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Chapter Overview
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Electrical power
Electrostatic discharge
Safety and electrical power
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Understanding Electricity and
Electrical Energy
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Electricity refers to
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The form of energy associated with moving
electrons and protons
The energy made available by the flow of electric
charge through a conductor
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Terminology
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Terminology (Cont.)
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Ohm’s Law
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Ohm’s law states that voltage equals the
product of the current times the resistance.
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Resistance: R=V/I
Current: I=V/R
Volts: V=IR
In these formulas, R = resistance in ohms,
V = voltage, and I = current in amperes.
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Personal Computers and Electrical
Power
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Alternating Current (AC)
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AC power is man-made, by use of generators.
A wire coil inside a generator rotates and
produces current that flows from one pole of
a magnet to the other, or alternates the
direction of the flow.
The number of revolutions made by the
generator is called its frequency.
In the United States, power companies run
their systems at 60 turns per second to
produce a high voltage of 60 Hz (cycles per
second) AC as they rotate.
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AC Power Uses Three Wires
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Direct Current (DC)
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AC is used for transporting low-cost power to
users, but a computer’s electronic
components require DC power.
A PC power supply’s main function is to
convert AC into DC. It does this by
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Using a step-down transformer
Using an AC/DC converter
DC flows in one direction from one pole (+)
to another (-) and thus is said to have
polarity.
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Measuring Electricity
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A multimeter is an essential troubleshooting
tool for a computer professional.
A multimeter measures several aspects of
electricity:
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AC voltage
DC voltage
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Continuity
Resistance
A multimeter consists of two probes and a
multiposition switch to select the type of test.
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Testing AC Power
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Failure to properly test AC outlets can
damage or destroy equipment.
When testing an AC power source, check
these three things:
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Is the hot wire sending the correct voltage, and is
it wired to the correct pin?
Is the neutral wire connected to ground and to the
correct pin?
Is the ground wire connected to ground and to the
correct pin?
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Setting Up a Multimeter
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Attach the black test lead to the negative (-)
marked hole.
Attach the red test lead to the volts (+) hole.
Set the selector switch to AC volts.
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Testing AC Outlets with a Multimeter
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Hot to neutral. Place one lead in hot and
the other in neutral. The reading should be
between 110 and 120 volts AC (VAC).
Hot to ground. Place one lead in hot and
the other in ground. The reading should be
between 110 and 120 VAC.
Neutral to ground. Place one lead in
neutral and the other in ground. The reading
should be 0 volts.
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Using AC Testers
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AC testers are small devices made especially
for testing outlets.
Inserting the tester into the outlet tests all
voltages at once.
Although not as accurate as a multimeter, an
AC tester is more convenient.
It provides a pass or fail indication rather
than an accurate voltage reading.
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AC Ripple
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When a power supply is working properly, it
produces a pure DC signal.
As a power supply ages, its ability to produce
pure DC falters.
Power supplies use electrolytic capacitors to
filter or smooth voltage after conversion.
When an electrolytic capacitor fails, it allows
more AC voltage through. This is called noise
or ripple.
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Testing for AC Ripple
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Set a multimeter to read AC.
Connect a .1 µfd (microfarad) capacitor to the
red lead.
With the power turned on, measure the DC
voltage.
Any ripple present is displayed as AC voltage.
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Testing Resistance
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Resistance, which is measured in ohms, is the
opposition to the flow of current through a conductor.
Place one lead of the meter on each side of the
circuit to measure resistance.
Measuring a component soldered in its circuit can
yield inaccurate readings.
Test resistance with the power off; do not connect
the meter to an electrical outlet.
If the meter is set too high or the resistance is too
high, the reading is inaccurate.
Before you take a reading, ensure that any charge
stored in a capacitor is properly discharged.
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Testing Continuity
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Continuity indicates if a connection exists between
two points in a circuit.
Continuity testing is used to determine the presence
of breaks in wires and electrical circuits.
If no continuity setting is available, you should use
the resistance setting.
Infinite resistance indicates that there is no continuity
and that there is a break in the line.
Little or no resistance indicates that there is
continuity and the circuit is complete.
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Testing DC Voltage
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Testing DC voltage is similar to testing AC voltage.
DC voltage has a positive pole (+) and a negative
pole (-).
It is important to place the positive (red) lead on the
positive side and the negative (black) lead on the
negative side of the circuit.
If leads are positioned backward, the reading gives
the incorrect polarity.
Connecting leads backward can damage an analog
meter.
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Testing a Power Supply
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Many computer problems blamed on the
operating system (OS) are really powerrelated problems.
Using an uninterruptible power supply (UPS)
can condition the line and resolve these
problems.
Test the power supply for proper functioning
before adding a UPS.
Find out if the client has any other powerrelated problems.
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Symptoms of a Bad Power Supply
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Intermittent lockups
Unexpected computer reboots
Erratic problems during booting
Changed or erased complementary metaloxide semiconductor (CMOS) settings
The destruction of data on mass-storage
devices
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Two Types of Tests for Power
Supplies
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Basic Power Supply Voltage Test
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The only purpose of this test is to verify the
existence and value of voltages.
