead into one of the two black wire sockets. You should get a reading
of ∼5 V. What do you have?
3. Now move the red lead to the yellow socket. What voltage do you get?
4. Testing the P1 connector is a little more complicated. You push the red and
black leads into the top of P1, sliding in alongside the wires until you bottom
out. Leave the black lead in one of the black wire ground sockets. Move the red
lead through all of the colored wire sockets. What voltages do you find? ATX
The original ATX power supplies had two distinguishing physical features: the motherboard power connector and soft power. Motherboard power came from a single cable
with a 20-pin P1 motherboard power connector. ATX power supplies also had at least
two other cables, each populated with two or more Molex or mini connectors for peripheral power.
When plugged in, ATX systems have 5 volts running to the motherboard. They’re
always “on” even when powered down. The power switch you press to power up the PC
isn’t a true power switch like the light switch on the wall in your bedroom. The power
switch on an ATX system simply tells the computer whether it has been pressed. The
BIOS or operating system takes over from there and handles the chore of turning the PC
on or off. This is called soft power.
Using soft power instead of a physical switch has a number of important benefits.
Soft power prevents a user from turning off a system before the operating system has
been shut down. It enables the PC to use power-saving modes that put the system to
sleep and then wake it up when you press a key, move a mouse, or receive an e-mail.
(See Chapter 21, “Portable Computing,” for more details on sleep mode.)
All of the most important settings for ATX soft power reside in CMOS setup. Boot
into CMOS and look for a Power Management section. Take a look at the Power On
Function option in Figure 10-19. This determines the function of the on/off switch. You
may set this switch to turn off the computer, or you may set it to the more common
ATX did a great job supplying power for more than a decade, but over time more
powerful CPUs, multiple CPUs, video cards, and other components began to need
more current than the original ATX provided. This motivated the industry to introduce
a number of updates to the ATX power standards: ATX12V 1.3, EPS12V, multiple rails,
ATX12V 2.0, other form factors, and active PFC. ch10.indd 374 12/11/09 12:22:28 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 375 Figure 10-19 Soft power setting in CMOS ATX12V 1.3 The first widespread update to the ATX standard, ATX12V 1.3, came
out in 2003. This introduced a 4-pin motherboard power connector, unofficially but
commonly called the P4, that provided more 12-volt power to assist the 20-pin P1
motherboard power connector. Any power supply that provides a P4 connector is called
an ATX12V power supply. The term “ATX” was dropped from the ATX power standard,
so if you want to get really nerdy you can sayaccuratelythat there’s no such thing
as an ATX power supply. All power suppliesassuming they have a P4 connectorare
ATX12V or one of the later standards.
The ATX12V 1.3 standard also introduced a
6-pin auxiliary connector—commonly called
an AUX connector—to supply increased 3.3connector
and 5.0-volt current to the motherboard (see
Figure 10-20). This connector was based on
the motherboard power connector from the
precursor of ATX, called AT.
The introduction of these two extra power connectors caused the industry some
teething problems. In particular, motherboards using AMD CPUs tended to need the
AUX connector, while motherboards using Intel CPUs needed only the P4. As a result,
many power supplies came with only a P4 or only an AUX connector to save money.
A few motherboard makers skipped adding either connector and used a standard Molex
connector so people with older power supplies wouldn’t have to upgrade just because
they bought a new motherboard (Figure 10-21).
The biggest problem with ATX12V was its lack of teethit made a lot of recommendations but few requirements, giving PSU makers too much choice (such as
choosing or not choosing to add AUX and P4 connectors) that weren’t fixed until
later versions. ch10.indd 375 12/11/09 12:22:28 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 376 Figure 10-21
Molex power on
motherboard EPS12V Server motherboards are thirsty for power, and sometimes ATX12V 1.3 just
didn’t cut it. An industry group called the Server System Infrastructure (SSI) developed
a non-ATX standard motherboard and power supply called EPS12V. An EPS12V power
supply came with a 24-pin main motherboard power connector that resembled a 20-pin
ATX connector, but it offered more current and thus more stability for motherboards.
