Temperature sensing circuit - Sample Essay

In this investigation I want to look into how a thermistor works, then experiment to find how temperature change affects its resistance. The main aim of this investigation is to design and build, using a thermistor, a temperature sensing circuit. This will be for use in a computer system. Because of the recent advances in the speeds of PCs there has been a big problem in to deal with the extra heat that their faster CPUs (central processing units) produce. Most manufactures have simply increased the surface area of heat sinks used and fitted larger and more powerful fans but even these are sometimes not enough.

If the same air is being forced through a heat sink over and over again it becomes quite warm and unable to remove more heat from the CPU so its temperature will rise. To solve this problem case fans are needed to refresh the air in the case but these can be noisy. Because the amount of heat produced varies and there is a big difference between the heat made on idle and when the CPU is under load (it can vary from 30w to 100w in some cases). This means the extra case cooling would only be needed when the CPU is under load and reaches a certain temperature.

Noise is that last thing you want when trying to concentrate at a computer so by only having the case fans on when needed to remove the hot air from the case I keep the computer as quite as possible whilst keeping the CPU at safe temperatures. Preliminary: How a thermistor works: Commonly, thermistors are made of semiconductor materials such as silicon, because this has a tendency to conducts better at higher temperatures, the resistance goes down as the temperature gets higher and the resistance will go up as it gets colder. Plan:

My preliminary investigation will involve gathering test data about the resistance of the thermistor I want to use at different temperatures. To do this I would wire up the thermistor and plug it into a multimeter and use it to test for resistance. This is a lot simpler than setting up a powered circuit and measuring the volts and amps to determine the resistance. It also reduces that chance of the current through the thermistor heating it up and altering the results or even blowing the thermistor and finally makes it safer because mixing mains power and water isn’t good practice. I will set up the experiment as shown in fig. 1

I will fix the temperature sensor from a multimeter to the thermistor with insulations tape to hold them close together. This will limit the temperature variation between the two. I will then pour a small amount of hot (80i?? C) water into a beaker and place the instrument into the water. I am using a small amount of water because it will cool faster so I don’t have to wait as long for the results, also I am not being exactly precise with the amount of water because I think it wont affect the overall results and if I add ice to the water to speed up getting to the low temperatures it will be changing the volume of water anyway.

I will take measurements of the resistance every 5i?? C starting at 75i?? C and dropping to 25i?? C. These are suitable temperatures because a CPU core will never get as low as room temperature, normally being at least 5i?? C above it, and once it reaches 75i?? C there is a high chance it will crash or even short out. It’s at this temperature that the warranty on the CPU becomes void. I will repeat the experiment 3 times to produce an accurate average and to avoid the chance of anomalous readings affecting the results.

Because of the properties of semiconductors like the ones used in thermistors I expect to see the resistance rise as the temperature falls. Results: See table fig. 2 and graph fig. 3 From my results I can see as expected the rise in resistance as the temperature falls. This does not have a straight line trend but is in a curve. I ended up testing the thermistor 4 times instead of the stated 3 because I noticed more varied results in the first 3 tests than expected and I wanted to do an extra test to get a better average.

Circuit design: Having collected the values I want for the resistance of the thermistor at different temperatures I can now plan my circuit for the fan controller. I will use a potential divider circuit with a variable resistor and the thermistor as R1 and R2. I will use this to turn on a relay when the temperature is high enough and the relay will switch on the fan. Here is my component list: Thermistor Rheostat Power supply x2 Cables Relay 80mm case fan Multimeter – to help set up the resistance of the rheostat.

The relay switches at 3v and the power supplies will mimic the power supply in a computer for this prototype. This means I have the choice of 2 input voltages: 5v and 12v (as is standard for an ATX Power supply). For the potential divider I will use 5v because 12v is un-necessary but I will power the fan with 12v because that is its rated voltage. The relay also has the ability to work in two ways – to break the circuit when powered or to make it. For this circuit I will use it to break.

I am using a rheostat instead of a fixed resistor because it gives me the opportunity to adjust the temperature at which the fan will come on. This is useful for using the design on different processors and in different situations as the safe temperature for different CPUs varies. I will set up my circuit as shown in fig. 4 The thermistor would be placed as close to the core of the CPU as possible to get the most accurate readings. For this investigation I will be setting it up to turn the fan on at 55i?? C which is a slightly high CPU temp.