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ELECTROMAGNETIC RELAYS

ELECTROMAGNETIC RELAYS. What is a relay ?. 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.

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ELECTROMAGNETIC RELAYS

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  1. ELECTROMAGNETIC RELAYS

  2. What is a relay ? Relay is an electrically operated switch.

  3. 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.

  4. 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.

  5. 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 .

  6. 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 .

  7. 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 .

  8. 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 .

  9. 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 .

  10. 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.

  11. 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 .

  12. 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 .

  13. Types of relays • Latching relay • Stepping relay • Reed relay • Mercury-wetted relay • Mercury relay • Polarized relay • Ratchet relay

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

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

  20. Protective relays • Main article: Protective relay • For protection of electrical apparatus and transmission lines , elctromechanical relays with accurate operating characteristics were used to detect overload , short-circuit and other faults. While many such relays remain in use ,digital devices now provide equivalent protective functions.

  21. Made By :- • Khushali Mistry – 13BEEEG078 • Sufaija Mangali – 13BEEEG073 • Shreya Parmar – 13BEEEG080 • Charmi Modi – 13BEEEG094

  22. Thank You…

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