Thermistor Investigation

Thermistor Investigation
For this experiment I am going to investigate the variation of the resistance of a thermistor with different temperatures. The resistance will be tested at approximately every 10 degrees, with a range from 80 C down to 30 C, and these results will be recorded. After recording the results, the experiment will be repeated at a total of 3 times, this is so we can get an average that should be correct and accurate. Scientific Knowledge: Thermistors are thermally sensitive resistors and have, according to type, a negative (ntc), or positive (ptc) resistance/temperature coefficient. A thermistor is a type of resistor whose resistance changes significantly when its temperature changes. A negative temperature coefficient (ntc) thermistor has a resistance that decreases with increase of temperature. A positive temperature coefficient (ptc) thermistor has a resistance that increases with increase of temperature. ntc thermistors are temperature dependant semiconductor resistors. When the temperature of a semiconductor is increased, the number of charge carriers increases as more valence electrons gain sufficient energy to break free from atoms to become conduction electrons.
The number of charge carriers increases as the temperature is increased so the resistance of the semiconductor falls. A thermistor is used to sense the alterations in temperature. It does this by altering its resistance, but not in proportion to the change of temperature. When the temperature increases the resistance of the thermistor drops, but not linearly. This is due to electrons in the thermistor gaining more energy because of the temperature increase. They then gain more kinetic energy i.e. a lot of movement, due to the temperature increase. When the electrons have more energy they are then more free to move. With more movement, conduction in the thermistor is easier as the electrons that would have been in the way of the flowing current are now able to contribute to the current. So if the temperature decreases, the opposite happens and the resistance will increase. Thermistors are used in temperature sensors and digital thermometers in systems when the resistance needs to drop drastically under a small amount of temperature change. An example of a system when a thermistor is needed is in incubators, when the temperature needs to be controlled and the temperature needs to be kept totally constant. If the temperature inside begins to decreases then the resistance will increase, therefore the cooling system will stop producing cold air so much. If the temperature begins to increase then the thermistors resistance will decrease, therefore the cooling system will work more, thus keeping the temperature inside the system constant. Prediction: I predict that when the temperature of the thermistor is increased the resistance will decrease. I think this because when a thermistor is heated there two competing effects which are; a) that, as in a wire, the atoms vibrate faster and collide with the flow of charge ( which is the flow of free electrons) slowing it down . b) b) because it is a semi conductor, the outer electrons get more energy as it is heated and are freed. I predict that the graph will be a curve. I think this because at a low temperature there are lots of electrons which can be freed as the atoms start to vibrate more. At a higher temperature the atoms have less electrons which can be freed as some have already been freed. Therefore I think that the reduction in decrease will be largest at low temperatures and then get smaller as the temperature rises. Apparatus: Ø Thermistor Ø Multimeter Ø Kettle Ø Glass Rod Ø Beaker Ø Leads Ø Thermometer Ø Ice Diagram: [image] Multimeter [image] [image]Thermistor Hot Water Method: First I will set the multimeter to KÎ?, unit for resistance. Next I will fill the beaker with boiling water from the kettle. Then place the thermometer into the beaker, to check the temperature. If the temperature is around 85 C then I will place the thermistor into the beaker. Before placing the thermistor into the beaker, I will take a reading of the room temperature and its resistance. Once the thermistor is placed I will start to take the readings from the thermometer for temperature and the multimeter for the resistance. I will take the readings every time when the temperature drops by 10 C. To speed up the experiment I will use ice or cold water to drop the temperature quickly because itÂ?s very time consuming when you let the temperature to drop by it self. Another thing I have to take in account is that when I add cold water or ice I have to thoroughly make sure the water is stirred to keep water at the same temperature, otherwise the water around the thermistor will be at a different temperature, to that around the thermometer. I will end the experiment by taking the last reading of 30 C and I will do this experiment three times. Then from the three results I will find the average. Results: Room temperature was at 24 C and the resistance was 1.81 KÎ?. Temperature q( C ) Readings Nominal q C KW 80 80 0.127 70 70.5 0.167 60 60.2 0.248 50 50 0.362 40 42 0.502 30 32 0.769 Temperature q( C ) Readings Nominal q C KW 80 80.2 0.16 70 71 0.166 60 60.5 0.234 50 51 0.348 40 41 0.498 30 32 0.774 Temperature q( C ) Readings Nominal q C KW 80 80.5 0.123 70 71 0.163 60 60.3 0.244 50 52 0.358 40 40 0.453 30 32 0.773 Temperature q( C ) Average Readings Nominal q C KW 80 80.2 0.137 70 70.8 0.165 60 60.3 0.242 50 51 0.356 40 41 0.484 30 32 0.772 Graph: [image] Analysis: From my graph I can clearly see that as the temperature increases the resistance decreases. This is because as the thermistor increases in temperature the atoms move faster and are colliding with the electrical current and slowing down the flow of charge. In a normal wire this would make the resistance decrease but because a thermistor is a semiconductor there is a second stronger effect which out weighs this. This effect means that the outer electrons are not free at room temperature but when heated the get more energy and are freed. This means that there are more electrons available to conduct. This does not apply in a normal wire because the outer electrons are free and the inner electrons are tightly held in the atom. The two competing effects in the thermistor mean that when the temperature is increased the resistance decreases. This tells us that the thermistor is a ntc. The graph shows the resistance dropping from about 0.772 KW to 0.484KW. This is a drop of 0.288 KW. By 10 C the resistance has already dropped to about 0.288 KW which proves that the resistance decreases most at lower temperatures and less at higher temperatures. It is because at a low temperature there are lots of electrons which can be freed as the atoms start to vibrate more. At a higher temperature the atoms have less electrons which can be freed as some have already been freed. Therefore the reduction in decrease will be largest at low temperatures and then get smaller as the temperature rises. Conclusion: As you can see from the results, as expected the resistance produced by the thermistor decreases drastically as the temperature increases. This is because the semiconductor material of the thermistor becomes more conductive as the temperature increases. In conclusion I think the results and the graph supported my prediction. Evaluation: In my experiment, the thermistor was quite good which gave accurate results eccept of some anomalies in my result. I think this was caused by human errors. Small unsystematic variations are present in all experimental data. Their size limits the precision with which a measurement can be made. Taking an average over repeated measurements can improve the final result, as long as the conditions can be kept the same. This is why I took recordings when the thermistor was in an environment when the temperature was increasing and then decreasing, this gave me two sets of recordings which allowed me to check my results were correct. Systematic error is very hard to detect, because detecting it means making another, even better, measurement. Systematic errors include zero error, and error due to disturbing influences, for example temperature. I think the thermistor could have been better by having a high resolution, appropriate output for a given input, rapid response time, small unsystematic fluctuations in results, and a small systematic error. The sensitivity of my thermistor is measured in kÎ?. Sometimes it is necessary to reduce the sensitivity so that the measuring instrument can deal with larger changes of input. This is why I decided to take my readings in kÎ? instead of Î?. In further investigation I would like to investigate resistance change and temperature for different components.

Thermistor Investigation 7.7 of 10 on the basis of 774 Review.