Relays

CHAPTER 7

Relay is an electrically operated switch.

History of relays

—The American scientist Joseph Henry invented a relay in 1835 in order to improve his version of the electrical telegraph developed earlier in 1831. It is claimed that the English inventor Edward Davy certainly invented the electric relay in his electric telegraph.

About Relays

—Relays use an electromagnet to operate a switching mechanism mechanically , but other operating principles are also used. Relays are used where it is necessary to control a circuit by low power signal or where several circuits must be controlled by 1 signal.

—The first relays were used in long distance telegraph circuits , repeating the signal coming in from 1 circuit and retransmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations .

—A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid state relays control power circuits with no moving parts instead using a semiconductor device to perform switching .

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Basic design and operation

—A simple electromagnetic relay consists of a coil of wire wrapped round a soft iron core , an iron yoke which provides a low reluctance path for magnetic flux , a movable iron armature , and one or more sets of contacts. The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts .

—It is held in a place by a spring so that when the relay is de-energized , there is an air gap in the magnetic circuit. In this condition one of the two sets of the contacts in a relay pictured is closed , and the other set is open .

—Other relays may have more or fewer sets of contacts depending on their functions . A relay in the picture also has a wire connecting the armature to the yoke . This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the PCB via the yoke , which is soldered to the PCB .

—When electric current is passed through the coil it generates a magnetic field that activates the armature , and the consequent movement of the movable contact either makes or breaks a connection with a fixed contact. —If the set of contacts was closed when the relay was de-energized , then the movement opens the contacts and breaks the connection.

—When the current to the coil is switched off , the armature is returned by a force , approx. half as strong as magnetic force , to its relaxed position . In a low voltage application relays reduces noise and in a high voltage application it reduces arcing .

—A solid state relay uses a thyristor or other solid state switching device activated by the control signals to which the controlled load , instead of a solenoid . A LED coupled with a photo transistor (optocoupler) can be used to isolate control and controlled circuits .

Types of relays

—Latching relay

—Stepping relay

—Reed relay

—Mercury-wetted relay

—Mercury relay

—Polarized relay

—Ratchet relay

Latching relay

—A latching relay has two relaxed states. These are also called “impulse”, “keep” or “stay” relays. When the current is switched off the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coil.

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Stepping relay

—A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges. In ELCB includes a specialized latching relay.

Applications of relays

—It is used for amplifying a digital signal, switching a large amount of power with a small operating power. A telegraph relay, repeating a weak signal received at the end of long wire. Switching to a standby power supply.

—Detecting and isolating faults on transmission and distribution lines by opening and closing circuit breakers. Controlling a high voltage circuit with a low voltage signal , as in some types of audio amplifiers.

—Controlling a high current circuit with a low current signal as in the starter solenoid of an automobile. Relays can be modified to delay opening or delay closing a set of contacts .A very short delay would use a copper disk between a armature and moving blade assembly.

—A 12 volt relay is often used to isolate any second battery in cars , forwards ,reverse and boats.

Source: svitvasad.ac.in

 

 

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