Experiment to determine characteristics of Thermistor

Experiment: Characteristics of Thermistor

This section provides you a detailed steps to determine the characteristics of thermistor.

Aim: To determine the characteristics of thermistor (resistance temperature characteristics).

Apparatus required: Bead type thermistor, multimeter, thermometer, water bath, heater connecting wires, etc.

bead type thermistor
bead type thermistor

Introduction:

Thermistors are the temperature sensitive resistors that exhibit a negative temperature coefficient of resistance. In other words electrical resistance of a thermometer will be reduced when it is placed in an environment of higher temperature likewise its temperature decreases. thus the characteristics of thermistor provides an information about how its resistance changes with the changes in temperature.

It is very essential in temperature measurement, thermistors are manufactured and formed into rods, discs, and washes, beads for special applications, they can be directly or indirectly heated. Temperature determines the resistance of those that are directly heated in environment. The resistance of indirectly heated thermistor is determined by temperature of self-contained heater.

Diagram:

types of thermistor
types of thermistor

Procedure:

Following are the steps to determine the characteristics of thermistor.

1)    Take water in container and place a heater to heat water.

2)    Immerse thermistor and thermometer in water bath.

3)    Switch on the power supply.

4)    Measure the temperature on the thermometer from room temperature (30 C) to 98 C and corresponding resistance of thermistor at that temperature.

5)    Switch off the power supply, and then take reading in decreasing order of temperature in an interval of 10 C.

6)    Plot a graph of temperature on X-axis and Resistance on Y-axis. This graph shows the characteristics of thermistor.

Observation Table:

By taking following readings we can plot the characteristics of thermistor on a graph paper.

Sr. No.: Temp in degree Celsius Resistance(Uploading) Resistance(Downloading)

When we plot characteristics of thermistor it will look like as follows:

Characteristics of thermistor
Characteristics of thermistor

Calculations:

We have To= 30 C, Ro=980?, ß=4000.

R(t) = Ro*exp [  ß* (1/T  –  1/To) ]

Conclusion:

1)    Input and output relationship is non-linear for thermistor (i.e. characteristics of thermistor are non-linear).

2)    In comparison with RTD change in resistance for a given change in temperature is very large.

Analysis:

1)    What is negative temperature coefficient of resistance ?

Answer: The property of a material in which resistance of a material decreases with increase in temperature that material is said to have negative temperature coefficient of resistance.

2)    What are common shapes of most commercially available thermistors.

Answer: The common shapes are bead type, disc type, rod type ans IC chips.

3)    What the difference between directly heated and indirectly heated thermistor?

Answer: In directly heated thermistor, there is direct contact between source and thermistor, but in indirectly heated thermistor there is no direct between source and thermistor.

4)    What is the relationship between thermistor resistance and temperature?

Answer: In thermistor, resistance is inversely proportional to the temperature.

5)    What is the range of temperature for thermistor?

Answer: The temperature range of thermistor is -50 C to 15C

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