It also came with an AUX connector, an ATX12V P4 connector, and a unique 8-pin
connector. That’s a lot of connectors! EPS12V power supplies were not interchangeable
with ATX12V power supplies.
EPS12V may not have seen much life beyond servers, but it introduced a number
of power features, some of which eventually became part of the ATX12V standard. The
most important issue was something called rails.
Rails Generally, all of the PC’s power comes from a single transformer that takes the
AC current from a wall socket and converts it into DC current that is split into three
primary DC voltage rails: 12.0 volts, 5.0 volts, and 3.3 volts. Individual lines run from
each of these voltage rails to the various connectors. That means the 12-volt connector
on a P4 draws from the same rail as the main 12-volt connector feeding power to the
motherboard. This works fine as long as the collective needs of the connectors sharing
a rail don’t exceed its capacity to feed them power. To avoid this, EPS12V divided the
12-volt supply into two or three separate 12-volt rails, each one providing a separate
source of power.
ATX12V 2.0 The ATX12V 2.0 standard incorporated many of the good ideas of
EPS12V into the ATX world, starting with the 24-pin connector. This 24-pin motherboard power connector is backward compatible with the older 20-pin connector so
users don’t have to buy a new motherboard if they use an ATX12V 2.0 power supply.
ATX12V 2.0 requires two 12-volt rails for any power supply rated higher than 230 watts.
ATX12V 2.0 dropped the AUX connector and required SATA hard drive connectors. ch10.indd 376 12/11/09 12:22:29 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 377 In theory, a 20-pin motherboard power supply connector will work on a motherboard with a 24-pin socket, but doing this is risky in that the 20-pin connector may
not provide enough power to your system. Try to use the right power supply for your
motherboard to avoid problems. Many ATX12V 2.0 power supplies have a convertible
24-to-20-pin converter. These are handy if you want to make a nice “clean” connection,
because many 20-pin connectors have capacitors that prevent plugging in a 24-pin connector. You’ll also see the occasional 24-pin connector constructed in such a way that
you can slide off the extra four pins. Figure 10-22 shows 20-pin and 24-pin connectors;
Figure 10-23 shows a convertible connector. Although they look similar, those extra
four pins won’t replace the P4 connector. They are incompatible! Figure 10-22
20- and 24-pin
connectors Figure 10-23
power connector Many modern ATX motherboards feature an 8-pin CPU power connector like the
one found in the EPS12V standard to help support high-end CPUs that demand a lot
of power. This connector is referred to by several names, including EPS12V, EATX12V,
and ATX12V 2x4. One half of this connector will be pin compatible with the P4 power
connector and the other half may be under a protective cap. Be sure to check the motherboard installation manuals for recommendations on if and when you need to use the ch10.indd 377 12/11/09 12:22:29 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 378 full 8 pins. For backward compatibility, some power supplies provide an 8-pin power
connector that can split into two 4-pin sets, one of which is the P4 connector.
Another notable connector is the auxiliary PCI Express (PCIe) power connector.
Figure 10-24 shows the 6-pin PCIe power connector. Some motherboards add a Molex socket for PCIe, and some cards come with a Molex socket as well. Higher-end
cards have a dedicated 6-pin or 8-pin PCIe power connector. The 8-pin PCIe connector
should not be confused with the EPS12V connector, as they are not compatible. Some
PCIe devices with the 8-pin connector will accept a 6-pin PCIe power connection instead, but this may put limits on their performance. Often you’ll find that 8-pin PCIe
power cables have two pins at the end that you can detach for easy compatibility with
PCI Express 6-pin
power connector Practical Application
Niche-Market Power Supply Form Factors The demand for smaller and
quieter PCs and, to a lesser extent, the emergence of the BTX form factor has led to the
development of a number of niche-market power supply form factors. All use standard
ATX connectors, but differ in size and shape from standard ATX power supplies.
