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Chapter Overview

Electrical power Electrostatic discharge Safety and electrical power

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Understanding Electricity and Electrical Energy

Electricity refers to 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

Ohm’s law states that voltage equals the product of the current times the resistance. 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) 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) 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 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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A multimeter is an essential troubleshooting tool for a computer professional.

A multimeter measures several aspects of electricity:

A multimeter consists of two probes and a multiposition switch to select the type of test.

Measuring Electricity

AC voltage DC voltage

Continuity Resistance

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Testing AC Power Failure to properly test AC outlets can

damage or destroy equipment. When testing an AC power source,

check these three things: 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

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

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

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 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

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 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 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 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 Many computer problems blamed on

the operating system (OS) are really power-related 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 power-related problems.

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Symptoms of a Bad Power Supply

Intermittent lockups Unexpected computer reboots Erratic problems during booting Changed or erased complementary

metal-oxide 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

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 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 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

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

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

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)

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

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. 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 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

Components are becoming smaller and operate at lower voltages.

These components are more susceptible to damage from ESD.

ESD causes three categories of problems: Catastrophic failure: “frying” or “smoking” a

part Upset failure: erratic fault in a component Latent failure: weakened transistor

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Preventing ESD 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. “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 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

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 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.) 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 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.