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Chapter 22, The basics of electrical energy, covers electricity and how it relates to the computer. A computer technician does not need to be an electrical engineer, but does need to be able to perform basic tests and to work safely. This chapter provides the reader with background in these issues.
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 Terminology Terminology (Cont.) 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. Personal Computers and Electrical Power Alternating Current (AC) AC power is manmade, 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 AC Power Uses Three Wires Direct Current (DC) AC is used for transporting lowcost 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 stepdown 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: AC voltage DC voltage Continuity Resistance A multimeter consists of two probes and a multiposition switch to select the type of test. 10 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. 29 Transistors 30 Transformers A stepdown transformer decreases the transformer’s voltage on the output side; a stepup transformer increases it In the PC power supply, the transformer’s secondary coils provide 12volt, 5volt, and 3.3volt outputs used by various components. 31 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. 32 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. 33 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. 34 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. 35 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 36 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. 37 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 38 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. 39 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. 40 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. 41 Common AC Wiring Color Codes in the United States 42 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. 43 ... oxide semiconductor (CMOS) settings The destruction of data on massstorage devices 21 Two Types of Tests for Power Supplies 22 Basic Power Supply Voltage Test The only purpose of this test is to verify the existence and value of voltages