Bài giảng Computer - Chapter 22

Chapter Overview Electrical powerElectrostatic dischargeSafety and electrical power 1Understanding Electricity and Electrical Energy Electricity refers to The form of energy associated with moving electrons and protonsThe energy made available by the flow of electric charge through a conductor 2Terminology 3Terminology (Cont.)4Ohm’s Law Ohm’s law states that voltage equals the product of the current times the resistance. Resistance: R=V/ICurrent: I=V/RVolts: V=IR In these formulas, R = resistance in ohms, V = voltage, and I = current in amperes. 5Personal Computers and Electrical Power6Alternating 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. 7AC Power Uses Three Wires8Direct 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 transformerUsing an AC/DC converter DC flows in one direction from one pole (+) to another (-) and thus is said to have polarity. 9A 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 voltageDC voltageContinuityResistance10Testing 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? 11Setting 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. 12Testing 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. 13Using 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. 14AC 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. 15Testing 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. 16Testing 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. 17Testing 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. 18Testing 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. 19Testing 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. 20Symptoms of a Bad Power Supply Intermittent lockupsUnexpected computer rebootsErratic problems during bootingChanged or erased complementary metal-oxide semiconductor (CMOS) settingsThe destruction of data on mass-storage devices 21Two Types of Tests for Power Supplies22Basic 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. 23Preparing 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. 24Isolating 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). 25Power Supply Operation26Fuse27Capacitor28Rectifiers 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. 29Transistors30Transformers 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. 31Testing 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. 32Inductors (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. 33Testing 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. 34Electrostatic 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. 35ESD 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 partUpset failure: erratic fault in a componentLatent failure: weakened transistor 36Preventing 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. 37Antistatic 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 devicesAntistatic pad: an insulating foam pad for individual chips with exposed pins 38Electrical 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. 39Safety 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. 40Safety 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. 41Common AC Wiring Color Codes in the United States42Chapter 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.  43