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thermistor compensation

Discussion in 'Electronic Design' started by daveatfernie, Apr 26, 2006.

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  1. daveatfernie

    daveatfernie Guest

    Hi everyone,
    I have some 0402 surface mounted thermistors which are causing me some
    bother. I am measuring an object in contact with the thermistor but am
    obviously measuring the temperature inbetween that of the PCB and the
    object. I would like to use another thermistor placed in the circuit
    (that will measure the temperature of the PCB) to allow me to offset
    this middle temperature to compensate. Can anyone help with a circuit
    example or ideas?

    Many thanks in advance

    David
     
  2. Phil Hobbs

    Phil Hobbs Guest

    Given the very small thermal mass of an 0402, and the huge thermal
    conductivity of the alumina body, solder, and copper pads, your readout
    signal error is going to be dominated by the quality of the thermal
    contact between the 0402 and the object being sensed. Given that,
    you're going to have real problems getting a good calibration from a
    second thermistor.

    Think about it as a voltage divider problem: analogically speaking,
    your sensor has a 1-ohm resistor go ground across its input, and you're
    connecting it to an unknown voltage (the object temperature) across a
    resistor of poorly controlled resistance that is certainly a good deal
    larger than 1 ohm (the thermal contact).

    This is not a recipe for a good measurement, regardless of how closely
    matched the two thermistor curves may be. You ought to figure out how
    to insulate the thermistor from the board.

    Cheers,

    Phil Hobbs
     
  3. daveatfernie

    daveatfernie Guest

    Thanks for the reply Phil.
    At the moment my jig is conducting the heat to the sensor well and
    repetitively. (I have a look up table to convert the resiatance into
    degres C and I'm getting a value approaching 0.1 degree C Accuraccy)
    I can read the temperature accurately with the sensor but it does vary
    with ambient.

    I have tiny PCB tracks (minimum my manufacturer will let me have 0.127)
    to try to minimise heatsinking.
    Unfortunately insulating an 0402 from the PCB is nigh on impossible.
    However I can improve the thermal conductivity of the ambient
    compensation sensor to the PCB (to accurately measure the temperature
    of the PCB).

    The offset of ambient is the final problem really. My thermistors have
    less than 1% variation.

    The layout is somthing like below:

    Object
    Thermal paste
    [ Real sensor ]
    ================================PCB
    [ambient sensor]

    Do you think I'm fighting a loosing battle?
     
  4. Do you have room to route a slot on each side of the sensing
    thermistor? I think if you could do that, and add a patch of thermal
    insulation (foam tape) to the back side of the board (and move the
    ambient sensor outside of that thermal insulation) you could improve
    your accuracy by an order of magnitude or more.
     
  5. John Larkin

    John Larkin Guest

    If you add the second thermistor and calibrate the fudgefactor
    somehow, you should be able to improve accuracy by, I'm guessing, 5:1
    or so. You'll have to do the thermistor curve thing to each thermistor
    independently, giving two temperatures, and then apply the fodge.

    I've seen similar stuff done in poorly-designed ovenized crystal
    oscillators, like from S**.

    John
     
  6. daveatfernie

    daveatfernie Guest

    Many thanks to both Johns,
    I'll look in to routing some of the PCB away. It is the fudge
    suggested by John Larkin I would prefer to add but electrically using
    perhaps a potential divider circuit but am having trouble getting my
    head around what I'm after. As the ambient increases the target sensor
    reads closer to the correct temperature so i need to further decrease
    the resistance of the target sensor using the ambient sensor.
    Many thanks again

    David
     
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