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favorite temp sensors

Discussion in 'Electronic Design' started by David Lesher, Apr 12, 2013.

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  1. David Lesher

    David Lesher Guest

    So we're looking to sense temps in the ~~0-40C range, and
    wondered what to use. In the past I've used LM35's and would
    again, but all the cool kids seem to using this digital stuph,
    so I was wondering what's a roughly parallel sensor that has
    some ilk of digital output?

    They'll feed a 32bit CPU so I'm sure we can scare up so IO lines...
     
  2. Bill Sloman

    Bill Sloman Guest

    Depends whether you want an accurate sensor.

    At the moment interchangeable glass-encapsulated thermistors leave everything else for dead at +/-0.2C (or better, if you are prepared to pay more).

    You've got to put together a bridge and an A/D to turn the resistance into a digital number, which is going to be a non-linear function of temperature

    http://en.wikipedia.org/wiki/Thermistor

    but sorting out the Steinhart-Hart relationship won't overload a 32-bit CPU\.
     
  3. David Lesher

    David Lesher Guest

    A few degrees won't matter....
     
  4. Guest

    The hobby 8-bit microcontroller people seem to like the Dallas DS18B20/
    DS18S20 and similar parts. They speak Dallas' "1-Wire" protocol, so
    you can string several of them on one I/O pin. The TO-92 case seems
    to be popular but you can also get them as SOIC. They claim +/- 0.5 C
    accuracy, and somewhere between 0.1 to 0.75 seconds per reading
    depending on how many (configurable) bits of resolution you want.

    I think those parts are popular in that area because they are always
    short of I/O port pins, and because there are ready-made software
    libraries that will handle bit-banging the protocol for you.

    These seem to be reasonably available 1 or 2 at a time (Digi-Key has
    them in minimum quantity 1), but they now come from Maxim. For other
    kinds of parts, I have heard varying things about Maxim's willingness
    to sell parts in small quantities, where "small" is defined as "less
    than 100,000 at a time".

    When PC motherboards started sprouting temperature sensors, a lot of
    them seemed to be National (now TI) LM75 or similar. The LM75 speaks
    I2C, claims +/- 2 C and comes in a couple of surface-mount packages.
    There are similar parts all the way from LM63 to LM94; some of the
    options are SPI, different resolutions, programmable alarm points,
    lower power consumption, etc. You need more than one I/O pin to make
    these work.

    These also seem to be reasonably available 1 or 2 at a time (Digi-Key).
    I haven't heard anything bad about TI's small-quantity availability.

    Standard disclaimers apply; I don't get money or other consideration
    from any companies mentioned.

    Matt Roberds
     
  5. David Lesher

    David Lesher Guest

    I'm all ears. TO92 case is nice, fanatical accuracy is not required,
    Price is part of the decision....we can always use just LM35's.....
     
  6. Bill Sloman

    Bill Sloman Guest

    Sure. Make the thermistor the frequency determining element in an astable, but then you have to cope with the temperature dependence of the reactive element.

    I was recommending an accurate solution, rather than a simple one.
    I may be a has-been, but I provided real - and potentially useful - information. You've just claimed to know a "better" way (probably incorrectly), without giving any indication of what it might be, and followed it up with gratuitous personal abuse. Definitely an exhibition of psychopathology, though you've skipped the criminal aspect on this particular occasion.
     
  7. Bill Sloman

    Bill Sloman Guest

    So why bother measuring it at all?
     
  8. Bill Sloman

    Bill Sloman Guest

    As I pointed out back in 1978

    Sloman, A.W. "On microdegree thermostats", Journal of Physics E: ScientificInstruments, 11, 967-968 (1978).

    platinum resistance thermometers have a lower thermal resistance to ambientthan thermistors, which does compensate - to some extent - for the lower sensitivity. Any place I'd have wanted to sue them I'd have had to use AC excitation to get the sensitivity I wanted. Reversing DC will do - but a bifilar wound Blumlein transformer bridge is a very neat way of setting up a very stable bridge, and that usually takes a couple of kHz if you want to usea nice compact ferrite cored transformer.
    Too true.
     
