3. Relays Contents
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3. Relays Contents 1 Relay 1 1.1 Basic design and operation ...................................... 1 1.2 Types ................................................. 2 1.2.1 Latching relay ......................................... 2 1.2.2 Reed relay ........................................... 3 1.2.3 Mercury-wetted relay ..................................... 3 1.2.4 Mercury relay ......................................... 3 1.2.5 Polarized relay ........................................ 4 1.2.6 Machine tool relay ...................................... 4 1.2.7 Coaxial relay ......................................... 4 1.2.8 Time delay .......................................... 4 1.2.9 Contactor ........................................... 4 1.2.10 Solid-state relay ........................................ 4 1.2.11 Solid state contactor relay ................................... 5 1.2.12 Buchholz relay ........................................ 5 1.2.13 Forced-guided contacts relay ................................. 5 1.2.14 Overload protection relay ................................... 6 1.2.15 Vacuum relays ........................................ 6 1.3 Pole and throw ............................................. 6 1.4 Applications .............................................. 7 1.5 Relay application considerations .................................... 8 1.5.1 Derating factors ........................................ 9 1.5.2 Undesired arcing ....................................... 9 1.6 Protective relays ............................................ 9 1.7 Railway signalling ........................................... 9 1.8 History ................................................. 10 1.9 See also ................................................ 10 1.10 References ............................................... 10 1.11 External links ............................................. 11 2 Solid-state relay 13 2.1 Coupling ............................................... 13 2.2 Operation ............................................... 13 i ii CONTENTS 2.3 Parameters ............................................... 13 2.4 Advantages over mechanical relays .................................. 14 2.5 Disadvantages ............................................. 14 2.6 Images ................................................ 14 2.7 See also ................................................ 14 2.8 References ............................................... 14 2.9 External links ............................................. 14 3 Contactor 15 3.1 Construction .............................................. 15 3.2 Operating principle .......................................... 16 3.3 Arc suppression ............................................ 16 3.4 Ratings ................................................ 17 3.4.1 IEC utilization categories ................................... 17 3.4.2 NEMA ............................................ 17 3.5 Applications .............................................. 17 3.5.1 Lighting control ........................................ 17 3.5.2 Magnetic starter ........................................ 17 3.5.3 Vacuum contactor ....................................... 18 3.6 References ............................................... 18 4 Reed relay 19 4.1 Memory device ............................................ 19 4.2 Crosspoint switch ........................................... 19 4.3 Other uses ............................................... 20 4.4 See also ................................................ 20 4.5 References ............................................... 20 4.6 External links ............................................. 20 4.7 Text and image sources, contributors, and licenses .......................... 21 4.7.1 Text .............................................. 21 4.7.2 Images ............................................ 21 4.7.3 Content license ........................................ 22 Chapter 1 Relay This article is about the electrical component. For other tor device to perform switching. Relays with calibrated uses, see Relay (disambiguation). operating characteristics and sometimes multiple operat- A relay is an electrically operated switch. Many relays ing coils are used to protect electrical circuits from over- load or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays". 1.1 Basic design and operation Simple electromechanical relay. A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which pro- vides a low reluctance path for magnetic flux, a movable Automotive-style miniature relay, dust cover is taken off iron armature, and one or more sets of contacts (there are two in the relay pictured). The armature is hinged to use an electromagnet to mechanically operate a switch, the yoke and mechanically linked to one or more sets of but other operating principles are also used, such as solid- moving contacts. It is held in place by a spring so that state relays. Relays are used where it is necessary to con- when the relay is de-energized there is an air gap in the trol a circuit by a low-power signal (with complete elec- magnetic circuit. In this condition, one of the two sets of trical isolation between control and controlled circuits), contacts in the relay pictured is closed, and the other set or where several circuits must be controlled by one sig- is open. Other relays may have more or fewer sets of con- nal. The first relays were used in long distance telegraph tacts depending on their function. The relay in the picture circuits as amplifiers: they repeated the signal coming in also has a wire connecting the armature to the yoke. This from one circuit and re-transmitted it on another circuit. ensures continuity of the circuit between the moving con- Relays were used extensively in telephone exchanges and tacts on the armature, and the circuit track on the printed early computers to perform logical operations. circuit board (PCB) via the yoke, which is soldered to the A type of relay that can handle the high power required PCB. to directly control an electric motor or other loads is When an electric current is passed through the coil it gen- called a contactor. Solid-state relays control power cir- erates a magnetic field that activates the armature, and the cuits with no moving parts, instead using a semiconduc- consequent movement of the movable contact(s) either 1 2 CHAPTER 1. RELAY Latching relay with permanent magnet being switched, and the relay contacts retain this setting across a power outage. A latching relay allows remote control of building lighting without the hum that may be Small “cradle” relay often used in electronics. The “cradle” term produced from a continuously (AC) energized coil. refers to the shape of the relay’s armature. In one mechanism, two opposing coils with an over-center spring or permanent magnet hold the contacts in position makes or breaks (depending upon construction) a con- after the coil is de-energized. A pulse to one coil turns nection with a fixed contact. If the set of contacts was the relay on and a pulse to the opposite coil turns the relay closed when the relay was de-energized, then the move- off. This type is widely used where control is from simple ment opens the contacts and breaks the connection, and switches or single-ended outputs of a control system, and vice versa if the contacts were open. When the current such relays are found in avionics and numerous industrial to the coil is switched off, the armature is returned by a applications. force, approximately half as strong as the magnetic force, Another latching type has a remanent core that retains the to its relaxed position. Usually this force is provided by contacts in the operated position by the remanent mag- a spring, but gravity is also used commonly in industrial netism in the core. This type requires a current pulse motor starters. Most relays are manufactured to operate of opposite polarity to release the contacts. A variation quickly. In a low-voltage application this reduces noise; uses a permanent magnet that produces part of the force in a high voltage or current application it reduces arcing. required to close the contact; the coil supplies sufficient When the coil is energized with direct current, a diode force to move the contact open or closed by aiding or op- is often placed across the coil to dissipate the energy posing the field of the permanent magnet.[2] A polarity from the collapsing magnetic field at deactivation, which controlled relay needs changeover switches or an H bridge would otherwise generate a voltage spike dangerous to drive circuit to control it. The relay may be less expensive semiconductor circuit components. Some automotive re- than other types, but this is partly offset by the increased lays include a diode inside the relay case. Alternatively, a costs in the external circuit. contact protection network consisting of a capacitor and In another type, a ratchet relay has a ratchet mechanism resistor in series (snubber circuit) may absorb the surge. that holds the contacts closed after the coil is momentar- If the coil is designed to be energized with alternating cur- ily energized. A second impulse, in the same or a sepa- rent (AC), a small copper “shading ring” can be crimped rate coil, releases the contacts.[2] This type may be found to the end of the solenoid, creating a small out-of-phase in certain cars, for headlamp dipping and other functions