Page 87 - CITS - Electronic Mechanic - TT - 2024

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