Relays

Relays are switches that are turned on and off by a small electrical current. They are ideal for using a small current to control a larger current. An example would be the horn circuit. A small current can be turned on and off by the horn switch in the steering wheel, using a light duty switch and wires. The small current is then used to activate the relay, which sends the larger current to the horn. Thus, larger wires are needed only up front where they can be shorter, and smaller wires can be used to run up the steering column. Most electrical components found in a motor vehicle are controlled by relays. ECUs use relays to control components such as fuel pumps, headlights, and the cooling fan. All of these circuits carry large electrical loads.

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FIGURE 36-40
Relay schematic.
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FIGURE 36-41
Spike-protected relays.

The relay is made up of an electromagnet, a set of switch contacts, terminals, and the case. The electromagnet is a winding of fine metal-insulated wire wrapped around an iron core. Each end of the winding is connected to one of the terminals. The contacts usually consist of three contacts—two fixed and one movable. The movable contact is fixed to a spring-loaded armature blade and is held against one of the fixed contacts, which is called the normally closed (NC) contact. When the relay coil is activated, the electromagnet pulls the movable armature blade contact away from the NC contact and against the normally open (NO) contact, sending power out to the controlled device. It also opens the NC contact points on that type of relay.

When the contact points are closed, current flows across the contact points and out to the electrical device to the rest of the circuit. As long as the small control current flows through the relay windings, the much larger current for the load will flow through the relay’s contact points. A solid-state relay acts like a mechanical relay but does not have any moving parts. This means that electronic relays do not use mechanical switches, but instead use transistorized circuitry to turn the circuit on and off. They also do not make any sound, unless they were specifically designed with components to do that.

Automotive relay terminals use one of two standard labeling systems: 85, 86, 30, 87a, and 87 or 1, 2, 3, 4, 5 Figure 36-40.

  • 85 or 1 = One end of the relay winding
  • 86 or 2 = The other end of the relay winding
  • 30 or 3 = Common—movable switch contact
  • 87a or 4 = NC fixed contact to the common
  • 87 or 5 = NO fixed contact to the common

When electromagnetic relays are de-energized, the collapse of the magnetic field induces a large voltage spike in the relay coil windings. If unchecked, this voltage can be transmitted back into the circuit where it can damage electronic components. Some relays deal with this danger by placing either a suppression diode or a resistor in parallel with the winding Figure 36-41. Doing so reduces the voltage spike by shunting the voltage spike from the output side of the coil back to the input side of the coil. This design prevents the voltage from spiking so high when the relay is de-energized. When replacing a relay that has spike protection, make sure to use a new relay that is specified for the application.