Since the 19th century, electro-mechanical relays have been built using different configurations and characteristics to suit different applications. Ever since, relays have been widely used in household appliances, computer interfaces, air and heating conditioning units, automobiles and in many other domestic and industrial electrical circuits that need some type of power control. To better understand how this type of remote switch operates, though, you need to know first what components make up a typical relay.
If you were to remove the housing of a *common relay-like the one used to power the fuel pump in your vehicle-you would find a small winding coil surrounding a soft iron core or electromagnet. You'd see that each side of the coil winding connects to a terminal at the bottom of the relay. When connected to an electrical circuit, one of these terminals receives a small electric current that travels through the coil and exits through the other terminal. This electric path is called the-low input power-control circuit.
In front of this coil in the control circuit, you would find a pair of spring-loaded contact points, each attached to a small, metallic arm. When the relay is not under operation, these two contact points-facing each other-remain separated. Each contact point, though, hooks to a terminal underneath the relay. Together, these two contact points-and their respective mounting arms-constitute the armature of the relay. The electric path formed by these two points is called the-high input-power circuit. This power circuit connects to those components that should run when the relay is activated through the control circuit, like the electric fuel pump in your vehicle.
How the relay operates
Now, you are ready to see an electro-mechanical relay in action. Let's take a look at your vehicle's fuel pump relay. When you hope into your vehicle and turn the ignition key to start the engine, you allow current to go through the control circuit of the pump relay. When electric current travels through the coil winding in the relay's control circuit, it forms a magnetic field around the relay's core. This magnetic field is strong enough to pull one of the contact points against the other contact point in the relay's power circuit. This action closes the electric path of the power circuit, allowing the fuel pump to work.
As long as there is current going through the control circuit, the power circuit-and any components connected to it-will remain energized. However, when you turn off the ignition key, you open the electric path to the control circuit. Then, the magnetic field around the core disappears and the contact points separate, opening the electric path to the power circuit and de-energizing the fuel pump.
Many relays come equipped with four terminals, like the one used to energize the electric fuel pump in a vehicle. However, there are relays with three terminals as well. In these relays, the case or housing serves as the grounding connection for one of the circuits. Furthermore, you can find remote switches built in many other designs and configurations, like the electronic, thermo-electric, piezo, reed and solid state relays.
Although electromechanical relays are very effective, they are not perfect. Relay problems usually stem from mechanical or electrical failures: the coil may break or the contacts points may wear out, burn or get stuck. On modern vehicles, many electric circuits are being replaced with electronic systems. Instead of relays, these systems incorporate *solid state components that use semiconductors, eliminating the need for movable parts and diminishing the chance of a mechanical failure.
Still, electro-mechanical relays continue to be very efficient electrical units with several advantages. Besides the capacity to control high power circuits remotely, a relay's control circuit ca use thin wires, making it easy to hide in tight spaces like under the dashboard and seats of your vehicle. At the same time, shorter lengths of expensive, heavy wire can be used for the high-power part of the circuit. In addition, during the last few decades, the relay has since a significant improvement in materials, capacity and technology. Thus, the electro-mechanical relay will continue to be used in the near future.