Chapter 22, The Basics of Electrical Energy
|1|Chapter Overview
A.Electrical power
B.Electrostatic discharge
C.Safety and electrical power
Chapter 22, Lesson 1
Power
|2|1.Understanding Electricity and Electrical Energy
A.Electricity refers to
1.The form of energy associated with moving electrons and protons
2.The energy made available by the flow of electric charge through a conductor
|3|B.Definitions
1.Electricity: the form of energy associated with charged particles, usually electrons
2.Electric charge: the fields generated when charged particles move in tandem, thereby producing energy
3.Electrical circuit: the path taken by an electrical charge
4.Electrical current (also known as electron flow): when an electric charge is carried, or flows through a conductor, it is known as a current.
5.Power: the rate at which an amount of energy is used to accomplish work
a.Electrical power is measured in watts, which is determined by multiplying voltage by current.
6.Conductors: materials that can carry an electrical current
a.Most are metals.
|4|7.Resistance: a quality of some materials that allows them to slow the speed of an electrical current, thereby producing heat and sometimes light
8.Insulators: materials that prevent or retard the electrical current of electrons
9.Ampere: a measurement of current strength, equal to 1 coulomb per second
10.Coulomb’s law: a law of physics stating that two charges will exert equal and opposite forces on each other. Opposite charges attract and like charges repel.
11. Ohm: a unit of electrical resistance
12.Volt: the unit of electromotive force, or potential energy, that, when steadily applied against a resistance of 1 ohm, produces a current of 1 ampere
12.Voltage: the potential energy of a circuit
|5|C.Ohm’s law
1.Computer professionals should know the formula known as Ohm’s law.
2.Ohm’s law states that voltage is equal to the product of the current times the resistance, or voltage = current x resistance.
a.Resistance: R=V/I, where R = resistance in ohms, V = voltage, and I = current in amperes
b.Current: I=V/R
c.Volts: V = IR
3.By memorizing any one of these formulas, you can easily derive the other two.
|6|2.Personal Computers and Electrical Power
A.A PC’s electrical power can come from
1.A wall outlet, in the form of AC
2.A battery, in the form of DC
|7|B.AC powers most of our lights and household appliances through wall outlets.
1.AC power is man-made, by use of generators.
2.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.
3.The number of revolutions made by the generator is called its frequency.
4.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.
|8|C.AC power is delivered using three wires.
1.Two of the wires are hot (meaning that they carry a charge).
2.One wire (running from the breaker box to the power pole) is neutral.
3.The measured voltage between two hot wires is between 220 and 240 volts AC (VAC).
4.The measured voltage between either hot wire and the neutral wire is between 110 and 120 VAC.
5.These voltages are called nominal because they can vary by plus or minus 10 percent.
6.Typical electrical outlets are connected between one of the hot wires and the neutral wire.
7.Wall power outlets use a three-prong connection where
a.The small rectangular hole is hot
b.The large rectangular hole is neutral
c.The small round hole is the ground
(1)The ground wire is used as a safety wire.
(2)If a short circuit occurs, a large flow of current is discharged all at one time.
(3)Electricity follows the path of least resistance to ground.
(4}By following the ground wire, the charge dissipates safely, causing less damage.
8.Ensure that the ground wire is properly installed.
a.A short circuit can cause physical damage to equipment and personnel.
b.The ground plug provides a direct connection to ground, giving the electricity an alternate path away from equipment and people.
9.It is unacceptable to use the following with a computer:
a.An electrical outlet without grounded plugs and the third ground wire
b.An extension cord without a ground wire
|9|D.DC power is used by a computer’s electronic components.
1.AC is used for transporting low-cost power to users, but a computer’s electronic components require a steady stream of DC power.
2.The PC power supply’s main function is to convert AC into DC. It does this by
a.Using a step-down transformer
b.Using an AC/DC converter
3.The AC adapters for laptops, ink-jet printers, and other consumer electronics turn AC into lower-voltage DC.
