To put it simply, an electromechanical relay is essentially just a switch. However, relays are controlled by electromagnets, whereas switches are manually actuated. They are used in a control circuit when a low power signal is required, or when several circuits need to be controlled. Inside the relay, a small current is used to create a magnetic field within the coil. This magnetic field is used to switch or control a much larger current. Relays come in a variety of sizes and can use different technology, but they operate with the same basic concept that makes them an electromagnetic relay.
It should be noted that like many other circuit diagram symbols, the symbols for relays can vary quite a bit. The most common way to see a relay symbol shows the relay coil as a box with protruding contacts shown as open or closed. Older circuit diagrams might show the replay as an actual coil. Regardless, it is a good idea when working with relays to know how they may or may not appear.
Although solid state switches are currently more popular due to their higher efficacy, relays are still ideal for many applications due to their unique properties. When a current flows through the coil box an electromagnetic field is created, pushing contact ends together, completing a full circuit. Relays have several advantages and disadvantages just as any other type of technology. On one hand, relays provide physical isolation between circuits and can withstand extremely high voltages. On the other, they are slower than semiconductor switches and have a limited lifetime.
Relay switches can be used in automotive applications, industrial applications, aerospace applications, large power operations, etc. Each relay is specific to the application in which it is to be applied and there are many to choose from on the market. So, it is important to weigh out the pros and cons when selecting any type of electronic component.
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