NOTE You’ll commonly find niche-market power supplies bundled with
computer cases (and often motherboards as well). These form factors are
rarely sold alone.
Here are some of the more common specialty power supply types:
• TFX12V A small power supply form factor optimized for low-profile ATX systems
• SFX12V A small power supply form factor optimized for systems using FlexATX motherboards (see Figure 10-25)
• CFX12V An L-shaped power supply optimized for microBTX systems
• LFX12V A small power supply form factor optimized for low-profile BTX
systems ch10.indd 378 12/11/09 12:22:30 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 379 Figure 10-25
SFX power supply EXAM TIP The CompTIA A+ exams test you pretty heavily on power
supplies. You need to know what power supply works with a particular
system or with a particular computing goal in mind.
Active PFC Visualize the AC current coming from the power company as water in a
pipe, smoothly moving back and forth, 60 times a second. A PC’s power supply, simply
due to the process of changing this AC current into DC current, is like a person sucking
on a straw on the end of this pipe. It takes gulps only when the current is fully pushing or
pulling at the top and bottom of each cycle and creating an electrical phenomenasort
of a back pressurethat’s called harmonics in the power industry. These harmonics create
the humming sound you hear from electrical components. Over time, harmonics damage
electrical equipment, causing serious problems with the power supply and other electrical
devices on the circuit. Once you put a few thousand PCs with power supplies in the same
local area, harmonics can even damage the electrical power supplier’s equipment!
Good PC power supplies come with active power factor correction (active PFC), extra
circuitry that smoothes out the way the power supply takes power from the power company and eliminates harmonics (Figure 10-26). Never buy a power supply that does not
have active PFCall power supplies with active PFC proudly show you on the box. Wattage Requirements
Every device in a PC requires a certain amount of wattage to function. A typical hard
drive draws 15 watts of power when accessed, for example, whereas some Athlon 64 X2
CPUs draw a whopping 110 watts at peak usage—with average usage around 70 watts.
The total wattage of all devices combined is the minimum you need the power supply
to provide. ch10.indd 379 12/11/09 12:22:30 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 380 Figure 10-26
PFC EXAM TIP The CompTIA A+ Certification exams do not require you to
figure precise wattage needs for a particular system. When building a PC for
a client, however, you do need to know this stuff!
If the power supply cannot produce the wattage a system needs, that PC won’t work
properly. Because most devices in the PC require maximum wattage when first starting,
the most common result of insufficient wattage is a paperweight that looks like a PC.
This can lead to some embarrassing moments. You might plug in a new hard drive for ch10.indd 380 12/11/09 12:22:31 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 381 a client, push the power button on the case, and nothing happens—a dead PC! Eek!
You can quickly determine if insufficient wattage is the problem. Unplug the drive and
power up the system. If the system boots up, the power supply is a likely suspect. The
only fix for this problem is to replace the power supply with one that provides more
wattage (or leave the new drive out—a less-than-ideal solution).
No power supply can turn 100 percent of the AC power coming from the power company into DC current, so all power supplies provide less power to the system than the
wattage advertised on the box. ATX12V 2.0 standards require a power supply to be at least
70 percent efficient, but you can find power supplies with better than 80 percent efficiency.
More efficiency can tell you how many watts the system puts out to the PC in actual use.
Plus, the added efficiency means the power supply uses less power, saving you money.
One common argument these days is that people buy power supplies that provide far more wattage than a system needs and therefore waste power. This is untrue.
A power supply provides only the amount of power your system needs. If you put a
1000-watt power supply (yes, they really exist) into a system that needs only 250 watts,
that big power supply will put out only 250 watts to the system. So buying an efficient,
higher-wattage power supply gives you two benefits. First, running a power supply at
less than 100 percent load lets it live longer. Second, you’ll have plenty of extra power
when adding new components.
As a general recommendation for a new system, use at least a 500-watt power supply.
This is a common wattage and gives you plenty of extra power for booting as well as for
whatever other components you might add to the system in the future.
