Download Chapter 36: Principles of Electrical Systems

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► Increasingly, vehicle technicians must have an understanding of the electrical principles involved in
vehicle system operation.
► Atoms contain negatively charged electrons moving around a nucleus, in which there are positively
charged protons and neutrons with no charge.
► Atoms with excess electrons have a negative charge and create a negative ion; those deficient in
electrons have a positive charge and create a positive ion.
► Free electrons can move from one atom to another if an electrical potential is applied.
► Materials with many free electrons are good electrical conductors.
► Copper is the most common conductor.
► Insulators are materials that do not conduct current easily; an example is plastic.
► Semiconductor refers to a material that conducts electricity more easily than an insulator, but not as well
as a conductor.
► Free electrons require a pathway or circuit, and a force to act upon them, such as a battery.
► Like charges repel, and unlike charges attract.
► The attraction of free electrons that creates a force is called voltage.
► The four factors that determine electrical resistance level are the type of material and the length, size,
and temperature of the conductor.
► Electrical resistance refers to the degree to which a material opposes the passage of an electrical current.
► Resistance is measured in ohms and is constant in an object unless the temperature changes.
► A semiconductor’s ability to conduct electricity depends on negative electrons and holes.
► The number of charge carriers in a semiconductor can be changed by adding small quantities of
impurities (doping).
► The PN junction of a semiconductor is located at the depletion layer.
► Semiconductors can prevent or allow current flow, depending on connection to a current source.
► Semiconductor materials include silicon, germanium, gallium-arsenide, and silicon carbide.
► Electrical circuits contain a power supply, a current flow on/off switch, a functional component, a
conductive pathway, and a protection device (e.g., a fuse).
► Voltage is the electrical pressure difference between two points in an electrical circuit.
► The ampere (amp) is the unit used to describe how much current is flowing at a given point within a
circuit when the functional component is operational.
► The ohm is the unit used to describe electrical resistance in a circuit or component.
► Direct current (DC) flows in one direction only; alternating current (AC) continuously changes its
direction of flow.
► Electrical components can work only on AC or DC, but not both.
► Circuits may be described in terms of continuity, open, short, and high resistance.
► Electrostatic energy occurs when two insulators are rubbed together, with one losing electrons to become
positively charged and the other gaining electrons to become negatively charged.
► Thermoelectric energy is produced by joining and heating two different metals.
► Electrochemical energy is produced via electrolysis, which is the immersion of two dissimilar metals in a
conducting liquid to break down chemicals into ions.
► Photovoltaic energy is produced via solar energy cells.
► Piezoelectric energy is produced when certain crystals are subjected to mechanical stress.
► Electromagnetic induction is created when a conductor cuts across a magnetic field.
► The effects of electricity include light (LED bulbs), heat (headlights), chemical reactions (lead-acid
battery), and magnetism (electric motors).
► Electromagnets are used in relays, solenoids, and motors, while electromagnetic induction is used in
ignition coils and transformers.
► Relays are used to control circuits that carry high current flow; they can be normally open (NO) or
normally closed (NC).
► Solenoids operate similarly to a relay, but create lateral movement rather than closing a circuit.
► Electric motors rely on magnetic fields to create rotary movement.
► Ohm’s law states that the total resistance of a circuit always equals the voltage divided by the amperage.
► The term “work” refers to transforming one form of energy into another.
► Power refers to the rate at which work is done, or the rate of transforming energy.
► The watt is the unit of power.
► Kirchhoff’s current law states that electrical current entering any junction is equal to the sum of the
current flowing out of the junction.
► In a series circuit, current can flow in only one path and all electrons flow at the same rate.
► Voltage drop refers to the pressure lost by driving the current through a resistor.
► The electrical properties of a series circuit are as follows: current flow is the same in all parts of the
circuit; the applied voltage is equal to the sum of the individual voltage drops; and total circuit resistance
is equal to the sum of the individual resistances.
► All components in a parallel circuit are directly connected to the voltage supply; hence the voltage across
each component is equal to battery voltage.
► Parallel circuit laws are as follows: the voltage across all branches of a parallel circuit are the same; the
total current equals the sum of the current flowing in each branch; the amount of current in each branch
is inversely proportional to the resistance of the branch; and the total resistance of a parallel circuit can
be calculated as RT = (R1 × R2) divided by (R1 + R2).Or, RT = 1/ 1/R1 + 1/R2 + 1/R3.
► Series-parallel circuits contain both a series circuit and a parallel circuit.
► Electrical components must be correctly connected onto circuits and may have numbered or marked
terminals to ensure proper connection.
► Circuit protection devices, which break the circuit during excessive current flow, are fuses, fusible links,
and circuit breakers.
► Resistors are used to control voltage that reaches various components because they resist the current
running through them.
► Resistor types include fixed, variable, thermistors, metal oxide varistors, and ballast resistors.
► Resistors are rated by both resistance value and power rating.
► Resistance value is indicated by colored bands, and tolerance is indicated by the number of identifying
bands.
► Variable resistor types are rheostats, potentiometers, and thermistors.
► Thermistors are a type of conductor in which resistance value is affected by temperature.
► Capacitors are used to store electrical energy and are rated by capacitance, which is the amount of
change stored in each plate for a given potential voltage between the plates.
► Diodes are used to restrict current flow to one direction only.
► Transistors (NPN and PNP types) are used as switches and to amplify currents.
► Control modules are designed to monitor multiple inputs from sensors and circuits and respond to those
inputs.
► Delay circuits can turn on or off an electrical device after a specified time delay.
► Microprocessors are designed to monitor and control most electrical systems on a modern vehicle.
► Most vehicle wires are braided copper with plastic insulation, but other types are shielded wires and
ribbon.
► Wire shielding to prevent noise (unwanted electromagnetic induction) can be twisted pair, Mylar tape, or
drain lines.
► Proper operation of electrical circuits requires correct wire size.
► Length and diameter of a cable determine its resistance.
► Copper has low resistance value, although as the wire length increases, so does resistance within the
wire; therefore, the cross-sectional area needs to increase to overcome resistance.
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Terminals and connectors are fitted to the ends of cables to provide low-resistance cable termination.
Terminals can be push-on spade terminals, eye ring terminals, and solder-type terminals.
Connectors can be permanent cable joiners, wiring harness connectors, or male or female connectors.
Wiring harnesses are used to bind wires together within a sheath of insulating tape or tubing.
Wiring diagrams are generally split up into systems and subsystems.
Microprocessors and their assorted components are mounted on circuit boards with conductive tracks
designed to connect electronic component leads with one another.
► Electronically controlled systems are integrated to a multiplexed serial communications network, such as
a controlled area network bus (CAN-bus).
► Fiber-optic cables can transmit light signals over very long distances.
► Electrical measurement tools include the ammeter, voltmeter, ohmmeter, and digital multimeter.