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Thermistors

Discussion in 'Electronic Design' started by Dave, May 15, 2005.

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  1. Dave

    Dave Guest

    Anyone know of a good app note for these? The ones I have seen are very
    vague.

    Thanks!
     
  2. Try the manufacturer's web site. If there isn't any, then try another
    manufacturer. Lots of info out there.


    Best regards,
    Spehro Pefhany
     
  3. Pooh Bear

    Pooh Bear Guest

    I think Epcos may have some app notes - also try Vishay BC.

    Thermistors *are* pretty vague btw. Considered a semiconductor solution ?

    Graham
     
  4. Genome

    Genome Guest

    YES

    We are talking lost resources here.

    The original manufacturers were Philips and Siemens who produce, or
    otherwise did produce, what have to be the best data sheets on the planet
    for passive components. They still produce good shit for the shit they do.

    You will notice that they are European companies.

    Not to say that you American or Japanese people don't know **** but, you
    couldn't find it in the first place. Wankers.

    The Philips stuff got sold off to Beyerschlag(? BC bought up by Vishay) and
    the Siemens stuff got sold off to Epcos. Both of which promptly fucked
    things up three times over.

    So.... it's all shit as defined by some suck cock beany fucker.

    DNA
     
  5. Fred Bloggs

    Fred Bloggs Guest

    That pretty much sums it up nicely- Thnx. And don't forget those British
    eff-yew-see-kays who bought up Gates batteries....
     
  6. Are they ?
    Not really. There are some with 1% matching or less.
    Then you measue temperatures, you have some problems
    anyway. How are you going to calibrate ?
    A PT100 ? With a precision current of voltage source ?
    You're good when you reach 6 digits for a current or
    voltage source. Then apply the PT100 to the location isn't
    trivial either.
    On the other hand, calibration with a temperature reference.
    Water at the tripple point, outgaz it with vacuum,
    apply vaccum to reach the tripple point and hope you have
    some good contact. Good. Then you may need a second point.
    A temperature normal. Freezing mercury, non-alloyed.
    Yuck. Another idea....

    I found NTCs good for repeatable 10mK accuracy around
    room temperature. Knowing where you are is a different matter
    but perhaps not really necessary.

    Rene
     
  7. Thermometrics Taunton Devon UK used to be STC they still make precision
    NTC and as described they can provide the best temperature information
    for many applications. Note I used to employ thermistors with good long
    term aging but un adjusted resistance to compensate crystal oscillators
    individually matching crystal/resistor/thermistor for best performance.
    Ignore demon address try
     
  8. Guest

    If you want interchangeable thermistors, Yellow Springs Instruments -
    YSI - were the original manufacuturers, and when I last looked you
    could still get some of their +/-0.05C tolerance parts of the shelf
    from Newark (via Farnell if you live outside the USA).

    For their data sheets and application notes look at

    http://www.ysi.com/temperature.htm

    In Europe, BetaTherm in Ireland have a decent range of parts at
    +/-0.2C- see

    http://www.betatherm.com/

    Thermometrics make some very good interchangeable thermistors - see

    http://www.thermometrics.com/

    If memory serves, their application notes were particularly good.
     
  9. Dave

    Dave Guest

    Thanks to all who made datasheet/app note suggestions. After some
    reading I'm getting the impression that all thermistors have a
    non-linear curve? Am I limited to temp sensor chips (lm35 etc) if I
    need a linear response?

    Thanks
     
  10. Joerg

    Joerg Guest

    Hello Dave,
    No. There are also plain diodes and thermocouples. You have to deal with
    rather small signals here but with clever offset nulling (clamping,
    chopper amp etc.) it can be done.

    Regards, Joerg
     
  11. There are compound thermistor devices that are linear, but with about
    1/10 the output, IIRC.
    Maybe you could linearize the response over the region of interest,
    using analog or digital means.



    Best regards,
    Spehro Pefhany
     
  12. Precious-metal RTDs are one of the best temperature sensors if you
    need an accurate linear response and don't mind a little cost. They
    have only a tiny bit of nonlinearity over a narrow range, and most of
    that is easily eliminated with a taste of +ve feedback. They are also
    made very accurately (initial tolerance) and can be very much free of
    hysteresis and drift, depending on the construction you choose. Base
    metal types are popular with the comfort heating and A/C crowd, and
    for monitoring heavy motors.

    There are plenty of other types of temperature sensors.



    Best regards,
    Spehro Pefhany
     
  13. Bill Sloman

    Bill Sloman Guest

    All thermistors seem to conform to the Steenhart-Hart fitting function,
    which is pretty non-linear.

    Over a sufficiently small temperature range, a thermistor-based sesnsing
    circuit can look reasonably linear.

    As Spehro Pefhany has pointed out, there are a number of options available
    if you need to measure temperature over a wider range.

    A thermistor plus Microchip Technology PIC microcontroller with a built-in
    A/D is probably the cheapest way to go if you need linear temperature
    measurements over a wider range.

    A platinum resistance thermometer is certainly the best way of getting a
    good temperature measurement, but you have to dissipate quite a lot more
    power in the sensor, and even then the sensitivity in volts per degree is
    quite a lot less, to the point where you find yourself stuck with building
    an AC-excited bridge circuit to get reliable resistance measurement. That is
    the way to go if you want to build a microdegree temperature controller, but
    it tends to be an overkill if you are less ambitious.
     
  14. Pooh Bear

    Pooh Bear Guest

    I have in the past simply used the temp coefficient of Vbe with a simple
    transistor sensor.

    It's linear with temp, you just need to select a device that has a
    reasonably consistent Vbe @ a given temp. Depends how accurately you're
    measuring.

    I understand this method has been used for measuring die temp in modern
    CPUs.

    Graham
     
  15. Except they don't use Vbe these days, they use delta Vbe at two
    different currents, which gives better interchangability. On the minus
    side, the signal level is an order of magnitude lower (more like
    200uV/K than 2mV/K).



    Best regards,
    Spehro Pefhany
     
  16. Ken Smith

    Ken Smith Guest

    Two 1N914s work quite will to get the difference in temperature. I used
    this fact in a controller for the pump in a solar hot water system many
    many years ago.
     
  17. The important questions with temperature sensors
    are "how many", and "do you already possess some
    means of temperature calibration".

    If the OP just wants one (or even just a few), then
    he goes for an already calibrated sensor, Pt100 or
    precision thermistor, at the appropriate price.

    If there is a production quantity then he can go for
    a cheap homebrew sensor, (of whatever sort is most
    convenient), and invest some money in a temperature
    calibration test setup.

    I like metallic resistive temperature sensors, (Pt,
    Ni, or even Cu in some apps), because the Beta-term
    is reasonably known and the linearising is simple.

    Afair there is even a combination of Pt+Ni where the
    correct proportions make the Beta-terms cancel, but
    have never seen a proprietry implementation of this.
     
  18. doug dwyer

    doug dwyer Guest

    If you want to control at a set temperature thermistors are good, YSI?
    Used to do a pair on thermistors that gave a linear voultage over a
    linited temperature range.
     
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