Don’t cut the specifications too tightly for power supplies. All power supplies produce less wattage over time, simply because of wear and tear on the internal components. If you build a system that runs with only a few watts of extra power available
from the power supply initially, that system will most likely start causing problems
within a year or less. Do yourself or your clients a favor and get a power supply that has
more wattage than you need. Installing, Maintaining, and
Troubleshooting Power Supplies
Although installing, maintaining, and troubleshooting power supplies take a little less
math than selecting the proper power supply for a system, they remain essential skills
for any tech. Installing takes but a moment, and maintaining is almost as simple, but
troubleshooting can cause headaches. Let’s take a look. Installing
The typical power supply connects to the PC with four standard computer screws,
mounted in the back of the case (Figure 10-27). Unscrew the four screws and the power
supply lifts out easily (Figure 10-28). Insert a new power supply that fits the case and
attach it by using the same four screws.
Handling ATX power supplies requires special consideration. Understand that an
ATX power supply never turns off. As long as that power supply stays connected to ch10.indd 381 12/11/09 12:22:31 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 382 Figure 10-27
for power supply Figure 10-28
system unit a power outlet, the power supply will continue to supply 5 volts to the motherboard.
Always unplug an ATX system before you do any work! For years, techs bickered about the
merits of leaving a PC plugged in or unplugged while you serviced it. ATX settled this issue
forever. Many ATX power supplies provide a real on/off switch on the back of the PSU
(see Figure 10-29). If you really need the system shut down with no power to the motherboard, use this switch.
When working on an ATX system, you may find using the power button inconvenient because you’re not using a case or you haven’t bothered to plug the power button’s leads into the motherboard. That means there is no power button. One trick when
in that situation is to use a set of car keys or a screwdriver to contact the two wires to
start and stop the system (see Figure 10-30). ch10.indd 382 12/11/09 12:22:31 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 383 Figure 10-29
On/off switch for
an ATX system Figure 10-30
Shorting the soft
on/off jumpers Your first task after acquiring a new power supply is simply making sure it works.
Insert the motherboard power connectors before starting the system. If you have video
cards with power connectors, plug them in too. Other connectors such as hard drives can
wait until you have one successful bootor if you’re cocky, just plug everything in! Cooling
Heat and computers are not the best of friends. Cooling is therefore a vital consideration when building a computer. Electricity equals heat. Computers, being electrical ch10.indd 383 12/11/09 12:22:32 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 384 devices, generate heat as they operate, and too much can seriously damage a computer’s
The power supply fan provides the basic cooling for the PC (Figure 10-31). It not only
cools the voltage regulator circuits within the power supply, but it also provides a constant flow of outside air throughout the interior of the computer case. A dead power
supply fan can rapidly cause tremendous problems, even equipment failure. If you ever
turn on a computer and it boots just fine but you notice that it seems unusually quiet,
check to see if the power supply fan has died. If it has, quickly turn off the PC and replace the power supply.
Power supply fan Some power supplies come with a built-in sensor to help regulate the airflow. If the
system gets too hot, the power supply fan spins faster. The 3-pin, 3-wire fan sensor connector plugs into the motherboard directly (Figure 10-32).
3-wire fan sensor
connector ch10.indd 384 12/11/09 12:22:33 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 385 Case fans are large, square fans that snap into special brackets on the case or screw
directly to the case, providing extra cooling for key components (see Figure 10-33).
Most cases come with a case fan, and no modern computer should really be without
one or two.
Case fan The single biggest issue related to case fans is where to plug them in. Most case fans
come with standard Molex connectors, which are easy to plug in, but other case fans
come with special three-pronged power connectors that need to connect to the motherboard. You can get adapters to plug three-pronged connectors into Molex connectors
or Molex connectors into three-pronged connectors. Maintaining Airflow
A computer is a closed system, and computer cases help the fans keep things cool:
everything is inside a box. Although many tech types like to run their systems with
the side panel of the case open for easy access to the components, in the end they are
cheating themselves. Why? A closed case enables the fans to create airflow. This airflow
substantially cools off interior components. When the side of the case is open, you ruin
the airflow of the system, and you lose a lot of cooling efficiency.
