Electromechanical relays use a small voltage to control a higher voltage or current connection, and are often used in vehicles and factory equipment. They have electrically separated contacts or poles that move when the coil is energized, and can handle high DC or AC voltages. However, they can be bulky and slow compared to solid-state semiconductor switches.
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 control signals from the signals being switched. These relays are often used in vehicles to turn 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 consisting of a coil of wire around an iron core moves an armature. This opens and closes a series of electrical contacts connected to the spring loaded armature. If a relay is normally closed, its contacts are connected when the relay coil is not energized. When sufficient current passes through the coil, the armature moves and disconnects the contacts until the coil is energized. An electromechanical relay with normally open contacts works in the opposite way.
Like switches, electromechanical relays have a number of electrically separated contacts or poles that move when the coil is energized. Some relays have only one pole; many have two and others may have four or more. Like switches, relays can be single or double throw.
In a two-throw relay, the poles touch a set of contacts when the coil is not energized. They disconnect from that set and touch the other set as long as the coil is energized. A one throw relay has only one set of contacts to touch. The poles are either connected to that set of contacts or they are disconnected and touching no one else.
Some electromechanical relays have coils designed to operate on alternating current (AC), while others use direct current (DC). Coil voltages are often relatively low, in some cases from a few volts to a few hundred volts. However, relays can often switch much higher DC or AC voltages. Some can handle up to 15,000 volts and currents up to several thousand amperes. The coil is electrically isolated from the switch contacts so that one can operate on DC while the other connects on 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 slower than a semiconductor device. A solid state relay isolates the control circuit from the load which is switched with an opto-isolator. A light emitting diode (LED) and photodetector operate the switch device. A transistor, silicon controlled rectifier (SCR), or triode for AC (TRIAC) switches the load electronically rather than mechanically.
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