Thermistor equasion confusion

Discussion in 'General Electronics Chat' started by ABandy, Feb 18, 2015.

  1. ABandy

    Thread Starter New Member

    Oct 8, 2014
    7
    0
    Hello all,

    I am trying to work through some of my predecessors work and I have come across this.
    There's a thermistor in a potential divider, I cannot tell if the thermistor is R1 or R2.
    The formula is then passed through something that look similar to the Steinhart & Hart equation...

    This is what I have in Excel.

    =3988/(LN(((($D$26*4990)/($D$26-D29))-4990)/10000)+3988/298)-273

    3988 being the thermistors B value.
    D26 being 5.09V
    D29 being the voltage from the potential divider
    4990 being the fixed resistor in the divider
    10000 Thermistor R at 25C
    273 is the conversion from K to C (298 is K at 25C)

    (replaced excel bits to make more readable)
    =thermB/(LogN((((Vin*R1)/(Vin-Vtherm))-R1)/RT25)+thermB/K25)-273

    I have tried to pull out and break down the voltage to resistance conversion here and tried to compare it to the normal formula's
    R1 = -(R2(Vout-5)/Vout)
    and R2 = (R1(Vin - Vout)/Vout)

    But the numbers do not match.

    Can anyone enlighten me here?

    Thanks.
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    It would be handy to have at least a sketch of the circuit you are referring to.

    Also, what is this cell in Excel supposed to be producing? The temperature in °C?
     
  3. ABandy

    Thread Starter New Member

    Oct 8, 2014
    7
    0
    Circuit is very simple really, USB voltage 5.09V going into a potential divider with a 4990R at the voltage end, and the thermistor at the ground end, divided voltage tapped off into a buffer, then into our DAQ system.
    And yes, the formula does produce a temperature in degrees C
     
  4. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    A schematic would help.
    You need an electronics person that is good with math AND Excel.
    Making him try to imagine the circuit at the same time is just too much for some people.
     
  5. ABandy

    Thread Starter New Member

    Oct 8, 2014
    7
    0
     
  6. #12

    Expert

    Nov 30, 2010
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    Oh, yay! WBahn made the 10,000 club. Big deal to some of us.
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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  8. WBahn

    Moderator

    Mar 31, 2012
    17,720
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    Thanks!

    I guess the next big milestone for some of us is to reach the half-Wookie point. :D
     
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