Thermistor - Voltage Circuit.

Discussion in 'General Electronics Chat' started by NMC, Mar 13, 2019.

  1. NMC

    Thread Starter New Member

    Nov 16, 2018
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    Hi All,

    I am looking to design a circuit using an Omega4404 thermistor which at 20 degree's outputs 0v.
    I want to draw a graph of corresponding voltages for 20-30 degrees.

    What type of circuit do I need to design?

    Thanks
    N
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi NMC,
    Do you have a datasheet for that 4404 sensor.?
    E
     
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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  4. Alec_t

    Expert

    Sep 17, 2013
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    Jeepers! That 44004 is expensive! There are MUCH cheaper thermistors available and which may do the job, unless you are constrained to have that particular resistance (2252Ω @ 25C).
    What tolerance is acceptable for your purpose?
    What is the purpose?
     
    Last edited: Mar 13, 2019
  5. MrChips

    Moderator

    Oct 2, 2009
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    Thermistors, in general, are non-linear resistors with negative temperature coefficients (NTC), i.e. resistance goes down as temperature goes up. The simplest way to get a linear response is to get the data into a computer and do the data to temperature conversion mathematically.

    If you want 0V output at any specific temperature, you can use a difference amplifier to subtract any DC offset voltage or use a bridge circuit.

    Personally, I prefer to convert the thermistor resistance to a frequency and get a microcontroller (MCU) to read the frequency. Without knowing your specific application it is difficult to suggest any such options.
     
  6. sghioto

    Well-Known Member

    Dec 31, 2017
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    Fahrenheit or celcius? Did you mean Omega 44004?
    SG
     
  7. NMC

    Thread Starter New Member

    Nov 16, 2018
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    Hi all,

    Thanks for the replies.
    Apologies it is an Omega 44004.

    I want to design a circuit that at 20 degrees Celsius the output voltage is 0V.
    I then need to determine the voltage for temperatures 21-30 degrees Celsius which have given resistances as below:

    20 degrees C = 2814 Ohms
    21 = 2690
    22 = 2572
    23 = 2460
    24 = 2354
    25 = 2252
    26 = 2156
    27 = 2064
    28 = 1977
    29 = 1894
    30 = 1815

    Below is a circuit which I tried to simulate -

    upload_2019-3-13_16-40-8.png
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    This is a bridge circuit.

    upload_2019-3-13_12-42-42.png

    You can make three resistors all 2814 ohms.
    Or you can make one resistor 2814 ohms (eg. Ra is thermistor, Rb = 2814 ohms).
    R1 = R2 = 1000 ohms, for example.

    or R1 = 2814 ohms,
    Rb = R2 = 1000 ohms, for example.
     
  9. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    Look at the LTC 2983 / 2084 and 2986 IC's. It's a pricey IC.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Here's the bridge circuit with an op amp differential to single-ended output (LTspice simulation below).

    Q1 and Q2 generate a current load on the output to help pull the output close to zero, which single-supply op amps don't generally do.
    If you have a plus and minus supply for the amp, then those aren't needed.

    The circuit gain is 2 as compared to the voltage across the same bridge connected to ground.

    Pot U2 is for offset adjustment.

    As you can see, the output voltage goes from ≈0V to ≈1V for a temp change of 20 to 30°C.

    upload_2019-3-13_12-17-13.png
     
    Last edited: Mar 13, 2019
    sghioto likes this.
  11. NMC

    Thread Starter New Member

    Nov 16, 2018
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    0
    Hi All,

    Thanks for the replies again.
    I'm still struggling to design a circuit for my thermistor-voltage circuit.
    Do I use a simple bridge circuit or do I need to use some sort of op-amp?

    N.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    So you don't understand the previous posts here?
     
  13. joeyd999

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    Why?
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    Because an MCU is better at counting pulses than measuring analog voltages.
     
  15. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Seriously: I would love to see your calibrationless T-to-F solution that gives milli°C resolution and centi°C accuracy with only one external component besides the MCU and thermistor.

    Edit: I've discussed my solution.
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    Seriously, no one is talking about calibrationless temperature measurement. The thermistor is non-linear and some form of transformation is required. Thermistors are interchangeable once the system is calibrated.

    My solution consists of thermistor, capacitor, and MCU.
     
  17. joeyd999

    AAC Fanatic!

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    And calibration. How's the stability against temperature and Vdd/reference voltage changes?
     
  18. crutschow

    Expert

    Mar 14, 2008
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    How does that accurately convert the thermistor resistance to a frequency? :confused:
     
  19. MrChips

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    I charge the capacitor then allow the capacitor to discharge through the resistor. I do this many times and then count how many times I can do this in a certain time period.
     
  20. crutschow

    Expert

    Mar 14, 2008
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    And you really think that's more accurate than using the processors A/D converter to measure an analog voltage, such as from the bridge circuit?
     
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