An important point to remember when implementing good airflow inside your
computer case is that hot air rises. Warm air always rises above cold air, and you can use
this principle to your advantage in keeping your computer cool.
In the typical layout of case fans for a computer case, an intake fan is located near the
bottom of the front bezel of the case. This fan draws cool air in from outside the case and
blows it over the components inside the case. Near the top and rear of the case (usually
near the power supply), you’ll usually find an exhaust fan. This fan works the opposite of
the intake fan: it takes the warm air from inside the case and sends it to the outside.
Another important part of maintaining proper airflow inside the case is ensuring
that slot covers are covering all empty expansion bays (Figure 10-34). To maintain good
airflow inside your case, you shouldn’t provide too many opportunities for air to escape. ch10.indd 385 12/11/09 12:22:33 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 386 Figure 10-34
Slot covers Slot covers not only assist in maintaining a steady airflow; they also help keep dust and
smoke out of your case.
EXAM TIP Missing slot covers can cause the PC to overheat! Reducing Fan Noise
Fans generate noise. In an effort to ensure proper cooling, many techs put several highspeed fans into a case, making the PC sound like a jet engine. You can reduce fan noise
by using manually adjustable fans, larger fans, or specialty “quiet” fans. Many motherboards enable you to control fans through software.
Manually adjustable fans have a little knob you can turn to speed up or slow down
the fan (Figure 10-35). This kind of fan can reduce some of the noise, but you run the
risk of slowing down the fan too much and thus letting the interior of the case heat up.
A better solution is to get quieter fans. Figure 10-35
Manual fan adjustment device Knob for adjusting fan speed ch10.indd 386 12/11/09 12:22:34 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 387 Larger fans that spin more slowly are another way to reduce noise while maintaining
good airflow. Fans sizes are measured in millimeters (mm) or centimeters (cm). Traditionally, the industry used 80-mm power supply and cooling fans, but today you’ll find
100-mm, 120-mm, and even larger fans in power supplies and cases.
NOTE When shopping for fans, remember your metric system: 80 mm = 8 cm;
120 mm = 12 cm. You’ll find fans marketed both ways. Many companies manufacture and sell higher-end low-noise fans. The fans have better bearings than run-of-the-mill fans, so they cost a little more, but they’re definitely
worth it. They market these fans as “quiet” or “silencer” or other similar adjectives. If
you run into a PC that sounds like a jet, try swapping out the case fans for a low-decibel
fan from Papst, Panasonic, or Cooler Master. Just check the decibel rating to decide
which one to get. Lower, of course, is better.
Because the temperature inside a PC changes depending on the load put on the PC,
the best solution for noise reduction combines a good set of fans with temperature sensors to speed up or slow down the fans automatically. A PC at rest uses less than half of
the power of a PC running a video-intensive computer game and, therefore, makes a lot
less heat. Virtually all modern systems support three fans through three 3-pin fan connectors on the motherboard. The CPU fan uses one of these connectors, and the other
two are for system fans or the power supply fan.
Most CMOS setup utilities provide a little control over fans plugged into the motherboard. Figure 10-36 shows a typical CMOS setting for the fans. Note that you can’t
tell the fans when to come on or offonly when to set off an alarm when they reach a
certain temperature. Figure 10-36 CMOS fan options ch10.indd 387 12/11/09 12:22:34 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 388 Software is the best way to control your fans. Some motherboards come with systemmonitoring software that enables you to set the temperature at which you want the fans
to come on and off. If no program came with your motherboard, and the manufacturer’s Web site doesn’t offer one for download, try the popular freeware SpeedFan utility
(Figure 10-37). Written by Alfredo Milani Comparetti, SpeedFan monitors voltages,
fan speeds, and temperatures in computers with hardware monitor chips. SpeedFan
can even access S.M.A.R.T. information (see Chapter 11, “Hard Drive Technologies”)
for hard disks that support this feature and shows hard disk temperatures, too, if supported. You can find SpeedFan at www.almico.com/speedfan.php. Figure 10-37
SpeedFan CAUTION SpeedFan is a powerful tool that does far more than work with
fans. Don’t tweak any settings you don’t understand!
