© 2012 Delmar, Cengage Learning Basic Electrical System Theory and Repairs Chapter 25

© 2012 Delmar, Cengage Learning Objectives Explain electrical principles Describe various electrical terms Understand and compare voltage, current, and resistance Use electrical meters and test instruments in a safe and correct manner

© 2012 Delmar, Cengage Learning Introduction Almost every system of the car uses electricity –Anti-lock brakes –Engine emission control devices –Dash warning lights and gauges –Electronic fuel injection –Electrically controlled transmissions Technicians in every area –Must understand electricity to be successful

© 2012 Delmar, Cengage Learning Electron Flow Matter: composed of atoms –Atoms: composed of protons, neutrons, electrons Electrons: negatively charged and orbit protons Protons: positively charged Neutrons: no charge Protons and neutrons: located in the nucleus –Number of protons and neutrons Determines the element Atoms try to remain electrically neutral –Equal number of protons and neutrons

© 2012 Delmar, Cengage Learning

Electron Flow (cont'd.) Electricity: flow of electrons from one atom to another

© 2012 Delmar, Cengage Learning Conductors and Insulators To remain in balance an atom will shed or attract electrons from neighboring atoms –Electrons flow between atoms to equalize charge Conductors: atoms with free electrons –Good conductors: silver, copper, and aluminum Insulators: few or no free electrons –Prevents flow of electrons between conductors –Good insulators: glass, rubber, and porcelain

© 2012 Delmar, Cengage Learning Basic Automotive Electrical System Electrical circuit –Complete circuit is needed to do work Requires a power source, conductor, and load –Automobiles: ground is provided by the frame –Electricity takes the path of least resistance With enough voltage, electricity jumps air gaps –Most wiring is insulated with polyvinyl chloride Control and protection devices –Switches, fuses, and circuit breakers

© 2012 Delmar, Cengage Learning

Electrical Terms Voltage –Electromotive force (EMF): force needed to push or pull an electron out of orbit Measured in volts using a voltmeter Current –Flow of electricity –Number of electrons flowing per second Measured in amperes

© 2012 Delmar, Cengage Learning Electrical Terms (cont'd.) Direct current –Electrons flow in one direction Alternating current –Oscillation from positive to negative and back Cannot be stored in a battery –Alternators make alternating current Converted to DC before recharges the battery Resistance –Obstruction to electrical flow One ohm is the resistance that will allow one ampere to flow when pushed by one volt

© 2012 Delmar, Cengage Learning Electrical Terms (cont'd.) Changes in current flow –Current flow inversely proportional to resistance –Current draw is the amount of current used to operate a load –Light, heat or motion energy result when resistance opposes the flow of current –Resistors are used to make heat or control load intensity Variable resistors control speed and intensity of electrical load

© 2012 Delmar, Cengage Learning

Circuitry and Ohm’s Law Series circuit: current flows equally through all parts –Resistances of all loads add up Parallel circuit: starts from a common point and branches –Total resistance is less than the sum of individual resistances Series-parallel circuits: combine two types Ohm’s law: voltage, amperage, and resistance relationship

© 2012 Delmar, Cengage Learning

Magnetic Fields Magnets have polarity –Like poles repel each other and unlike poles attract Electromagnetism –Magnetic field is created around outside of conductor Electromagnetic induction: –Electricity is produced by moving magnetic field over a conductor Relay: magnetically controlled switch

© 2012 Delmar, Cengage Learning

Capacitors Store electricity –Used to absorb voltage changes Connected in parallel in a DC circuit –In AC circuit electricity flows through capacitor as part of the wiring Made of two pieces of foil separated by insulator –Ground side connected to one piece of foil –Positive side connected to the other

© 2012 Delmar, Cengage Learning

Automotive Electronics Electrical components: use mechanical parts –Electronic systems: use solid state parts Semiconductor: acts as insulator and conductor –Common materials are silicon and germanium Diode: allows electricity to flow in only one direction Transistor: electronic relay –Resists electrical flow or allows a predetermined amount of current to flow

© 2012 Delmar, Cengage Learning Basic Electrical Tests Meters: permanent magnetic (analog) or digital –Analog meter has a needle moved by a magnet –Digital volt-ohmmeters have only a voltmeter and ohmmeter –Digital multimeters are popular today Voltmeter tests: –System voltage at the battery or alternator Voltage drop testing –Difference in voltage between two points –Excessive voltage drop due to resistance

© 2012 Delmar, Cengage Learning Basic Electrical Tests (cont'd.) Ammeter: measures amperage (current flow) –Must be hooked in series with the load –Amp draws are tested with system under load Current probe: available for low- and high-amps Ohmmeter: measures resistance –Must be calibrated Hand calibrate by connecting its wires together to read zero resistance –Never connect an ohmmeter across an energized circuit

© 2012 Delmar, Cengage Learning Basic Electrical Tests (cont'd.) Circuit problems –Open circuit: break in path of electrical flow –Short circuit: electrical path has been shortened –Grounded circuit: current flows directly to ground

© 2012 Delmar, Cengage Learning Basic Electrical Tests (cont'd.) Jumper wire: simple wire with alligator clips –Finds open circuits Never use a jumper lead smaller than the circuit Never use to bypass a high resistance load Only use to temporarily bypass a component Test lights: tests for open and short circuits when power is disconnected –Self-powered test light: if light glows the circuit or part has continuity Power probe: combines features of test probes