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Temperature measurement

Discussion in 'Electronic Basics' started by Abbie, Nov 24, 2003.

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  1. Abbie

    Abbie Guest

    Can someone recommend the simplest circuit for temperature
    measurement with a passive thermistor. I also need the theory
    and the mathematics. I recall that I need two clibration points
    and two variable resistors.
  2. Rich Webb

    Rich Webb Guest

    The simplest would probably just be a voltage divider with the
    thermistor as the lower (ground side) resistor. A starting point would
    be to use an upper (Vcc side) resistor that's around 1/2 the nominal
    value of the thermistor, giving you a voltage range from about 2/3 of
    Vcc down towards ground as the thermistor heats up.

    Measure the voltage, calculate the thermistor resistance as
    Rt = R1 * Vm / (Vcc - Vm). Look up or calculate the temperature as a
    function of the resistance. That can range from a simple linear
    approximation, to a two- or three-segment linear approximation, all the
    way up to a Steinhart-Hart approximation (google for it).
  3. Suraj

    Suraj Guest

    I did a similar project by interfacing a 10 K NTC to an 8 Bit ADC a
    couple of years ago. Literally learnt everything off this group, even the
    thermistors were sent by somebody on this group!!
    I implemented the Steinhart's approx in C++. If you need some of the helpful
    posts I got, I'll send them to you.

  4. Bob Masta

    Bob Masta Guest

    If you are not absolutely locked into using a thermistor,
    consider using an ordinary junction diode instead.
    The forward resistance is extremely linear over a wide
    temperature range. The only caveat is that each unit
    needs individual calibration (ice water and boiling
    water, etc). So this approach is probably better for
    a one-off project than a production item. But it
    eliminates the need for linearity correction, which
    can be a problem with thermistors.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  5. Baphomet

    Baphomet Guest

    "forward resistance is extremely linear"? Did you mean forward voltage drop?
  6. Bob Masta

    Bob Masta Guest

    The foward voltage drop is only linear with temperature when it is
    passing a constant current, hence my use of "resistance".
    It's not that resistance *as such* is linear with temperature,
    but at any given measuring current it is. You just can't
    compare measurements taken with different currents.

    Incidentally, the linear range for this technique is reputed
    to go down to a few degrees Kelvin. On the high end it
    is limited by melting of the device or its contacts, but
    I imagine it should be about as good as the original
    device rating.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
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