Study of RELAYS
Relays
A relay can be described as an electrically operated switch. This implies that a relay consists of a mechanism to make or break connection in an electric circuit. This mechanism can be electromagnetic, thermal or electronic type. Moreover, this mechanism does not belong to the electric circuit being switched. Thus, relay can be operated by one circuit to switch another circuit. Thus it provides complete electrical isolation between controlling circuit and controlled circuit.
Relay can be defined as �A device that opens or closes an auxiliary circuit under some predetermined condition in the main circuit�. The object of relay is to act as a sort of electric magnifier i.e. to say, it enables a comparatively weak current to bring into operation a much stronger current.
RELAY CONSTRUCTION
Relay consists of three basic elements:
(i) An actuating element called exciting coil,
(ii) Linkage to transfer the actuation/de-actuation of input to output,
(iii) Output elements or the contacts.
The figure shows the construction of general purpose electromagnetic relay. It contains core surrounded by coil of wire: the core is mounted on metal frame. The movable part of relay is called armature when a voltage is applied to the coil, current flowing through it produces a magnetic field in the core. In other words, the core acts as electromagnet and attracts the metal armature. When the armature is attracted to the core, the magnetic path is from the core through armature, through the frame and back to the core. On removing the voltage, the spring attached to armature returns the armature to its original position. In this position there is small air gap. Hence more power is needed to pull in the armature than that needed to keep its hold in the attracted position.
The relay contacts and terminals are mounted on an insulated board. When coil is actuated by flow of current, armatures along with the contact arm assembly moves downwards so that the contact arm touches the bottom contact.
CLASSIFICATION OF RELAYS
Relays can be classified in many ways depending upon their function, input, output, applications and performance characteristics.
(i) Relay classification by function. According to function relays are categorized as:
(a) Monostable relay. A relay whose contacts return to the original position after the excitation is switched off. They are further neutral, polarized and AC voltage types.
(b) Bistable relay. Relays whose contacts remain in the charged position after the excitation is removed. The contacts change over to other position by an energizing current of opposite direction. These are further of polarized and remanent types.
(ii) Relays classified by output. Relays are classified as follows according to output.
(a) Low wattage dc load.
(b) Medium wattage dc load.
(c) High wattage dc load.
(d) Low wattage ac load.
(e) Medium wattage ac load.
(f) High wattage ac load.
(g) Specialized loads.
(iii) Relays classified by input. According to input relays are classified as:
(a) Direct current
(b) Alternating current.
(iv) Relay classified according to application. The relays are classified as follows on the basis of application:
(a) commercial
(b) industrial
(c) military
(d) communications
(e) railway
(v) Relays classified by performance. Relays are classified by what they can do and are required to do. These are:
(a) general purpose
(b) special purpose
(c) definite purpose.
FORMS OF CONTACTS:
The basic contacts are shown in the figure:
The arrows indicate the contact points. The main arm is moved by the moement of armature. This movement arm is indicated by black below it at one end.
Forms A and B are simplest. Form A is normally open contact which closes when the relay gets actuated and contact gets broken when the relay is energized. Form B is normally closed contact. Form C is single pole change over contact. Form D is make break contact. The long arm is contacted first. Then it is pushed away from short arm. Form E is basically a D contact combined with a B contact. Forms F,G,H and J are piles ups where multiple contacts are made or broken when the relay picks up.
A single relay can have a pile of contacts and operate all of them together. The contact assembly can contain a variety of contact forms.
COMMON TYPES OF RELAYS:-
Electromagnetic relays are manufactured in a wide variety of sizes, shapes and types to meet different requirements. Basic principle of operation remains same. The commonly used relays are classified below.
1. LATCHING RELAY: These are used in overload and alarm circuits. One relay is energized or de-energized, it stays in that position. These are like clapper relays, but they do not need continuous current through coil to keep them in open or closed position. A short pulse of sufficient amplitude is enough to close the relay. It has two coils and a mechanical ratchet for latching. A small current pulse to coil causes the armature to pull in and stay latched. A short pulse when applied to coil 2 causes the armature to go for normal position.
