The following points highlight the six main types of relays used in electronic equipments. The types are: 1. Electromagnetic Relay 2. Power Relays 3. Telephone Relay 4. Card Actuated Relay 5. Sensitive Relay 6. Solid State Relays.
Type # 1. Electromagnetic Relay:
Most of the relays used are electromagnetically operated. The parts of these relays are an iron core and its surrounding coil of wire, an iron yoke which provides a low reluctance path for magnetic flux-the yoke being shaped so that the magnetic circuit can be closed by a movable piece of iron called the armature, and a set of contacts. The armature is hinged to the yoke and is held by a spring in such a way that there is an air gap in the magnetic circuit.
Fig 8.3. Basic diagram showing the operating principle of a relay.
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Fig. 8.3 shows the principle of operation of this relay. When an electric current flows in the coil, the armature is attracted to the iron core. Electrical switching contacts are mounted on the armature. When the relay coil is energised, these movable contacts break their connection with one set of fixed contacts and close a connection to previously open contacts.
When electric power is removed from the relay coil, a spring returns the armature to its original position. The relay coil is bobbin-wound with insulated copper wire. The resistance of the coil, and thus the current that can flow in the coil for any energising voltage, is determined by the gauge of the wire and the length used.
The coil mmf is proportional to the number of turns of wire on the coil times the current flowing through the wire. The more is the number of turns put on the bobbin, the less is the current needed to operate the relay. Standard voltages for dc relays are 6, 12, 24, 48 and 110 V and for ac relays are 6, 12, 24, 48, 120 and 240 volts.
Type # 2. Power Relays:
Power relays are heavy duty clapper relays capable of multiple switching of resistive loads of 15 A to 25 A per pole at either 24 V dc or upto 230 V ac. They are designed for use in industrial equipment for controlling fractional horse power motors, solenoids, and heating elements or to energise a high power contactor.
Type # 3. Telephone Relay:
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The telephone relay is the most reliable, time proven and having a versatile design. It is made in variety of sizes from standard to subminiature. These types of relays are used in large numbers in telephone exchange. The contact springs, interleaved with insulators, are clamped to the frame and actuated by insulated studs attached to the armature.
Type # 4. Card Actuated Relay:
In this type of relay, the movable contact arms protrude through holes in a rigid plastic card attached to the armature. When the relay is operated the card moves either by flexing the movable contact arms to make contact with fixed arms of by flexing the movable contact arms to make contact with fixed arms or by allowing pre-stressed contact arms to make with fixed contact arms. Card actuated relays are of two general types-industrial and wire spring. Industrial card actuated relays are similar in design to telephone type except for the card actuation.
In wire spring relays, the contact arms are thin round wires rather than flat springs. A single wire spring relay can transfer as many as 51 circuits at the same time, replacing, and four or more conventional relays.
Type # 5. Sensitive Relay:
A sensitive relay is a relay that can operate on a comparatively small amount of power or on a small pulse signal for continuously powered relays. Conventional electromagnetically actuated relays can be made sensitive by increasing the turn on the coil resulting in a stronger magnetic flux, and by increasing the magnetic efficiency of the iron core and magnetic structure both by configuration and choice of materials.
Type # 6. Solid State Relays:
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Solid state relays consist of a solid state switching device driven by either an amplifier, an LED optical coupler, or a transformer. The magnitude of the load current and whether it is ac or dc determines the choice of a semiconductor switching device. For low level dc switching FET are used. Bipolar devices are used for intermediate and power level dc switching. Triacs are used for ac or dc power switching.
Advantages of solid-state relays are:
(i) High reliability
(ii) Long life
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(iii) No known wear-out modes
(iv) Silent operation
(v) No contact bounce or arcing which makes operation relatively transient free.