Page 87 - CITS - Electronic Mechanic - TT - 2024
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ELECTRONICS MECHANIC - CITS




           Thermistor
           A thermistor is a resistance thermometer, or a resistor whose resistance is dependent on temperature. The term
           is a combination of “thermal” and “resistor”. It is made of metallic oxides, pressed into a bead, disk, or cylindrical
           shape and then encapsulated with an impermeable material such as epoxy or glass.
           Thermocouple
           A thermocouple is a sensor that measures temperature. It consists of two different types of metals, joined together
           at one end. When the junction of the two metals is heated or cooled, a voltage is created that can be correlated
           back to the temperature.

           Principal Of Thermocouple
           The thermocouple working principle is based on the See back Effect. This effect states that when a closed circuit
           is formed by jointing two dissimilar metals at two junctions, and junctions are maintained at different temperatures
           then an electromotive force (e.m.f.) is induced in this closed circuit.
           Working Of Thermocouple
           A typical circuit diagram of a thermocouple is shown in Figure. In the Figure, two dissimilar metals ‘A’ and ‘B’ are
           joined at the two junctions ‘P’ and ‘Q’. Here the ‘P’ junction is measuring junction or hot junction whereas the
           junction ‘Q’ is the reference junction or cold junction. And a PMMC instrument is connected in this arrangement
           as shown in Figure.
































           When these junctions are kept at different temperatures, generally cold junction is kept at 0oC and measuring
           junction is kept at an unknown temperature which we want to measure (i.e. the temperature of the junction is
           raised by heating it). An e.m.f. will be generated in this circuit due to the temperature difference of the junctions.

           These are used to measure the temperature up to 1400oC. There are different combinations of metals which are
           used in thermocouples. Different combinations generate different e.m.f
           Types of Thermocouple
           T-Type Thermocouple

           Positive wire — Cu Negative wire — Constantan It can be used up to 350oC. It is very stable and inexpensive.
           Generally, it is used for very low-temperature applications.
           E – Type Thermocouple

           Positive wire — Chromel Negative wire — Constantan It can be used up to 850oC. It is most sensitive thermocouple.
           It generates a high output voltage.



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                                    CITS : E & H - Electronics Mechanic - Lesson 43 - 47
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