4.DC flows in one direction from one pole (+) to another (-), and thus is said to have polarity.
|10|3.Measuring Electricity
A.The multimeter
1.Is an essential troubleshooting tool for a computer professional
2.Is used to measure several aspects of electricity:
a.AC voltage
b.DC voltage
c.Continuity
d.Resistance
3.Consists of two probes:
a.An analog or digital meter
b.A multiposition switch to select the type of test
4.Is also called a volt-ohm meter (VOM) or a digital volt-ohm meter (DVOM)
|11|B.Testing AC power
1.Failure to properly test AC outlets can damage or destroy equipment or injure or electrocute the computer technician.
2.When testing an AC power source, check these three things:
a.Is the hot wire sending the correct voltage, and is it wired to the correct pin?
b.Is the neutral wire connected to ground and to the correct pin?
c.Is the ground wire connected to ground and to the correct pin?
|12|C.Setting up a multimeter
1.Attach the black test lead to the negative (-) marked hole.
2.Attach the red test lead to the volts (+) hole.
a.If this lead is placed in the wrong hole (ohm or amp), it can permanently damage the meter.
3.Set the selector switch to AC volts.
a.If there are several selections, use the highest setting possible (if the voltage is unknown), or select one level higher than the estimated voltage.
b.Select 200 VAC for standard household outlets.
c.Some digital meters use “auto-range” and don’t need any selection except AC volts.
|13|D.Testing AC outlets with a multimeter
1.Hot to neutral
a.Place one lead in hot (smaller of two vertical slots) and the other in neutral (larger of the vertical slots).
b.The reading should be between 110 and 120 VAC.
2.Hot to ground
a.Place one lead in hot (smaller of the two vertical slots) and the other in ground.
b.The reading should be between 110 and 120 VAC.
3.Neutral to ground
a.Place one lead in neutral (larger of the two vertical slots) and the other in ground.
b.The reading should be 0 volts.
|14|E.Using AC testers
1.AC testers are small devices made especially for testing outlets.
2.Inserting the tester into the outlet tests all voltages at once.
3.Although not as accurate as a multimeter, an AC tester is more convenient.
4.Provides a pass or fail indication rather than an accurate voltage reading
|15|F.AC ripple
1.When a power supply is working properly, it produces a pure DC signal.
2.As a power supply ages, its ability to produce pure DC falters.
3.Power supplies use electrolytic capacitors to filter or smooth the voltage after conversion.
4.When an electrolytic capacitor fails, it allows more AC voltage through. This is called noise or ripple.
|16|5.Testing for AC ripple
a.Set a multimeter to read AC.
b.Connect a .1 µfd (microfarad) capacitor to the red lead.
c.With the power turned on, measure the DC voltage.
d.Any ripple present is displayed as AC voltage.
|17|G.Testing resistance
1.Resistance is an opposition to the flow of current through a conductor and is measured in ohms.
2.Place one lead of the meter on each side of the circuit to measure resistance.
3.Measuring a component that is soldered in its circuit can yield inaccurate readings.
4.You should test resistance with the power off.
5.A meter that is set up to read resistance can be damaged if it is connected to an electrical outlet.
6.If the meter is set too high or the resistance is too high, you get an inaccurate reading.
7.Before taking a reading, ensure that any charge stored in a capacitor is properly discharged.
|18|H.Testing continuity
1.Continuity indicates whether or not a connection exists between two points in a circuit.
2.Used to determine the presence of breaks in wires and electrical circuits
3.If no continuity setting is available, use the resistance setting.
4.If the multimeter measures infinite resistance, there is no continuity.
a.This indicates a break in the line.
5.If the multimeter shows little or no resistance, there is continuity and the circuit is complete.
|19|I.Testing DC voltage
1.Testing DC voltage is similar to testing AC voltage, but with one important difference: DC voltage is sensitive to polarity.
2.DC voltage has a positive pole (+) and a negative pole (-).
3.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.
4.If leads are positioned backward, the polarity of the reading is opposite what it should be.
5.If you use an analog meter, connecting the leads backward could cause damage to the meter.
|20|4.Testing a Power Supply
A.Many computer problems blamed on the operating system (OS) are really power problems.
