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Electromechanical relays are devices used to make and break electrical connections. A relay usually uses a small voltage to control a higher voltage or higher current connection. Unlike some electronic switches, electromechanical relays electrically isolate the control signals from the signals being switched. These relays are often used in vehicles to switch high-current ignitions, motors and lights on and off. They are also used in factory equipment to control valves, pumps and motors.
In most electromechanical relays, an electromagnet formed by a coil of wire around an iron core moves an armature. This opens and closes a set of electrical contacts attached to the spring-loaded armature. If a relay is normally-closed, its contacts are connected when the relay coil is not energized. When enough current passes through the coil, the armature moves and disconnects the contacts for as long as the coil is energized. An electromechanical relay with normally-open contacts works in the opposite manner.
Like switches, electromechanical relays have a number of electrically-separate contacts or poles which move when the coil is energized. Some relays have only one pole; many have two and others may have four or more. Also similar to switches, relays can be either single-throw or double-throw.
In a double-throw relay, the poles touch one set of contacts when the coil is not energized. They disconnect from that set and touch the other set for as long as the coil is energized. A single-throw relay only has one set of contacts to touch. The poles are either connected to that set of contacts or they are disconnected and not touching any others.
Some electromechanical relays have coils which are designed to operate on alternating current (AC), while others use direct current (DC). Coil voltages are often relatively low, from just a few volts to a few hundred volts in some cases. Relays can often switch much higher DC or AC voltages, however. Some can handle up to 15,000 volts and currents as high as several thousand amperes. The coil is electrically isolated from the switching contacts so one can operate on DC while the other connects AC if needed.
An electromechanical relay device typically has several drawbacks compared to a solid-state semiconductor switch. It can be bulky and expensive and switch much more slowly than a semiconductor device. A solid-state relay isolates the control circuit from the load being switched with an optoisolator. A light-emitting diode (LED) and photodetector drive the switching device. A transistor, silicon-controlled rectifier (SCR) or Triode for AC (TRIAC) switches the load electronically instead of mechanically.
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