Temperature Sensor

Discussion in 'General Electronics Chat' started by R!f@@, Apr 4, 2011.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    My hot gun broke so I decided to repair it and make it better or rather customize it for my needs.

    I dismantled the hot gun and did a little experimenting on the heater and how the air is supplied to it. My one just does not seem to heat up.

    [​IMG]

    I found an Atmega8L-8PU μC inside. Probably for the digital temp display and air flow adjustment.

    [​IMG]

    Something is freaking me out. The little temp sensor at the tip of the gun gives a 1Ω reading at my room temp and when I heat it, my fluke gave a negative resistance reading. :eek:
    Why the heck does the meter gives a negative sign in the resistance region. When I reverse the probe it goes positive.

    [​IMG]

    Is this possible or is my meter playing an April fool prank on me
     
    Last edited: Apr 4, 2011
  2. debe

    Well-Known Member

    Sep 21, 2010
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    Hi R!f@@, For high temp sensing it will use a thermocouple which when heated will generate a voltage & this is what is upsetting the res reading. It will be very low voltage generated. Will look up volt/temps for a K type thermo. 100deg C-4.1Mv. 250deg C-10Mv. 300deg C-12Mv. 350deg C-14Mv. 400deg C-16Mv. 450deg C-18Mv. 500deg C-20.3Milli volt.
     
    Last edited: Apr 4, 2011
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  3. someonesdad

    Senior Member

    Jul 7, 2009
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    It means there's a voltage in the circuit. Bench multimeters often have "offset compensated" resistance measurements to subtract out this effect. They either disconnect the constant current source and measure the voltage (and then correct for it) or measure the resistance both ways and average the results.

    Alas, many people don't know about this effect, even some EEs. When testing unknown circuits for resistance, it's wise to test with the DMM's leads reversed. If you see two different positive readings, you're either biasing a semiconductor junction or there's a voltage in the circuit. If one reading is negative, then there's a voltage in the circuit.

    Sometimes it's only a mV or so that can throw your readings off.
     
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  4. Markd77

    Senior Member

    Sep 7, 2009
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    Sounds like a thermocouple. It creates a (tiny) voltage when heated so will give you funny resistance readings.
     
  5. Markd77

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    ...and then three come along at once.
     
  6. R!f@@

    Thread Starter AAC Fanatic!

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    Ah so that is what a K type thermocouple thingy is all about.
    I have wondered about it but haven't any time to study it...

    OK !! back to the drawing board...
     
  7. R!f@@

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    By the way....I am having a hard time viewing this document.

    Been trying for hours but it just don't seem to be downloading after a few secs.
     
  8. Markd77

    Senior Member

    Sep 7, 2009
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    pdf sent by email.
     
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  9. R!f@@

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    Hey Mark.. thanks for the pdf.

    Can you tell me what I would need to write the codes for AVR as in PIC.
    Like to know how I can find everything. That is like Assembler and compiler and lots.
    Is AVR same as PICs
     
  10. R!f@@

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    I made a table for the thermocouple I have. I heated it with my iron so readings are taken by adjusting the iron tip temperature

    Reading order is, iron tip temp set value -- Thermocouple Voltage --- thermocouple resistance.

    Iron at 200°C -- 4.0mV -- 1.55Ω
    Iron at 250°C -- 4.8mV -- 1.67Ω
    Iron at 300°C -- 6.0mV -- 1.89Ω
    Iron at 350°C -- 7.0mV -- 1.95Ω
    Iron at 400°C -- 8.0mV -- 2.38Ω
    Iron at 450°C -- 9.0mV -- 2.60Ω.

    Since this is my first attempt using a thermocouple, I like to know whether the mV is the one to use or the resistance change is the one I should go for.
     
  11. debe

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    Sep 21, 2010
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    Not knowing the circuit i would assume a comparator would be looking at the volage produced by the thermocouple.
     
  12. debe

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    Couple of thermocouple thermometers one is for checking thermostats in electric irons & frypans.
     
  13. R!f@@

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    Ya...the circuit has a amp for the thermocouple..I going thru it right now
     
  14. BillB3857

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    Feb 28, 2009
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    debe likes this.
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