B.Using an uninterruptible power supply (UPS) can condition the line and resolve these problems.
1.Test the power supply for proper functioning before adding a UPS.
C.Find out if the client has any other power-related problems such as flickering lights, or a power strip that is overloaded with too many electronic devices.
|21|D.A bad power supply can cause
1.Intermittent lockups
2.Unexpected computer reboots
3.Erratic problems during booting
4.Changed or erased complementary metal-oxide semiconductor (CMOS) settings
5.The destruction of data on mass-storage devices
|22|E.Two types of tests for power supplies
1.Basic test is used to verify voltages.
2.Advanced test is used to check the internal components.
|23|F.Basic power supply voltage test
1.The only purpose of this test is to verify the existence and value of voltages.
2.With time, most power supplies show a reduction in voltage.
3.The drop in voltage shows in both the 5-volt and 12-volt outputs.
4.The drop is more pronounced in the 12-volt output.
|24|5.Preparing the meter and testing the voltages
a.Connect the black lead to the common (-) connector and the red lead to the voltage (+) connector.
b.Turn the test selector to DC volts.
(1)If the meter has an AC/DC switch, set it to DC.
(2)If the meter uses “auto-range,” set the range to 15–20 volts.
c.For P8/P9 (ATX power connectors), place the meter’s black (ground) lead on the black wire connection and its red (positive) lead on the yellow (+12-volt) connection.
d.Record the voltages.
(1)A good power supply provides a voltage between 11–13 volts DC.
e.Replace the power supply if the voltage reading is less than 10.
f.If no voltage is present, the problem might not be the power supply.
(1)Another piece of hardware might be causing an excessive load on the system.
|25|6.Isolating the problem
a.Disconnect the Molex leads from the power supply.
b.Connect the meter leads.
c.Turn off the AC power.
d.Disconnect all the Molex plugs from the devices.
e.Turn the power back on.
(1)If power is present at the motherboard, one of the devices is bad and is draining the power supply.
f.Reconnect each Molex plug, one at a time, and test the power. When the power drops out, you have located the offending device.
|26|G.Advanced testing requires a working knowledge of power supply operation.
1.There are three sections to a power supply: the switching network, the transformer, and the voltage regulator.
2.The switching network
a.Smoothes out and conditions the line against spikes, which are sudden increases in voltage; and sags, which are decreases in voltage.
b.The main components are
(1)Fuse
(2)Capacitors
(3)Rectifiers
(4)Switching transistors
3.The transformer reduces the voltage of the square wave DC into separate 12-volt and 5-volt square wave AC circuits.
4.The voltage regulator
a.Receives low-voltage AC outputs of the transformer and converts them to clean DC power
b.Performs the following functions:
(1)Uses rectifiers to convert AC output to DC output
(2)Regulates the voltage and uses capacitors to remove ripples
(3)Monitors the amount of current used by the computer circuits and adjusts for variations in load on the power supply
|27|5.Electronic Components
A.A fuse serves one purpose: to fail and cut the flow of power if a current load exceeds the safe capacity that system components can absorb.
1.A PC fuse is almost always a small, clear, glass tube with metal caps on each end and a wire connecting the two caps.
a.The thicker the wire, the more current a fuse can conduct before failing.
2.If the wire melts or breaks, the fuse is “blown.”
a.If a fuse fails, first try to replace it with another fuse of the same rating.
b.If a fuse fails repeatedly, the system is overloaded and further troubleshooting is required.
3.The amperage (A) rating stamp on the metal cap indicates the maximum current the wire is rated to conduct.
a.Do not exceed the rated limits because an excess power load can damage or destroy the system.
|28|B.A capacitor is an electrical component that holds an electrical charge.
1.Capacitors are used in electronic flashes in photography.
2.In PCs, capacitors regulate the flow of current to areas of the system circuits for a short time.
3.The amount of current a capacitor can hold is called capacitance (measured in microfarads).
4.An electrolytic capacitor can hold a significant charge for long periods.
a.Any residual charge must be released.
b.Failure to follow safety procedures can result in injury or death.
|29|C.Rectifiers and diodes
1.A rectifier is a device that converts AC power into a DC form (in a process called rectification).
