Thermistor and the calculas?

grasshopper

I am using a thermistor and my mind has lost its focus.

As of now I can read the sensor and it changes with a change in temperature. I am using a Steinhart Hart equation but my concern is this.

1. If my Voltage divider is as follows in picture 1 then my total current is 125.0uA. Vin is 5volts



2. if my voltage divider is as in picture 2 then my total current is 69.4uA. Vin is 5volts



My problem is that in order for me to correctly read the thermistor I must know the current. In order to know the current I must know the Resistance. But how can i know the resistance if i can get the current. My voltage is the same but my mind is lost. I was going to implement some sort of derivative but i am at a loss.


3. I wont know the resistance because i cant calculate the voltage in efforts to get the current.



My prop code

pub Calc_Volts | amps

  amps := 0.00026497
  microVolts := 0.000076295
  
  Volts := MAT.FMul(mat.ffloat(Board_temp), microvolts)
  Resistance := Mat.Fdiv(Volts,amps)
  Steinhart_Hart
    
pub Steinhart_Hart   | ScrapBit1 

    '1/K = [noparse][[/noparse]A + B log ( r ) ] + [noparse][[/noparse]C log ( R ) ^3]
  
     
     ScrapBit := G.Log10(resistance)
     Scrapbit1 := G.Fmul (scrapbit , StHeart_B)
     ScrapBit1 := G.Fadd (Scrapbit1, StHeart_A)
     
     ScrapBit2 := G.Fmul(ScrapBit, ScrapBit)
     ScrapBit2 := G.Fmul(ScrapBit2, ScrapBit)'cube
     ScrapBit3 := G.Fmul(ScrapBit2, StHeart_C)'mul to C
     
     ScrapBit3 := G.Fadd(scrapBit1,ScrapBit3)

     Kelvin := G.Fdiv ((G.FFloat(one)),scrapBit3)
     Celsius  := G.Fsub(Kelvin, Con_Celsius)
     G.stop

Mike Green

Measure the voltage across the thermistor (call it Vt). The voltage across the 10K resistor depends on the voltage across the thermistor and the supply voltage (Vdd) which I hope is fixed and known. The same current flows through the 10K resistor and the thermistor (I = V / R = (Vdd - Vt) / 10K). The thermistor value is R = V / I = Vt / ((Vdd - Vt) / 10K).

Tracy Allen

Algebra, not calculus (lucky you!)

               10k                Rt
+5 ------/\/\/\-----o-----/\/\/\----gnd
                         Vt

Vt = 5000 * Rt/(Rt+10000) voltage divider equation, Vt is measured across Rt

keep it in ohms and millivolts for integer scaling,

solve for Rt

Rt = 10000 * Vt / (5000 - Vt)

For example, if you see Vt = 2501 milivolts, then R = 10008 ohms.

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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 6/21/2008 7:02:21 PM GMT

grasshopper

I guess what i am trying to say is that how can I find Total Current when R2 resistance will vary depending on temperature. I can do the voltage divider concept, but since R2 is never know then I am at a loss. I think that the current will change depending on R2s resistance that also changes.

Mike Green

Please reread my previous note. As long as you know the supply voltage, you don't need to know the actual current.

grasshopper

damn !

Sorry i see what you mean now. Took a cold beer and now i understand.

R = V / I = Vt / ((Vdd - Vt) / 10K)

Perfect way to put it.

I got to slow down at times. Thanks again fellas