With time, most power supplies show a
reduction in voltage.
The drop in voltage shows in both the 5-volt
and 12-volt outputs.
The drop is more pronounced in the 12-volt
output.
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Preparing the Meter and Testing the
Voltages
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Connect the black lead to the common (-) connector
and the red lead to the voltage (+) connector.
Turn the test selector to DC volts. If the meter has an
AC/DC switch, set it to DC.
Place the meter’s black (ground) lead on the black
wire connection and its red (positive) lead on the
yellow (+12-volt) connection.
Record the voltages. A good power supply provides
11–13 volts DC.
Replace the power supply if the voltage reading is
less than 10.
If no voltage is present, check for problems other
than the power supply.
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Isolating the Problem
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Disconnect the Molex leads from the power supply.
Connect the meter leads.
Turn off the AC power.
Disconnect all the Molex plugs from the devices.
Turn the power back on and test for power on the
motherboard.
Reconnect each Molex plug, one at a time, and test
the power each time.
Identify the offending device (the one connected to
the meter when the power drops out).
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Power Supply Operation
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Fuse
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Capacitor
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Rectifiers and Diodes
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A rectifier is a device that converts AC power
into a DC form (rectification).
A diode is a device that lets current flow in
only one direction.
Test for resistance across both leads of the
diode with the power off.
A good diode exhibits low resistance in one
direction and high resistance in the other.
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Transistors
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Transformers
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A step-down transformer decreases the
transformer’s voltage on the output side; a
step-up transformer increases it.
In the PC power supply, the transformer’s
secondary coils provide 12-volt, 5-volt, and
3.3-volt outputs used by various components.
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Testing a Transformer
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Disconnect the power.
Discharge all capacitors.
Ensure that all leads have been disconnected.
Configure the multimeter to measure
continuity (or resistance).
Simultaneously touch each lead of the
multimeter to one of the pairs of contacts.
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Inductors (Coils)
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Inductors (also called coils) are loops of
conductive wire.
Current passing through the inductor sets up
a magnetic field.
Inductors are tested for continuity in the
same way as a transformer.
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Testing a Coil
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Visually check the wire for deterioration.
Turn the system power off and then
disconnect one lead to the coil.
Connect one meter lead to each end of the
coil.
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A null or low reading indicates continuity.
A reading of high or infinite resistance indicates
the coil should be replaced.
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Electrostatic Discharge
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The human body can generate a tremendous
amount of voltage, called static electricity.
Electrostatic discharge (ESD) is the transfer of
electrons from one object to another.
Buildup of energy with nylon clothes can
easily reach 21,000 volts.
About 750 volts can produce a visible spark.
A mere 10 volts can ruin a computer chip.
External factors determine the amount of
energy released.
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ESD Damage
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Components are becoming smaller and
operate at lower voltages.
These components are more susceptible to
damage from ESD.
ESD causes three categories of problems:
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Catastrophic failure: “frying” or “smoking” a part
Upset failure: erratic fault in a component
Latent failure: weakened transistor
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Preventing ESD
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The leading cause of ESD damage is improper
handling of electronic devices.
The key to ESD prevention is to keep all
electronic components and yourself at a
common electrical potential.
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“Ground” yourself by touching the metal computer
chassis.
Do not move around while installing or handling a
part.
Use ESD suppression devices when working with
exposed parts.
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Antistatic Devices
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Antistatic mat: a nonconducting pad placed
on the work surface
Antistatic wristband: a wristband with a
grounding strap that connects to the PC
chassis
Antistatic pouch: a sealed, nonconducting
pouch used to store electronic devices
Antistatic pad: an insulating foam pad for
individual chips with exposed pins
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Electrical Safety Is Your
Responsibility
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Standard wall outlets in the United States
provide a nominal 120 VAC.
You can receive a lethal shock from much
lower voltages than 120 VAC.
Inside a computer and a monitor, voltages as
high as 30,000 volts can exist, even after the
power is turned off.
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Safety Guidelines
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If you are not sure how to safely service a
part of the computer, do not do it.
Always use grounded outlets and power
cords.
Switch the power off and disconnect all
equipment from its power source before
removing the cover.
Always replace blown fuses with fuses of the
correct rating and type.
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Safety Guidelines (Cont.)
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Do not work alone. You might need help in an
emergency.
Remove all jewelry and wristwatches. They
can cause short circuits.
Have trained personnel service computer
power supplies and monitors.
Work with one hand. Using two hands can
cause a direct circuit, via your heart, from
one object to another.
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Common AC Wiring Color Codes in
the United States
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Chapter Summary
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Ohm’s law states that voltage = current  resistance.
Electricity is delivered as AC; computers use DC.
Electricity always seeks the path of least resistance to ground.
For safety reasons, you should always use an electrical outlet or
extension cord with a ground wire with a PC.
A multimeter measures electrical voltage, current, resistance,
and continuity.
Familiarity with electronic components is important to a
computer technician.
ESD can damage computer parts, but it is easy to prevent.
Follow safety guidelines when working with electrical
components.
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