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Mike Meyers’ A+ Guide to Managing and Troubleshooting PCs Instructor’s Manual
Chapter 8
Chapter 8
Power Supplies
Learning Objectives
In this lesson, you’ll identify various power supplies and install and test power supplies. By the end of
the lesson, you should be able to do the following:

Explain the basics of electricity

Describe the details about powering the PC

Install, maintain, and troubleshoot power supplies
Pre-test
1.
Volts × amperes = what?
2.
Why should you test the AC outlet before you plug in your PC or before you ground yourself to
it?
3.
Why should you be concerned with how many watts a power supply can output?
4.
What are some things to check if the system doesn’t power up?
5.
Are motherboards powered by DC or AC power?
Lecture Outline
Historical/Conceptual
I.
Understanding Electricity
A. Electricity: The flow of negatively charged particles (electrons) through a conductor.
B. Current: The amount of electrons flowing measured as amperes, or amps.
C. Voltage: The “pressure” of the electrons flowing in the wire measured in volts.
D. Wattage: The amount of amps and volts needed by a particular device (measured in watts).
E. Relationship between voltage, current, and wattage is expressed mathematically as V × A = W.
F.
Resistance to the flow of electrons is measured in ohms.
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G. Two types of electricity
1.
Direct current (DC): Electricity flows in one continuous direction around the circuit. It is
required by PC components.
2.
Alternating current (AC): Electricity flows back and forth in the circuit. It is supplied by
the power company.
Discussion Topic
Power supply
All PCs run on DC. All utilities companies provide AC, either as 110–120 VAC or 220–230
VAC, depending on the country you are in. The U.S. uses 110–120 (commonly called
115 VAC). Other countries use 220–230 VAC (commonly called 230 VAC). Most power
supplies have a switch in the back that can be set to either voltage. Setting it to 230
VAC in the U.S. gives it only half the power it needs (it’s expecting 230, but the wall
socket provides 115). It simply doesn’t work. However setting it to 115 in a country that
provides 230 will damage the power supply.
Essentials
II. Powering the PC
A. Supplying AC
1.
Power supply attaches to the AC outlet and converts AC to DC.
2.
AC provided by electric company.
a)
In U.S., AC is ~115 V
b) In other countries, AC is ~230 VAC
3.
An improperly wired AC outlet can destroy the PC and the testing equipment, or cause
electrocution.
a)
AC outlet has three holes:
(1) Large hole is the neutral.
(2) Small hole is the hot.
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(3) Round hole is the ground.
b) From hot to neutral or hot to ground should measure ~115 V.
c)
From neutral to ground should measure 0 V.
d) If readings are within range, you have a good and safe AC power supply. If you do not
read 0 V from neutral to ground, you should call an electrician.
4.
Test the AC outlet using a multimeter or volt-ohm meter (VOM), which may be analog or
digital.
a)
Most multimeters provide four kinds of measurements: AC voltage, DC voltage,
continuity, and resistance.
(1) Continuity tests to see if electricity can flow from one end of the wire or
component to the other, or said another way, it checks to see whether the wire is
continuous and not broken. Be sure to turn the power off before testing for
continuity.
(2) Resistance is measured in ohms. Parts being tested for resistance must not have
power. You can use resistance to check continuity. While “infinite” resistance
has no continuity (what you’d expect from a broken wire), “0” resistance has
continuity.
b) Multimeters have two probes: the black lead is considered “negative” and the red lead
is considered “positive.” Multimeters may display results either as digital or analog.
Discussion Point
Testing equipment
A decent multimeter (or VOM) can be purchased inexpensively at most electronics or
hardware stores; it can test for AC and DC voltage, amperage, continuity, and
resistance. Also available is a less-expensive circuit tester to test your AC outlet. This
simply plugs into the AC outlet and checks for proper voltage, proper wiring, and proper
grounding. A series of LED lights on the module shows the results.
5.
A circuit tester can test the wiring of the wall outlet. It is a small plug that inserts into a
wall outlet and indicates the status of the power with a series of LED lights.
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6.
Chapter 8
Two devices are essential for handling sags and spikes in the AC supply:
a)
Surge suppressor
(1) Power surges (where power exceeds 120 VAC) can cause damage to the power
supply.
(2) Surge suppressors are inserted between power supply and the outlet to protect
the system from power surges.
(3) Rated in joules—unit of electrical energy. More joules gives better protection
against higher surges.
(4) A surge suppressor has a joule rating to indicate how much energy it can handle
before it fails. It should have at least 800 joules, but the more, the better.
(5) Each surge suppressor should be replaced as recommended by the manufacturer.
A surge suppressor will generally continue to supply power even after it has lost
its protecting ability. By then, it is little more than an extension cord with
multiple outlets.
b) Uninterruptible power supply (UPS): Select UPS based on the wattage you need to
power the PC, monitor, external drives, and modem long enough to protect your data
from being lost.
(1) Measured in watts and volt-amps
(2) Acts as surge suppressor and power conditioner
B. Supplying DC
1.
Power supply to motherboard and internal devices. Modern motherboards use more than one
connector for power:
a)
P1 socket (20 or 24 wires)
b) Extra power connector (4, 6, or 8 wires)
2.
Power to peripherals:
a)
Molex supplies 5 or 12 V. It is the most common type and connects to most internal
devices such as hard drives, CD-ROMs, and ZIP drives.
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b) Mini supplies 5 or 12 V. It is commonly used only for 3.5-inch floppy drive.
c)
SATA 15-pin power connector provides 3.3, 5, 12 V.
d) Splitters and adapters may be used.
(1) Splitter can convert one connector to two.
(2) Adapter can convert Molex connector to SATA connector.
