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Thermocouple mux

Discussion in 'Electronic Design' started by Marco Trapanese, Nov 25, 2012.

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  1. Hello!

    I'm working on a simple board that uses the recommended Analog's design
    to acquire a tc:

    1. what do you think about this circuit? IMO, it's quiet expensive but
    it should be reliable

    2. I need to acquire two tc: how to mux both sensors? I mean: I want to
    use the same front-end and select what tc to acquire.

  2. rickman

    rickman Guest

    I don't know how expensive this circuit is, but I seem to recall that
    you can get a decent TC chip for $5 that gives you a digital output with
    cold junction compensation. Turns out the hard part of a TC measurement
    is the cold junction compensation. How do you measure the temperature
    of the cold junction to ±0.5­°C? The last time I looked at this I found
    that ±2°C was about as good as you can get without spending more bucks.

    You can mux the TC wires using an analog switch, but you need to decide
    up front how you are going to handle the grounding. Some probes expect
    one side of the TC to be hard grounded, others need to be floated. The
    most common compromise is to ground one side with a 1 Mohm resistor like
    show in the app schematic. So you may want to switch both legs so the
    two probes can be different.

  3. Il 25/11/2012 23:29, rickman ha scritto:

    Do you recall any part number?

    Ok thanks!
  4. Il 25/11/2012 22:06, John Larkin ha scritto:

    I was afraid about the change of materials, but I was wrong because all
    tc will change at the same point. Very easy, indeed.

    Got it.

    The ds proposes an hardware solution to compensate the cold-junction
    using a thermistor and a precision resistor. I guess this requires a
    precise selection of them (and perhaps I need to adjust values among
    production batches).

    In fact, you suggested to do the compensation in software.

    How to handle this? I mean: I read the absolute Vtc using ADC (e.g. in
    uV). Then I know the temperature of the cold-junction in °C.

    In order to get the temperature at the reference junction do I need to
    "invert" the tc equation? So I get the voltage a tc should exhibit at
    the cold-junction temperature, I subtract it from the actual Vtc and
    finally convert this into a temperature again.

    Am I right?

  5. Il 26/11/2012 10:14, Spuckle ha scritto:

    Very interesting!
    The Larkin's solution is more accurate and flexible. This one is very
    straightforward. Anyway some parameters are provided by design only -
    not tested.

    Have you ever used this chip?

  6. whit3rd

    whit3rd Guest

    It's usually the case that the cold junction is expected to be at
    room temperature (like, 20C +/- 5) but the hot junction can be
    at -200C to 800C... so one can get away with crude compensation.
    That's a good reason not to trust the all-in-one chip solutions,
    unless you really HAVE to make the output show up on a moving-needle
    meter (or other panel voltmeter).
  7. rickman

    rickman Guest

    I'm not sure this is the right question. It is easy to deal with the
    cold junction if you know its temperature. How do you plan to get the
    cold junction temperature? That is the hard part, at least if you want
    to get an accurate reading of temperature (better than ±1°C). Getting
    the local temperature accurately is not so easy to do inexpensively and
    accurately. That is why the TC specific devices seem a little pricey.

  8. Careful, that's not quite correct.

    To be a bit more explicit, the thermocouple transfer function or
    tables assume a certain reference (cold-junction) temperature, usually
    (but not always) 0°C (the temperature of an distilled ice-water
    slurry, generally close enough anyway).

    The inverse transfer function gives you the voltage you would read if
    the "hot" junction was at the terminal block temperature and the cold
    junction was at the reference temperature (say 0°C). For terminal
    block temperature > 0°C that will be a positive voltage.

    You then ADD that voltage to the measured voltage from the
    thermocouple, and turn the crank attached to the transfer function to
    get the temperature at the hot junction.

    For example, suppose the terminal block is at 20°C we measure 4.360mV
    from a type R (Platinum-Rhodium) thermocouple.