Even if you don’t want to mess with your fans, always make a point to turn on
your temperature alarms in CMOS. If the system gets too hot, an alarm will warn you.
There’s no way to know if a fan dies other than to have an alarm. When Power Supplies Die
Power supplies fail in two ways: sudden death and slowly over time. When they die
suddenly, the computer will not start and the fan in the power supply will not turn. ch10.indd 388 12/11/09 12:22:34 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 389 In this case, verify that electricity is getting to the power supply before you do anything.
Avoid the embarrassment of trying to repair a power supply when the only problem is
a bad outlet or an extension cord that is not plugged in. Assuming that the system has
electricity, the best way to verify that a power supply is working or not working is to use
a multimeter to check the voltages coming out of the power supply (see Figure 10-38). Figure 10-38
Testing one of the
5-volt DC connections Do not panic if your power supply puts out slightly more or less voltage than its
nominal value. The voltages supplied by most PC power supplies can safely vary by as
much as ±10 percent of their stated values. This means that the 12-volt line can vary
from roughly 10.5 to 12.9 volts without exceeding the tolerance of the various systems
in the PC. The 5.0- and 3.3-volt lines offer similar tolerances.
NOTE Many CMOS utilities and software programs monitor voltage, saving
you the hassle of using a multimeter. Be sure to test every connection on the power supply—that means every connection on your main power as well as every Molex and mini. Because all voltages are
between –20 and +20 VDC, simply set the voltmeter to the 20-V DC setting for everything. If the power supply fails to provide power, throw it into the recycling bin and get
a new oneeven if you’re a component expert and a whiz with a soldering iron. Don’t
waste your or your company’s time; the price of new power supplies makes replacement
the obvious way to go. No Motherboard
Power supplies will not start unless they’re connected to a motherboard, so what
do you do if you don’t have a motherboard you trust to test? First, try an ATX tester. ch10.indd 389 12/11/09 12:22:35 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 390 Many companies make these devices. Look for one that supports both 20- and 24-pin
motherboard connectors as well as all of the other connectors on your motherboard.
Figure 10-39 shows a power supply tester. Figure 10-39
supply tester Switches
Broken power switches form an occasional source of problems for power supplies that
fail to start. The power switch is behind the on/off button on every PC. It is usually secured to the front cover or inside front frame on your PC, making it a rather challenging
part to access. To test, try shorting the soft power jumpers as described earlier. A key or
screwdriver will do the trick. When Power Supplies Die Slowly
If all power supplies died suddenly, this would be a much shorter chapter. Unfortunately, the majority of PC problems occur when power supplies die slowly over time.
This means that one of the internal electronics of the power supply has begun to fail.
The failures are always intermittent and tend to cause some of the most difficult to
diagnose problems in PC repair. The secret to discovering that a power supply is dying
lies in one word: intermittent. Whenever you experience intermittent problems, your
first guess should be that the power supply is bad. Here are some other clues you may
hear from users:
• “Whenever I start my computer in the morning, it starts to boot, and then locks
up. If I press CTRL-ALT-DEL two or three times, it will boot up fine.”
• “Sometimes when I start my PC, I get an error code. If I reboot, it goes away.
Sometimes I get different errors.”
• “My computer will run fine for an hour or so. Then it locks up, sometimes once
or twice an hour.”
Sometimes something bad happens and sometimes it does not. That’s the clue for
replacing the power supply. And don’t bother with the voltmeter; the voltages will show
up within tolerances, but only once in a while they will spike and sag (far more quickly ch10.indd 390 12/11/09 12:22:35 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 391 than your voltmeter can measure) and cause these intermittent errors. When in doubt,
change the power supply. Power supplies break in computers more often than any
other part of the PC except the floppy disk drives. You might choose to keep power supplies on hand for swapping and testing. Fuses and Fire
Inside every power supply resides a simple fuse. If your power supply simply pops and
stops working, you might be tempted to go inside the power supply and check the fuse.
