All relays are switches, but not all switches are relays. These components are vital in a wide range of electronic and other control systems. Invented in 1835, relays not only helped give rise to the telegraph, but also to the transistor. They may be unassuming to look at, but relays are everywhere you need indirect control of a process.What Relays Do
While there are many different kinds of relays, they all do basically the same thing. A relay uses the current from one circuit to open or close another. This can happen either manually or automatically depending on the nature of the relay. Hook a relay to a thermometer and it can turn off your dryer when it overheats. Other uses exist anywhere you need to control something at a distance. Relays are also useful when controlling high currents because you can control the low current device and let it deal with the high current one.Solid State Relays
As the name implies, solid state relays are entirely electronic in nature, containing no moving parts. The biggest advantage of going solid state is that the relays are much faster; they also draw less current than other relays. At the same time, you cant just replace the contacts; you have to replace the whole relay.Electromechanical Relays
Electromechanical relays use the initial current to initialise an electromagnet, which then closes a physical switch. They are extremely popular in industrial settings as well as many safety devices. For example, the circuit breaker in your house is a form of relay. Many general purpose relays work on electromechanical principles.Opening and Closing Relays
Most safety devices are opening relays. They open a circuit and prevent the flow of current when a certain threshold is reached. This is useful for protection devices and follows the same principle as a safety valve on a boiler. Closing relays activate circuits, and are more common in controls than protection applications.