2.A diode is a device that lets current flow in only one direction.
a.Two or more diodes connected to an AC supply can convert the AC voltage to DC voltage.
3.Diodes can be tested with a multimeter.
a.With the power turned off, test for resistance across both leads of the diode.
b.A good diode exhibits low resistance in one direction and high resistance in the other.
|30|D.Transistors
1.The efficient, compact, reliable transistor replaced large, inefficient vacuum tubes.
2.Transistors are a pair of diodes connected in series with an on-off switch.
3.Varying the voltage sent to a transistor turns the switch on or off.
4.Transistor testing involves special equipment and is not normally performed by computer technicians.
|31|E.Transformers
1.A step-down transformer decreases the transformer’s voltage on the output side; a step-up transformer increases it.
2.In the PC power supply, the transformer’s secondary coils provide 12volt, 5-volt, and 3.3-volt outputs used by various components.
|32|3.To test a transformer:
a.Disconnect the power.
b.Discharge all capacitors.
c.Check the bottom of the circuit board of the power supply to be sure all leads are disconnected.
(1)The primary connections and the secondary connections are located below the transformer.
d.Configure the multimeter to measure continuity (or resistance).
e.Simultaneously touch each lead of the multimeter to one of the pairs of contacts.
(1)A good transformer shows a reading of low resistance.
(2)A high reading might indicate a broken coil.
|33|F.Inductors (coils)
1.Inductors (also called coils) are loops of conductive wire.
2.Current passing through the inductor sets up a magnetic field.
a.This field reduces any rapid change in current intensity.
3.Inductors are tested for continuity in the same way as a transformer.
|34|4.To test a coil:
a.Visually check the wire for deterioration.
b.Turn off the system power and then disconnect one lead to the coil.
c.Connect one meter lead to each end of the coil.
(1)A null or low reading indicates continuity.
(2)A reading of high or infinite resistance indicates the coil should be replaced.
Chapter 22, Lesson 2
Electrostatic Discharge
|35|1.Causes of ESD
A.Under the right conditions, the human body can generate a tremendous amount of voltage called static electricity.
B.ESD occurs when an imbalance in the amounts of positive and negative electrical charges on the surface of an object is released.
1.The buildup of energy with nylon clothes can easily reach 21,000 volts.
2.About 750 volts can produce a visible spark.
3.A mere 10 volts can ruin a computer chip.
4.Factors such as type of materials and humidity determine the amount of energy released.
|36|C.ESD damage
1.Components are becoming smaller and operate at lower voltages.
2.These components are more susceptible to damage from ESD.
3.ESD causes three categories of problems:
a.Catastrophic failure
(1)Often called “frying” or “smoking” a component
(2)Mishandling and misapplying a power source are the most likely causes.
(3)Can be prevented by exercising care in opening, installing, cabling, and testing
b.Upset failure
(1)Erratic fault that is very difficult to detect because it is intermittent
(2)Diagnose this problem by removing and replacing suspected parts until the failure stops occurring.
c.Latent failure
(1)Weakens the transistor
(2)Is difficult to isolate because the affected part can operate normally and pass quality control and conformance tests
|37|D.Preventing ESD
1.The leading cause of ESD damage is improper handling of electronic devices.
2.The key to ESD prevention is to keep all electronic components and yourself at a common electrical potential.
a.“Grounding” yourself is as easy as touching the metal computer chassis before handling components.
b.Do not move around while installing or handling a part; this can generate additional voltages.
|38|3.Use ESD suppression devices when working with exposed parts. These include
a.Antistatic mat: a nonconducting pad placed on the work surface and on the floor in front of the work area
b.Antistatic wristband: a wristband with a grounding strap that connects to the chassis of the PC
c.Antistatic pouch: a sealed, nonconducting pouch used to store sensitive electronic devices
d.Antistatic pad: an insulating foam pad in which individual chips with exposed pins should be embedded when they are not installed in a computer
Chapter 22, Lesson 3
Safety and Electrical Power