Power relays use an electromagnet to open or close a circuit. They can operate with low or high voltage and are used in various applications, including automobiles. There are different types of power relays, including reed and mercury bath relays, but the latter is rarely used due to its high cost and limitations.
A power relay is a switch that uses an electromagnet to open or close a circuit. The basic design of a relay uses an electromagnet coil, an armature, a spring, and one or more contacts. If the power relay is designed to be normally open, the circuit is not completed when it is turned off.
When power is applied to the power relay, usually from a battery source, the electromagnet attracts the armature, a moving arm often made of iron. The armature, held in place by the spring, is pulled towards the coil until it makes contact, thus closing the circuit. If the relay is normally closed, the coil pulls the armature away from the contact, opening the circuit.
A power relay can be operated using a low amount of voltage, but it can also conduct a higher amount of voltage. For this reason, power relays are used for many different applications including audio amplification, automotive electronics and telephone systems. Power relays were also used in early computer systems and telegraph systems to transmit signal from one circuit to another.
Automobiles contain many electrical devices and systems, all powered by a 12-volt battery, making power relays ideal for use in automobiles. Many cars contain 20 or more relays that operate everything from the horn to the transmission system and windshield wipers. While an automobile’s relays can be located throughout the vehicle, the ones that can wear out and are most likely to need replacement are often found in the fuse box. This location makes it easy to find and replace power relays as needed.
There are many types of power relays, including electromechanical, reed, and mercury bath relays. A reed power relay uses a coil wound around two reed switches, surrounded by an inert gas, and enclosed in a glass tube. When the coil is energized, the overlapping ends of the switches that contain the contacts move towards each other. Once they are no longer energized, the switches move away, breaking the circuit.
A mercury bath power relay works in much the same way as a reed relay. The difference is that instead of being surrounded by an inert gas, reed switches are bathed in mercury. Mercury bath power relays are very expensive and must always be kept vertical. Because of their high cost and physical limitations, mercury filled relays are rarely used.
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