3.
Power supply standards
a)
ATX
(1) Previous standard
(2) Two distinguishing features: P1 motherboard power connector and soft power
(3) Soft power: 5 V constantly provided to motherboard allows computer to control
power. Features set in BIOS
b) ATX12V 1.3. First widespread update to ATX standard. Introduced P4 4-pin
connector supplying 12 V. Also added AUX connector—6-pin 3.3 V and 5 V
connectors
c)
EPS12V. Provided extra power often needed by server motherboards. Used 24-pin P1,
an ATX12V P3, an AUX connector, and a unique 8-pin connector.
d) Rails
(1) Traditional power supplies have three rails: 12 V, 5 V, and 3.3 V.
(2) EPS12V divided 12 V rail into multiple 12 V rails. Remember 12 V used for
motors such as those needed for hard drives. Multiple 12 V rails provides
support for more hard drives.
e)
ATX12V 2.0
(1) Incorporated many of the ideas of the EPS12V into ATX supplies
(2) 24-pin connector is backward-compatible with 20-pin connector.
(3) Has two 12 V rails for power supplies rated at higher than 230 watts
(4) Dropped AUX connector and added SATA connectors
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IT Technician
f.
Niche market power supply form factors
(1) Many different power supplies currently available. Different power supplies are
targeted toward specific needs of specific form factors.
(2) Active Power Factor Correction (PFC). Eliminates harmonics that can damage
electrical components. Feature found in most quality power supplies.
g.
Wattage requirements
(1) Make sure the power supply provides sufficient wattage for all the devices in the
PC.
(2) Power supplies typically are only about 70 percent efficient. Don’t expect fully
400 watts capability out of 400 watt power supply.
(3) Recommended wattage is at least 400 watts.
III. Installing, Maintaining, and Troubleshooting Power Supplies
A. Installing power supplies. Most power supplies have only four screws holding them in. Remove
the screws to replace the power supply.
Safety Alert
Remember, the ATX Power Supply never turns off as long as it is plugged in. It continuously
provides 5 volts to the motherboard. Always unplug an ATX system before you do any
work.
B. Cooling
1.
Electricity causes heat, so cooling techniques are essential for the computer.
a)
Power supply fan provides basic cooling.
b) Case fans provide supplemental cooling.
c)
Biggest issue related to case fans is where to plug them in. Some have Molex
connectors to plug directly into power supply Molex connector. Others have threepronged connectors that connect to motherboard.
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2.
Chapter 8
Maintaining airflow
a)
Keep case closed. Open case prevents flow of air over components.
b) Replace slot covers if devices are removed. Open slots contribute to loss of airflow
and contribute to dust and smoke getting into the case.
3.
Reducing Fan Noise
a)
Cheaper fans (with cheaper bearings) are noisier. Can be replaced with more
expensive quieter fans. Larger fans that spin slower can also be used.
b) Manual fan adjustment devices can be installed inline to slow down the speed of a fan
manually.
c)
CMOS settings allow you to monitor health of PC by observing speed of fans.
Safety Alert
Danger in opening power supply
Although it is possible to replace the power supply cooling fan, be careful. The power supply
has capacitors that store power for a long time even after the power supply is
unplugged. It is dangerous to touch any electrical components inside the power supply.
It is not a good idea to open the power supply at all, even to replace a fuse.
C. When power supplies die
1.
Power supplies die in two ways:
a)
Sudden death. When the fan doesn’t turn and no voltage is present, verify that you
have good AC power first and then replace the power supply.
b) Slowly over time. Manifests with intermittent errors. It is more difficult to diagnose
because it may show good voltage since power supply may cause a small sag or spike
in voltage that is too quick to be seen even by a multimeter. Computers with dying
power supplies may fail to boot, display error codes, and experience intermittent
crashes.
Discussion Point
Faulty power supplies
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Power supplies fail more often than almost any other component in a PC except for floppy
drives. When you identify a bad power supply, it is generally cheaper to replace the
power supply considering the time and effort it would take to repair it.
2.
Power supplies need a load to test
a)
Connect the power supply to a motherboard.
b) Connect the power supply to a tester.
Discussion Point
Troubleshooting the power supply
The power supply must be “under a load” for it to be tested. Something must be hooked up to
it—either a tester or a motherboard—to test it properly. A faulty motherboard or faulty
device can shut down the power supply. If you do not detect power at the Molex
connectors, first turn off the power and disconnect all devices except the motherboard
and power backup. If you get power, methodically plug in one device at a time until you
no longer get power. Cut power each time before you plug in a new device. When you
discover the unit killing your power, you have discovered your problem. If you unplug all
your devices (except the motherboard) and you still have no power, try plugging in a few
devices and unplugging the motherboard. If you get power, the motherboard was the
cause for not getting power. If a power supply stopped working after you added a new
device to your system, check the wattage rating of the power supply. It may not be able
to provide enough power for all your devices.
3.
Broken power switches can make the power supply look faulty. Consider shorting the soft
power jumpers to verify.
D. Fuses and fire
1.
Fuses blow for a reason and may prevent fires.
2.
Fire extinguishers are divided into three classes. A Class C fire extinguisher is used for live
electrical equipment.
IV. Beyond A+
A. You can get upgrades for power supplies for aesthetic reasons, such as glowing power supplies.
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B. Modular power supplies allow users to add only the power cables needed.
C. Rail power: be aware of how much power is provided to 12 V rails. Minimum of 16 to 18 A
needed for each 12 V rail.
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