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Chapter Four
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Figure 4-19.
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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