2. RATCHET RELAY: This is also a clapper type relay which does not need continuous current thorough its coil to remain in operation. It operates like latching type relay. Instead of holding down an armature as in a latching relay, each current pulse through relay coil causes a sprocket and cam assembly to move one position. The insulated cams on either end of cam shaft are made from Bakelite and hold the contacts open or closed.
3. ROTARY STEPPING RELAY: It is combination of a ratchet relay and rotary switch and is used to connect one or more input circuits to an output circuit chosen from a sizeable group of circuits. It responds to current pulses supplied externally.
Rotary stepping relay consists of three parts:
(a) Driving mechanism, (b) bank assembly, and (c) wiper assembly.
These stepping switches are stepped by a pawl and ratchet mechanism, making one step for each current pulse applied to switch coil. Pawl is connected to the relay armature. It causes wiper arm to step from one contact to another when a voltage is applied to the relay coil. The wiper can be positioned to any one of the twenty-four wiper contacts.
They are used extensively in machine tool control, conveyor systems, test equipment, and communication switching.
4. POLARIZED RELAY: In ordinary relay, its operation is not affected by the direction of flow of current. The armature moves irrespective of direction of current in coils. However, polarized relays are sensitive to the polarity of applied voltage. The contact closure they provide varies with the polarity of the applied voltage. They have fixed magnetic flux in addition to the flux provided by relay coil. This fixed flux is provided by the permanent magnet. It provides sufficient initial pull and hence small additional pull is required. This small pull can be derived from magnetic flux generated by small current in the coil. This relay requires less power. It is very sensitive and very fast operating.
5. SOLENOID RELAYS: It differs from capper relay in the way that a part of core moves to close the electrical contact. These are characterized by large armature travel and are used in high power circuits to handle large currents. It has large contacts and requires great pressure to produce a good low resistance connection.
Solenoid relays are used in applications where a large travel of the armature is required, such as closing of hydraulic value, automatic reversal of tape movement in a cassette recorder.
6. THE DRY REED RELAY: A reed relay is formed by placing a coil of wire wound around the glass tube. When d.c. is passed through the coil the magnetic field will close the reed switch contacts,. When the current through coil stops, the reed relay opens. The whole assembly forms a very compact electromagnet. These are available in open frame and dual in line (DIP) packaging. Dry reed devices are susceptible to the external noise. Hence to improve coupling of the coil to switch, many relays use some form of magnetic shielding. Metal casing serves this function.
7. MERCURY WETTED REED RELAY: It is similar in construction asdry reed relay except a small amount of mercury is added to the glass tube before the switch is sealed.
The main purpose in adding the mercury to capsule is to reduce and stabilize contact resistance and to increase contact load rating and life. The maximum allowable load varies from one reed to another.
8. SOLID STATE RELAY: They make use of semiconductor devices like SCR, triacs, etc. and these relays are mostly used for a.c. loads. They have longer life, very small noise, speed is upto nano seconds and operate with very less power.
9. GENERAL PURPOSE RELAY: It has clapper type armature, leaf springs, button contacts and an L or U shape heelpiece, with the coil directly pulling on the clapper type armature and movable contacts attached to the armature. They have advantage of cost and availability. They are used in the fields of air-conditioning and heating equipment, in household appliances, coin operated machines and lighting controls.
10. POWER TYPE RELAY: It is like general purpose relay. Usually, the insulation used is thick and of superior material and terminals are larger. Contacts are adequate for heavy currents, with large armature strokes and contact gaps. These relays can handle large currents and are easy to repair. They are used in air-conditioning heat equipment, electric appliances, motor control and machine tool control.
PACKAGING: Relays are packaged in a wide variety of contact arrangement and many package configurations using can package and crystal cans of relatively large enclosure, as well as plastic encapsulation and open construction with plastic dust covers. The packaging used depends on environmental and reliability considerations. If environment is controlled, a relatively chap and simple package can be used. In military or aerospace applications hermetically sealed or plastic encapsulated package becomes essential to avoid corrosion of contacts.