RTD / Thermocouple - PIC help

Discussion in 'The Projects Forum' started by urb-nurd, Aug 21, 2014.

  1. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    Hey guys.
    I am trying to control the temperature of a heater through PWM using the output from a PIC that reads the temp from either an RTD or Thermocouple.

    Currently i have the heater, the pic and development board (mikroC ready for PIC).
    I have a few mofsets that should operate just fine for my application.
    I also have ONE RTD - i understand the comparable differences between RTD's and thermocouples and aim to implement both so i can asses their viability myself (and learn on the way!).
    The question burning at hand after all that hot air exudation is: what will i need to interface my sensors with my PIC.
    I have seen some off the shelf IC's that might fit the bill however my knowledge in this field is particularly lacking.
    I shall continue to research and i hope any input here will hasten the process.

    Thanks alot AAC!
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    the rtc is a resistor, easily read with an analog inpput. the thermocouple generates a very small voltage which unless your pic has a thremocouploe input, will have to be amplified.
     
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  3. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    Thanks!
    I just pulled up a thread implying what you said about the RTD, this is good news for me as it means i can start tinkering with what i have.
    As for Thermocouple input, i will investigate my pic - research the types and then team one with an amplifier.
    Thank you for your help.

    One last question for the many tubes of the interwebs - how could i go about fixing my sensor to my heater?
    I cant really hold it in place and im not sure of any safe heat resistant adhesives or epoxy that would work without emitting noxious gasses as the temperature rises.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,995
    745
    I would secure it with a p-clip type fixing,using a threaded nut bolt,or self tapper, higher temps are better with a K- thermocouple using a pre calibrated chip AD595,gives 10mV per deg C.
     
  5. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    Could you please elaborate on p-clip?
    I have looked and seen what i think you mean.
    However i should of elaborated on my heater before asking such questions.
    My heater is ceramic and cylindirical, however the rdt sensor i have is minuscule - and flat.
    I think for my application i may have to find a custom arrangement or possibly have a thermocouple/RTD made encased in ceramic and attach that to my heater.
    The thermal paste i have seen is only good to 200C whereas i would like a nicer buffer zone for safety as my heater will likely be running near 220c

    Thanks for your help!
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,393
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  7. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    guys i got a question.
    I have my RDT here, it has leads about 15mm long. maybe 10mm.
    How can i connect the sensor to my PIC without fouling the resistance?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    What is the nominal resistance of your RTD?
     
  9. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    Thanks for you assistance!
    Here is the Data Sheet

    the model i have is 362-9834

    the nominal resistance is 38.5 ohms
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You are misinterpreting what Interval means. You need to read this
     
  11. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
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    my bad, i glanced and obviously misread.
    Is the answer you were looking for in that datasheet?
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Nominally, the resistance is near 100Ω. You could be pretty sloppy with external wiring and not greatly effect the readings..

    This is covered in the Wiki
     
  13. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    yeah i read that wiki again.
    Cheers for that, ill try make sure i can get as little resistance as possible.
    I guess i can always measure the offset and compensate in my code.

    One last little question, the wiki there shows a Wheatstone bridge.
    I assume i am to design a bridge to operate over my temperature range and then have the v.ref as an analogue input on my pic?
     
  14. ebeowulf17

    Active Member

    Aug 12, 2014
    678
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    Sounds like you're on the right track for RTDs already, so I don't have anything to add there...

    For TCs, the Analog Devices thermocouple amp ICs are great. I looked into the first Adafruit model listed above (MAX31855) and wasn't happy with the resolution and accuracy specs. I ended up using the AD595 mentioned above with great success (using a lookup table rather than straight linear conversion for better accuracy.) The AD595 seems to be getting phased out (or at least not recommended as much) in favor of the AD8495, which is cheaper and, if I remember correctly, more accurate. When I was building my last project, I wasn't ready to work with SMT components so I passed on it at the time, but now that Adafruit has a ready-made breakout based on it (second Adafruit link above,) I would highly recommend it. The breakout already has all the filter capacitors, resistors etc. built-in, so there's very little circuit left to build. For me, the breakout is perfect - the AD595 DIP IC costs more than twice as much and required a fair bit of additional circuitry around it to make it perform well, where the breakout is smaller, cheaper, and pre-built. Of course, if part of what you're going for is learning more about the guts of these circuits, then the AD595 might be better so that you can actually get more hands-on with the whole system. Either one should provide good performance.
     
    Last edited: Aug 26, 2014
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