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Homework Help: As level sensor coursework-thermistor-linearity

  1. Dec 30, 2005 #1
    hi guys,
    Basically im having a little problem with linearity on this coursework. First off i did a simple resistance against temperature circuit in series, then i was told to put this in a potential divider circuit,so id measure the voltage output of the thermistor and see how this was affected by resistance and temperature change. I was told that this would produce more linear results,although i cant understand why. Also instead of using trial and error to bundle a pair of resistors together to get good linearity ,is there another way to predict which combination of resistors will produce the most linear results. Also in my conclusion on my different combinations of resistors,instead of just saying that it looks quite or not very linear,is there a way to prove that its linear, and also to prove which is the most linear,because for some graphs its hard to tell.
  2. jcsd
  3. Dec 31, 2005 #2
    anybody,please. its in for Tuesday,so id be really grateful if someone could help me out just a little
  4. Jan 1, 2006 #3
    First of all, almost all metals have positive temperature coefficient of resistance but there are some materials such as Carbon and Germanium which have negative temperature coefficient.
    Thermistors are basically semiconductor devices. In case of semiconductors resistivity decreases with increasing temperature which means semiconductors exhibit negative temperature coefficient of resistance. The change in resisitivity with respect to temperature takes place in a reproducible manner. The temperature characteristics of these elements are always non-linear. If you plot a graph of the Voltage against the Current, you will find the graph remains in the Ohmic region for sometime initial readings but goes into the negative resistance region after sometime.
  5. Jan 1, 2006 #4
    well i only did between 10 Celsius to 80 Celsius so for me alot of the graphs are linear at this specific temperature range,sorry i should have stated that at the beginning. So i dont really have to talk about other regions only about the results i gained. So is there a way to say why certain combinations of resistors in a potential divider circuit provide linear results between 10-80 Celsius?
    Also thanks for replying.
  6. Jan 2, 2006 #5
    Thermal characteristics:
    Well let us look at the formula giving the relationship between the temperature and the resistance:
    [tex]R_T = R_0 e^{\left[B\left(\frac{1}{T} - \frac{1}{T_0}\right)\right]}[/tex]

    where RT and R0 are the resistances at T and T0 degree Kelvin. B is a material constant.
    The formula clearly shows a non-linear relationship between the temperature and the resistance.

    When you connect it to an electric circuit you deal with the electrical characteristics. The voltage-current graph is linear upto a certain limit but the temperature-voltage graph is non-linear. Can you show me what formulae you have used?
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