    For 20°C. the corresponding voltage is +111uV

    Add 4.360mV + 0.111mV = 4.471mV

    Turn the crank, and get +500°C for the hot junction temperature.
  9. What do you pay (approx.) for a platimun RTD? And where do you get

    What about a 'regular' ~10k thermistor?
    (Yeah I know, the 1 degree ones cost more.)

    Or just a diode, the slope is (pretty much) known it's the 'offset'
    that is different piece to piece. So maybe a single point 'zero'
    where you stick the TC onto the cold junction would work.

    George H.
  10. The thin film ones are cheap and plentiful these days- even Digikey
    has them for a couple dollars. They don't have much hysteresis (their
    big failing) if you keep them around room temperature. Wirewound ones
    are relatively expensive (and slow).
    The slope is more important than the absolute accuracy, because you
    want to avoid having to test each instance of the product in a thermal
    chamber- too slow and expensive. Platinum RTDs are pretty good.
    Cheapie "accurate" thermistors seem to be trimmed for the zero, but
    have rather poorly controlled slope.
    Same thing applies to the RTD, in spades. Just zero it out in an ice
    bath and Robert's your uncle. You don't need really high precision

    If you really want to use a junction, a transistor-connected diode is
    better than a diode, and transistor-connected diode measured at two or
    three currents is even better (but the signal level goes down, so the
    measurements have to be better).

    For most food and paint baking and most plastics machinery, few care
    about a few degrees C. They're good enough, and the whole BOM
    including power supply, case and output stage has to come in at less
    than those boutique chips cost. It's hard to make the device that
    isothermal without adding cost and subtracting ease of replacement and
    multi-sensor type ability, so the accuracy of measurement is a moot
    point in many cases.

    We used a small coil of copper wire as well as special resistors as
    RTDs. Worked pretty well. Platinum for the rich customers. These days,
    they just have to be well-heeled.
  11. OK dat's nice. I'm use to the sire wound variety ~$10.
    OK, I've
    Grin, Yeah I'm using pnp's in to-220 pac's as T sensor's (positive
    current source) Using the decade current difference measurement gives
    the absolute T to something better than 1% (a degree or two).

    But mostly I'd like to put the TC and cjc inside a temperature control

    An all analog circuit would make things simple for me,

    For my control loop application I need stability ...
    but no accuracy.

    George H.

    - Hide quoted text -
  12. Much thanks,

    (It's seems weird paying more for some special R's and C's
    than for opamps floating around 'em.)

    George H.
  13. Il 26/11/2012 21:02, rickman ha scritto:

    Good point.
    In my application the "cold" junction (by the way about 50 °C) and the
    board are enclosed in a small thermally-isolated box.
    I need to know the internal temperature for other purposes and I'm going
    to use a quite good temperature sensor to get a measure better than ±1 °C.

    I will put the sensor very close to the tc connector. Due to the small
    box I may assume the temperature will not change a lot in few centimeters.

  14. Guest


  15. Easier said than done!
  16. Guest

    Yep. If you dig wayyy back in sed(*), I think you'll find I predicted
    that, you suspected it, then you quickly tested, confirmed, and put it
    to use.

    But, that's a diode-connected transistor. All my transistor-connected
    diodes(**) used two diodes, and had gains less than one.

    (*) 5/2/09 "PAD5", 6/12/09 "Collector-base diode..."
    (**) inventions of my youth.
  17. Guest

    2 x 1n914's weren't much better. ;-)
  18. Yeah, I bet a lot of us tried that one. Two 1N60* diodes don't have
    much gain.

    * dating myself, but the equipment was old enough to be scrapped even
  19. Fred Bartoli

    Fred Bartoli Guest

    Spehro Pefhany a écrit :
    When I was a kid (11 or 12) when I read how a transistor was done I had
    to try a transistor-connected-diodes-pair...

    Do I need to say how that worked? :)

    If I were that same kid nowadays I guess I'd be even more confused.
    MMBT3906 vs BAV70: same package, same multimeter test result but not
    same working...
  20. Fred Bartoli

    Fred Bartoli Guest

    a écrit :
    The OA70s germanium transistor didn't either.

    Seems to be a pattern there...
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