  9. For a bit more money I saw a project that had a whole bunch of
    DS18B20's that did a temperature profile of metal plate. (Maxim
    though...)

    George H.
     
  10. Guest


    I've been using the AD7414. With appropriate wiring and board
    stuffind, I can get eight on an I2C interface (used for thermal
    testing only).
     
  11. Nico Coesel

    Nico Coesel Guest

    Microchip has some pretty cheap ones. Like everything from Microchip
    they are a bit crappy but if accuracy isn't the prime goal...
     
  12. Not a digital solution, But I've been using diode connected
    transistors (in TO-220 packs) as temp sensors. I like that it comes
    with a mounting hole :^)
    I did a calibration run (versus a spendy lakeshore diode) and then a
    single point calibration after that. (all transistors with the same
    date code.) I've got to do another calibration in the near future,
    and it will be interesting to see how different the next batch is.
    How important are all the process variations for a diode connected
    transistor?

    George H.
     
  13. Guest

    Seiko has some cheap ones, too.
     
  14. Is this accurate to +/- 0.2'C (without calibration)?
     
  15. Not in my experience. (but my experience is fiarly limited... a few
    transitors tested.)

    I always got a number that was a bit off ~0.3%, so about 1 degree at
    room temp. I always assumed the error was due to the transistor
    beta... Since the current is Ic and Ib. (I think I got a temperature
    that was always a bit high, but I'd have to check my notebook.) You
    could add some beta 'fudge factor'.... but then beta changes with
    temperature too.

    It also depended a bit on the collector current. (1 uA to 10uA were
    'nice' currents)

    George H.
     
  16. David Lesher

    David Lesher Guest

    So midst the noise I've seen the following mentioned:

    LM71
    DS18B20
    LM75
    AD7414

    Of those, only a few seem to come in non-surface mount packages
    and I'm not sure how we'd couple surface mount devices to the
    copper cooling lines.

    Given that we just want to measure the coolent temperature on
    both sides of the liquid-cooled heatsink, and not reinvent
    anything [1]; it looks like the DS18B20 may do the trick.

    It's 2-4X the price of LM35's but still affordable.


    Thanks for the suggestions.

    1] Or rather "we are inventing enough on the product; we don't
    need to do so on the test instrumentation too..."
     
  17. whit3rd

    whit3rd Guest

    It's an absolute-temperature measurement, so +/- 0.2 C implies better
    than a part-per-thousand error band. Voltage references and resistor
    values for the current pulse would have to be better than 0.1%, so probably
    the answer is NO. The only way to get components with that accuracy
    is with trimming, and that means ALL qualified sensors are dependent
    on calibration or trimming. You can choose which, but you
    must pay, either way.

    Calibration after soldering is better than trimming before soldering.
     
  18. whit3rd

    whit3rd Guest

    On Friday, April 12, 2013 9:27:51 AM UTC-7, David Lesher wrote:
    [about temperature measurement, with mention of 0.2C accuracy]
    If you want the difference to be accurately measured, that's
    actually an ideal place to put in a thermocouple, or several
    thermocouples in series. If you use a single batch
    of thermocouple wire, it'll have a good zero-temp-difference
    characteristic guaranteed by design. Input pipe is hot
    junction, output pipe is cold junction, and an autozero op amp
    can condition the signal easily.
     
  19. Guest

    Which section of your specification was that in? ;-)
     
  20. Jon Kirwan

    Jon Kirwan Guest

    A published source on this is found here:
    http://cds.linear.com/docs/en/application-note/an45f.pdf

    Starting on top of page 7.

    ....

    I take George's recent experiences using these seriously,
    though. I'll add a short note to his writing on the topic.

    Jon
     
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