This is not a good idea. First off, the capacitors in most power supplies carry high voltage charges that can hurt a lot if you touch them. Second, fuses blow for a reason. If a
power supply is malfunctioning inside, you want that fuse to blow, because the alternative is much less desirable.
Failure to respect the power of electricity will eventually result in the most catastrophic
of all situations: a fire. Don’t think it won’t happen to you! Keep a fire extinguisher
handy. Every PC workbench needs a fire extinguisher, but make sure you have the right
one. The fire prevention industry has divided fire extinguishers into four fire classes:
• Class A Ordinary free-burning combustible, such as wood or paper
• Class B Flammable liquids, such as gasoline, solvents, or paint
• Class C Live electrical equipment
• Class D Combustible metals such as titanium or magnesium
As you might expect, you should only use a Class C fire extinguisher on your PC if
it should catch fire. All fire extinguishers are required to have their type labeled prominently on them. Many fire extinguishers are multi-class in that they can handle more
than one type of fire. The most common fire extinguisher is type ABC—it works on all
common types of fires. Beyond A+
Power supplies provide essential services for the PC, creating DC out of AC and cooling the system, but that utilitarian role does not stop the power supply from being an
enthusiast’s plaything. Plus, server and high-end workstations have somewhat different
needs than more typical systems, so naturally they need a boost in power. Let’s take a
look Beyond A+ at these issues. It Glows!
The enthusiast community has been modifying, or modding, their PCs for years, cutting
holes in the cases, adding fans to make overclocking feasible, and slapping in glowing
strips of neon and cold cathode tubes. The power supply escaped the scene for a while,
but it’s back. A quick visit to a good computer store off- or online, such as http://directron
.com, reveals power supplies that light up, sport a fancy color, or have more fans than
some rock stars. Figure 10-40 shows a see-through PSU. ch10.indd 391 12/11/09 12:22:35 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 392 Figure 10-40
power supply that
glows blue On the other hand, you also find super-quiet stealth power supplies, with single or
double high-end fans that react to the temperature inside your PC—speeding up when
necessary but running slowly and silently when not. One of these would make a perfect
power supply for a home entertainment PC because it would provide function without
adding excessive decibels of noise. Modular Power Supplies
It’s getting more and more popular to make PCs look good on both the inside and the
outside. Unused power cables dangling around inside PCs creates a not-so-pretty picture. To help stylish people, manufacturers created power supplies with modular cables
power supply ch10.indd 392 12/11/09 12:22:36 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 393 Modular cables are pretty cool, because you add only the lines you need for your system. On the other hand, some techs claim that modular cables hurt efficiency because
the modular connectors add resistance to the lines. You make the choice; is a slight
reduction in efficiency worth a pretty look? Rail Power
When you start using more powerful CPUs and video cards, you can run into a problem I call “rail power.” Every ATX12V power supply using multiple rails supplies only
a certain amount of power, measured in amps (A), on each rail. The problem is with
the 12-V rails. The ATX12V standard requires up to 18 A for each 12-V rail—more than
enough for the majority of users, but not enough when you’re using a powerful CPU
and one or more PCIe video cards. If you have a powerful system, get online and read
the detailed specs for your power supply. Figure 10-42 shows sample power supply
specs. Many power supply makers do not release detailed specsavoid them! Figure 10-42
Sample specs ch10.indd 393 12/11/09 12:22:36 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 394 Look for power supplies that offer about 16 to 18 A per rail. These will be big power
supplies400 W and up. Nothing less will support a big CPU and one or two PCIe
Watch out for power supplies that list their operating temperature at 25ºC—about
room temperature. A power supply that provides 500 W at 25ºC will supply substantially less in warmer temperatures, and the inside of your PC is usually 15ºC warmer
