Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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60 Chapter Four
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85 87 87A
Figure 4-19.
Electromagnetic relay.
Most relays contain a device that protects circuitry
from the voltage spike that occurs when the
coil is de-energized. In older vehicles, the protective
device is usually a diode (as in the circuit on
the left in Figure 4-19). A diode is a semiconductor
device that can be useful in several ways. In a relay,
the diode is located in parallel with the coil, where
it dissipates the voltage spike. (You’ll learn more
about how diodes work in a Chapter 10.)
Today many automobile relays include a resistor,
rather than a diode, to protect the control circuit
(as in the circuit on the right in Figure 4-19). The
resistor dissipates the voltage spike in the same
way that a diode does. For more information about
relays, see the section on “Diagnostic Strategies”
in Chapter 4 of the Shop Manual.
ISO Relays
In Figure 4-19, on the right, the five terminals with
specific numbers assigned to them (#85, #86, etc.)
show that this relay, like many others now used in
vehicles, is an ISO relay. ISO relays, as required
by the International Organization for standardization
(ISO), are the same size and have the same
terminal pattern. They’re used in many majorcomponent
circuits, and are often located in a
vehicle’s underhood junction block or power distribution
center. For more information about ISO
relays, see the section on “Diagnostic Strategies”
in Chapter 4 of the Shop Manual.
ELECTROMAGNETIC
INDUCTION
Only a decade after the discovery of magnetic
fields surrounding current-carrying conductors,
more discoveries were made about the relationship
between electricity and magnetism. The
Figure 4-20. Voltage can be induced by the relative
motion between a conductor and a magnetic field.
modem automotive electrical system is based in
great part upon the principles of electromagnetic
induction discovered in the 1830s. Along with
creating a magnetic field with current, it is also
possible to create current with a magnetic field.
Magnetic flux lines create an electromotive
force, or voltage, in a conductor if either the flux
lines or the conductor is moving (relative
motion). This process is called electromagnetic
induction, and the resulting electromotive force
is called induced voltage (Figure 4-20). If the
conductor is in a complete circuit, current exists.
It happens when the flux lines of a magnetic
field cut across a wire (or any conductor). It does
not matter whether the magnetic field moves or
the wire moves. When there is relative motion
between the wire and the magnetic field, a voltage
is produced in the conductor. The induced voltage
causes a current to flow; when the motion stops,
the current stops.
Voltage is induced when magnetic flux lines
are broken by a conductor (Figure 4-20). This
relative motion can be a conductor moving across