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# Experiments with a thermistor

Extracts from this document...

Introduction

S E N S O R   P R O J E C T

EXPERIMENTS WITH A THERMISTOR

AMIRUL OMAR

INTRODUCTION

What is a thermistor?

A thermistor is a type of temperature-sensitive resistor, meaning that its resistance is affected by a change in temperature. Thermistors can be categorised by their temperature coefficient of resistance, which is ratio of increase in resistance to its resistance temperature rise. Therefore, there are 2 main types, namely positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors. In a PTC thermistor, resistance increases with increasing temperature, whereas in a NTC thermistor, its resistance decreases instead. The graph below illustrates this fact:

Operating over a range of -200°C to + 1000°C, they are supplied in glass bead, disc, chips and probe formats. NTC thermistors should be chosen over PTC thermistors when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity.

The next question is, how does an NTC thermistor work? Why does its resistance decrease when temperature is increased? When the thermistor is subjected to an increase in temperature, the number of electrons able to move about and carry charge increases - it promotes them into the conducting band. Therefore it will be able to carry more current and as a result, its resistance falls.

Middle

2.70

2.80

2.75

50

2.33

2.51

2.42

60

2.19

2.15

2.17

70

1.80

2.00

1.90

80

1.65

1.61

1.63

90

1.40

1.48

1.44

100

1.25

1.45

1.35

Evaluation of Graph

When a graph of voltage/V against temperature/oC was plotted, a ‘best-fit’ curve can be drawn. From the line, it can be deduced that the voltage decreases with the increase in temperature. As temperature increases, the resistance of the thermistor decreases.

Error Report

• Not all the points fit into the curve, meaning that there were some inaccurate values. This is probably due to some heat loss or gain from and to the surroundings although a polystyrene cup and lid was used to minimise it.
• When I used the melting ice to record the voltage reading, the temperature was not exactly at 0oc. The ice cubes were not fully grinded to give a larger surface area and were virtually still in a solid state, and therefore there might be a little error in its voltage reading.
• For each temperature, the voltage reading on the multi-meter slightly fluctuated, therefore I had to repeat the experiment twice and calculate the average to minimise the percentage error of the values.

EXPERIMENT #2

Investigation

For the second experiment, I performed a similar kind of experiment, but from a different sort of perspective. In this case, instead of using a liquid medium with varying temperatures, I have used a source of heat which releases a reasonably constant and significant amount of heat.

Conclusion

Sensitivity = change of output / change of input

= 5.01 – 1.35 / 0 – 100

= 0.0366V/oC

Using calculations as stated below, it can also be deduced that the thermistor has a varying sensitivity, and is more sensitive at lower temperatures.

0 - 50oC          Sensitivity = 5.01 – 2.42 / 50 – 0

= 0.0518V/oC

50 – 100oC        Sensitivity = 2.42 – 1.35 / 100 – 50

= 0.0214V/oC

Response time

The response time is the time taken by a sensor to respond to a change in input. When a sudden temperature change was applied, the bead thermistor was able to respond to it 3 times faster than the thermistor probe, therefore comparatively, its response time is very rapid. When the thermistor is subjected to a change of temperature of 80oC, it took 7.30 seconds to show a steady value on the multi-meter.

Resolution

The resolution is the smallest change it can detect in the quantity it is measuring, or the precision with which the measurement can be made. When performing the experiment, the voltage value on the multi-meter constantly fluctuated, therefore an average had to be taken.

Random error

The graphs plotted were best-fit curves, whereby there were several values that did not coincide with the curve. Therefore there are small unsystematic variations present in the experimental data.

Systematic error

There is only a small systematic error because disturbing influences are minimised so that the thermistor would not be affected in terms of its resistance.

Overall comment

Due to its high sensitivity and rapid response time, it can be deduced that the thermistor is a good sensor system.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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