A power relay is a switch that uses an electromagnet to open or close a circuit. The basic design of a relay utilizes an electromagnet coil, an armature, a spring and one or more contacts. If the power relay is designed to normally be open, the circuit is not completed when in the off state.
As power is applied to the power relay, generally from a battery source, the electromagnet attracts the armature, a movable arm often made of iron. The armature, which was held in place by the spring, is pulled in the direction of the coil until it reaches a contact, thus closing the circuit. If the relay is normally closed, then 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 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 in telegraph systems to relay the signal from one circuit to the next.
Automobiles contain many electrical devices and systems, all powered by a 12-volt battery, which makes power relays ideal for use in automobiles. Many cars contain 20 or more relays that operate everything from the horn to the power train system and windshield wipers. While the relays in an automobile can be situated throughout the vehicle, those that can wear out and are most likely to be replaced 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 wetted relays. A reed power relay uses a coil wrapped around two reed switches, surrounded by an inert gas and encased in a glass tube. When the coil is energized, the overlapping ends of the switches that contain the contacts move toward each other. Once they are no longer energized, the switches move apart, breaking the circuit.
A mercury wetted power relay works in much the same way as a reed relay. The difference is that instead of being surrounded by an inert gas, the reed switches are wetted in mercury. Mercury wetted power relays are very expensive and have to be kept vertical at all times. Because of their high cost and physical limitations, mercury wetted relays are seldom used.