Interference suppression is now a critical tive engineering task because the modem automo-bile has an increased need for EMI suppression.
The increasing use of cellular telephones, as well as onboard computer systems, are only two of the factors that have made interference suppression extremely important.
Even small amounts of EMI can disrupt the operation of an onboard digital computer, which operates on voltage signals of a few millivolts (thousandths of a volt) and milliamperes (thou-sandths of an ampere) of current. Any of the inter-ference transmission modes discussed earlier are capable of creating false voltage signals and excessive current in the computer systems. False voltage signals disrupt computer operation, while excessive current causes permanent damage to micro-electric circuitry.
As the complexity and number of electronic sys-tems continues to increase, manufacturers are using multiplex wiring systems to reduce the size and number of wiring harnesses, which also reduces EMI. Multiplexing is a method of sending more than one electrical signal over the same channel.
SUMMARY
Electricity can be generated in several ways. The most important way for automotive use is by mag-netism. Magnetism is a form of energy caused by the alignment of atoms in certain materials. It is indicated by the ability to attract iron. Some mag-netic materials exist in nature; others can be artifi-cially magnetized. The magnetic properties of some metals, such as iron, are due to electron motion within the atomic structure. Reluctance is resistance to the movement of magnetic lines of force: iron cores have permeability and are used to reduce reluctance in electromagnetic fields.
Lines of force, called flux lines, form a magnetic field around a magnet. Flux lines exit the north pole and enter the south pole of a magnet. Magnetic flux lines also surround electrical conductors. As current increases, the magnetic field of a conductor becomes stronger. Voltage can be generated by the interaction of magnetic fields around conductors.
The relative movement of a conductor and a magnetic field generates voltage. This process is called induction. Either the conductor or the mag-netic field may be moving. The strength of the induced voltage depends on the strength of the magnetic field, the number of conductors, the speed of the relative motion, and the angle at which the conductors cut the flux lines. Electromagnetic induction is used in generators, alternators, electric motors, and coils. Magnetomotive force (mmf) is a
measure of electromagnetic field strength. The unit of measure used for mmf is ampere-turns. The principle of a transformer can be summarized by describing it as flowing current through a primary coil and inducing a current flow in a secondary or output coil.
Electromagnetism can also generate electro-magnetic interference (EMI) and radiofrequency interference (RFI). Such interference can disrupt radio and television signals, as well as electronic system signals. Many devices are used to sup-press this interference in automotive systems.
Review Questions
1.Which of the following can store energy in the form of an electromagnetic charge?
a. Thermocouple b. Induction coil c. Potentiometer d. Capacitor
2. Current flows through a heated thermocouple because of the two-way flow of
a. Electrons between dissimilar materials b. The blockage of free electrons between
the metals
c. The one-way transfer of free electrons between the metals
d. Random electron flow
3. Which of the following forms of generating electricity is not widely used in an
automobile?
a. Heat b. Pressure c. Chemistry d. Magnetism
4.The lines of force of a magnet are called a. Flux lines
b. Magnetic polarity c. Magnetic lines d. Flux density
5.A material through which magnetic force can easily flow has a high
a. Reluctance b. Permeability c. Capacitance d. Magnetic attraction
6.The left-hand rule says that if you grasp a conductor in your left hand with your thumb pointing in the direction of the electron ( to +) flow,
a. Your fingers will point in the direction of the magnetic flux lines.
b. Your fingers will point in the opposite direction of the magnetic flux lines.
c. Your fingers will point at right angles to the magnetic flux lines.
d. Your fingers will point at a 45-degree angle to the magnetic flux lines.
7.The left-hand rule is useful to determine a. The direction of current flow
b. The length of the magnetic flux lines c. The strength of the magnetic field d. Flux density
8.When two parallel conductors carry electrical current in opposite directions, their magnetic fields will
a. Force them apart b. Pull them together c. Cancel each other out
d. Rotate around the conductors in the same direction
9.The motor principle of changing electrical energy into mechanical energy requires
a. Two semiconductors carrying current in opposite directions
b. Two semiconductors carrying current in the same direction
c. Two conductors carrying current in opposite directions
d. Two conductors carrying current in the same direction
10. Which of the following will not increase induced voltage?
a. Increasing the strength of the magnetic field
b. Increasing the number of conductors cutting flux lines
c. Increasing the speed of the relative motion between the conductor and the flux lines
d. Increasing the angle between the flux lines and the conductor beyond 90 degrees 11. The _________ in an automobile DC
generator rectifies AC to DC.
a. Armature b. Commutator c. Field coil d. Loop conductor
12. In an ignition coil, low-voltage primary current induces a very high secondary voltage because of
a. The different number of wire turns in the two windings.
b. An equal number of turns in the two windings
c. The constant current flow through the primary winding
d. The bigger wire in the secondary winding 13. To reduce EMI, manufacturers have done all
of the following except:
a. Using low resistance in electrical systems b. Installing metal shielding in components c. Increasing the use of ground straps d. Using capacitors and choke coils
LEARNING OBJECTIVES
Upon completion and review of this chapter, you should be able to:
• Define a series circuit.
• Identify the series circuit laws and apply Ohm’s Law for voltage, current, and resistance.
• Define a parallel circuit.
• Identify the parallel-circuit laws and apply Ohm’s Law for voltage, current, and resistance.
• Define Kirchhoff’s Voltage Drop Law and Current Law.
• Define a series-parallel circuit.
• Identify the series and parallel circuit loads of a series-parallel circuit.
• Using Ohm’s Law, solve series-parallel cir-cuits for voltage, current, and resistance.
KEY TERMS
Circuit
Kirchhoff’s Law of Current Kirchhoff’s Law of Voltage Drops Parallel Circuit
Series Circuit
Series-Parallel Circuits Voltage Drop
INTRODUCTION
In this chapter you will apply the individual series and parallel circuit laws learned in previous chapters to a combination circuit consisting of some compo-nents connected in series and some in parallel.
Series/parallel electrical circuits seem complicated, but are in fact fairly simple to understand if you remember which circuit laws apply to each circuit load component. Most vehicle electrical circuits used today contain several series-parallel circuits, or portions of a series-parallel circuit.