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Thread: Resistor on thermistor question

  1. #1
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    Default Resistor on thermistor question

    This may be a dumb question but I will ask anyway since I am not a master of electronic design. In the manual for hooking up a thermistor it says you should add a resistor between the adc pin and 0 volts, but why? arnt you just adding resistance?

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    It's not dumb if you don't know why !

    The thermistor is a variable resistance so with it between pin and +V and with a resistor between pin and 0V that gives a potential divider. As the temperature changes, the resistance changes, the ratio of thermistor resistance to that of the the pull-down determines the voltage put to the ADC pin.

    You've now got a temperature-to-voltage device which can be read by READADC then you can convert the voltage ( READADC returned value ) back to a temperature.

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    You make a
    Voltage divider.

    Please have a look at:
    http://www.kpsec.freeuk.com/vdivider.htm

    e

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    Thanks,
    Hippy, your definition makes alot of sense, and electric the web site helped alot. So lets see if i got it. If I have a temp. probe with a min resistance of 100.1 Ohm and a max resistance of 102.1 Ohm using the formula on the web site I need a 10 Ohm resistor? max and min are correct values....Ohm not k Ohm

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    Quote Originally Posted by buntay View Post
    Thanks,
    Hippy, your definition makes alot of sense, and electric the web site helped alot. So lets see if i got it. If I have a temp. probe with a min resistance of 100.1 Ohm and a max resistance of 102.1 Ohm using the formula on the web site I need a 10 Ohm resistor? max and min are correct values....Ohm not k Ohm
    A typical thermistor has a range of
    thousands of Ohms.

    See, for example:
    http://www.kpsec.freeuk.com/components/other.htm

    You will need to look at the
    Datasheet for your thermistor.

    Otherwise follow Manual 3, and use a 10k
    then start experimenting. :-)


    e

  6. #6

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    Thermistors come in all sorts of values.

    If you take a look at the listings in Farnell, for axample, your mind will boggle.

    Choose one which which has a value of a few k at your favourite temperature. As hinted at by Ec.
    And PLEASE take his advice about Data Sheets.
    And get a calculator and pen and paper.
    We must take the GUESSWORK out of these basic designs.

    At this stage in your 'career' avoid putting low resistors in series.
    Why?
    Check out the current calcs.
    The current may heat up the thermistor which will screw your thermometry.
    Or, worse, excess current will pop your thermistor - an unhappy day.

    So, i would suggest that you have a look at a couple of thermistors , tell us your app, tell us your proposed circuit and give us some links to Data Sheets for the favourite thermistors.

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    Quote Originally Posted by buntay View Post
    If I have a temp. probe with a min resistance of 100.1 Ohm and a max resistance of 102.1 Ohm using the formula on the web site I need a 10 Ohm resistor?
    Technically you can use any resistor but that will affect the voltage range which the voltage output will cover; at 5V, 10R pull-down, 100.1R => 0.4541V, 102.1R => 0.446V. That's going to give an extremely narrow range of readings from READADC10 even under perfect conditions.

    For voltage divider use it would be best to choose a thermistor which has a wide resistance swing across the temperature's you're measuring.

    For lower / narrower resistances I'd guess one would use an op-amp with the thermistor to adjust the gain of an applied signal and measure the result, or some Wheatstone Bridge configuration.

    An indication of your application and what you're intending to do may help give better answers.

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    In a Voltage Divider setup the Larger resistance always has the most Voltage across it,
    = available voltage swing to measure.

    E.g. 10K Resistor(Neg) + 100K Thermistor(Pos) (@25DegC) (5V Supply) = 0.45V and 4.54V,
    with Equal Resistances the Max you can get is 50/50.

    Also a voltage Divider wastes power, so using low value components will waste a lot
    more power. (Hint - Battery Apps )
    The total resistance of both components should be taken into account at all temperatures
    that may occur during use.

    Some Thermistors are as low as 10 or 27 Ohms (@25DegC), not really suitable for this type of App.

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    It sounds to me like this part is a 100 ohm RTD, not a thermistor. With a span of only 2 ohms, it will be hard to get a workable range of values for the ADC with just a simple voltage divider. Some signal conditioning is required.
    JaguarJoe

    If Einstein and Pythagoras were both right then E = M(a^2 + b^2)

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    @buntay note there is another issue when measuring temperature with a temperature variable resistance device - self heating. Note that a power of V*V/R is being dissipated in the device and this - depending on your needs of accuracy and thermal mass - may affect your readings - ideally you want low voltage on the temperature sense unit or a high resistance in that unit.

    Good luck with your project!

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