than the outside air. Sadly, many power supply makerseven those who make good
power suppliesfudge this fact. Chapter Review Questions
1. What is the proper voltage for a U.S. electrical outlet?
A. 120 V
B. 60 V
C. 0 V
D. –120 V
2. What voltages does an ATX12V P1 connector provide for the motherboard?
A. 3.3 V, 5 V
B. 3.3 V, 12 V
C. 5 V, 12 V
D. 3.3 V, 5 V, 12 V
3. What sort of power connector does a floppy disk drive typically use?
4. Joachim ordered a new power supply but was surprised when it arrived because
it had an extra 4-wire connector. What is that connector?
A. P2 connector for plugging in auxiliary components
B. P3 connector for plugging in case fans
C. P4 connector for plugging into modern motherboards
D. Aux connector for plugging into a secondary power supply
5. What should you keep in mind when testing DC connectors?
A. DC has polarity. The red lead should always touch the hot wire; the black
lead should touch a ground wire.
B. DC has polarity. The red lead should always touch the ground wire; the
black lead should always touch the hot wire. ch10.indd 394 12/11/09 12:22:36 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10
All-In-One Chapter 10: Power Supplies 395 C. DC has no polarity, so you can touch the red lead to either hot or ground.
D. DC has no polarity, so you can touch the black lead to either hot or neutral
but not ground.
6. What voltages should the two hot wires on a Molex connector read?
A. Red = 3.3 V; Yellow = 5 V
B. Red = 5 V; Yellow = 12 V
C. Red = 12 V; Yellow = 5 V
D. Red = 5 V; Yellow = 3.3 V
7. Why is it a good idea to ensure that the slot covers on your computer case are
A. To maintain good airflow inside your case.
B. To help keep dust and smoke out of your case.
C. Both A and B are correct reasons.
D. Trick question! Leaving a slot uncovered doesn’t hurt anything.
8. A PC’s power supply provides DC power in what standard configuration?
A. Two primary voltage rails, 12 volts and 5 volts, and an auxiliary 3.3 volt
B. Three primary voltage rails, one each for 12-volt, 5-volt, and 3.3-volt
C. One primary DC voltage rail for 12-volt, 5-volt, and 3.3-volt connectors
D. One voltage rail with a 12-volt connector for the motherboard, a second
voltage rail with a 12-volt connector for the CPU, and a third voltage rail for
the 5-volt and 3.3-volt connectors
9. What feature of ATX systems prevents a user from turning off a system before
the operating system’s been shut down?
A. Motherboard power connector
B. CMOS setup
C. Sleep mode
D. Soft power
10. How many pins does a SATA power connector have?
D. 15 ch10.indd 395 12/11/09 12:22:36 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 10 CompTIA A+Certification All-in-One Exam Guide 396 Answers
1. A. US outlets run at 120V.
2. D. An ATX12V power supply P1 connector provides 3.3, 5, and 12 volts to the
3. B. Floppy drives commonly use a mini connector.
4. C. The P4 connector goes into the motherboard to support more power-hungry
5. A. DC has polarity. The red lead should always touch the hot wire; the black
lead should touch a ground wire.
6. B. A Molex connector’s red wires should be at 5 volts; the yellow wire should be
at 12 volts.
7. C. Both A and B are correct reasons: Keeping the slots covered helps keep a good
airflow in your case and keeps the dust and smoke away from all those sensitive
8. B. The standard PC power supply configuration has three primary voltage rails,
one each for 12-volt, 5-volt, and 3.3-volt connectors.
9. D. The soft power feature of ATX systems prevents a user from turning off
a system before the operating system’s been shut down.
10. D. SATA power connectors have 15 pins. ch10.indd 396 12/11/09 12:22:37 PM ...
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This note was uploaded on 04/27/2010 for the course COMPTIA 1201 taught by Professor N/a during the Spring '10 term at Galveston College.
- Spring '10