Chapter Overview • Electrical power • Electrostatic discharge • Safety and electrical power
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
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.
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.
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.
Measuring Electricity • 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. • AC voltage • DC voltage • Continuity • Resistance
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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).
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.