temp coefficient of resistance

Discussion in 'General Electronics Chat' started by haditya, Sep 30, 2004.

  1. haditya

    Thread Starter Senior Member

    Jan 19, 2004
    220
    0
    is the temp coefficient of resistance constant at all values of temp?? or does it vary with temp??
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    A material's tempco is its change in resistance as the temperature changes. Usually given in parts per million, or as a precentageof total resistance. The normal reference point is 25 degrees C. The second question makes no sense, as you can see from the above.
     
  3. Iodem_Asakura

    Senior Member

    Sep 14, 2004
    140
    0
    I thought it was a logarithmic (non-linear) relation, just like a capacitor charge or discharge (not all time at the same rate, at start the change is fast but later become slower).
    So i thought the change at the resistance (Ohm/ºC[F]) will be different at each point of temperature. Of course always there will be a part that is almos linear. In these cases the manufacturers base on that part to give a coeficient that will be more exact for most cases. I think there should be graphics for that (or maybe it's standar the logarithmic funtion?).
    Well, sorry if you don't understand what i'm trying to say. This is not my born language.
     
  4. haditya

    Thread Starter Senior Member

    Jan 19, 2004
    220
    0
    ill put it in another way

    R2=R1[1+(alpha)(t2-t1)]
    alpha here is called the temp coeff of reistance defined at a certain temp

    i want to know at what temp is it defined and is the above formula valid for all temp t1 and t2, when alpha is defined for a value way out of the range of t1 or t2
    in short i want to know if alpha is a function of temp with any definite mathematical relationship
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    143
    This is a more complicated question than it seems on the face of it!

    The variation of resistance R with temperature T for most (not all) metallic materials is represented by the general equation:

    R = Ro(1 + a1T + a2T^2 + anT^n....)

    Where:

    Ro is the resistance at T = 0.
    And there are several temperature coefficients of resistance dependant upon serval material factors.

    You can probably see the similarity between your equation and the generalised form. An important point to note is thet the number of terms neccessary in the summation is dependant on the material, the accuracy of the calculation and finally the temperature range.

    Basically the extent to which you calculate the generalised form determines the temperature range over which your calculation is valid. For example, using a common material as platinum and assuming a1 = 0.004 and all subsequent a set to 0, the non-linearity is only 0.5% over the temperature range of -40 to 140 C. More accuarcy in the calculation would yield a greater temperature range.

    These considerations become very important when designing instrumentation resistive temperature transducers. The calculations become more complicated if you come to design Thermistors where the above equation becomes the product of an exponential function and we introduce the characteristic temperature constant for the material and start differentiating the original function - oh it gets so confusing!

    Although I haven't specifically answered your question, I hope I've shown you that the temperature range of a temperature coefficient of resistance isn't as plain as it may seem. I would assume that if you are given a value for the temperature coefficient of resistance that, unless stated otherwise, it would be true for all values of temperature.
     
  6. haditya

    Thread Starter Senior Member

    Jan 19, 2004
    220
    0
    ok...thanks a lot...that pretty much clears some more questions in my head
     
Loading...