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Homework Help: Calculate the voltage for a diode

  1. Nov 9, 2008 #1
    1. The problem statement, all variables and given/known data
    I'm trying to calculate what voltage change will compensate for a temperature change of 1 degree for a diode.
    I have set up the circuit, taken down results and plotted the graph (its non-linear). So I re-plotted it using natural logs of I, and got a straight line with a gradient. Now I'm lost as to how to answer this question.


    2. Relevant equations
    I= Is x (e^(eV/kT) -1)
    It's the characteristic equation for V vs I for a diode.


    3. The attempt at a solution
    I took natural logs to get:
    ln I = lnIs x (eV/kT - 0)

    Then said the gradient is equal to e/kt.
    But then where do I go from there?

    A little push in the right direction would be great!

    Thanks in Advance!
     
  2. jcsd
  3. Nov 9, 2008 #2

    tiny-tim

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    Hi Lavace! :smile:

    If you mean I= Is (eeV/kT -1),

    then I/Is + 1 = eeV/kT:smile:
     
  4. Nov 9, 2008 #3
    Re: Diodes

    But then I'd end up with ln(I - Is) + 0 = eV/kT

    Then the gradient is equal to e/kT, so how do we calculate what voltage change will compensate for a temperature change of 1 degree for a diode for this? I have a value of the gradient as well!
     
  5. Nov 9, 2008 #4

    tiny-tim

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    No … ln(I/Is + 1) is not ln(I/Is) + log(1), is it? :smile:
     
  6. Nov 9, 2008 #5
    Re: Diodes

    Sorry to make it so slow but where des this come into finding the voltage change for 1 degree?

    Thanks for the advice!
     
  7. Nov 10, 2008 #6

    tiny-tim

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    oooh, that's electronics, isn't it? :confused:

    i'm just here for the maths. :redface:
     
  8. Nov 10, 2008 #7
    Re: Diodes

    ot sure if this helps or hinders.
    But there is Wien's Law that allows you to determine Peak wavelength for a given temperature in Kevin
    ie

    [tex]
    \lambda _{peak} T = 2.90 \times 10^{ - 3} meters.Kelvin
    [/tex]

    The energy gap is the energy required by a semiconductor needed before starting to conduct, the energy is related to the wavelength of the EM wave.

    So

    [tex]
    E_g = hf = \frac{{hc}}{\lambda }
    [/tex]

    where c is the speed of light
     
  9. Nov 10, 2008 #8

    Redbelly98

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    Re: Diodes

    It sounds like you want to maintain the same current I, when there is a temperature change, by changing V.

    Okay, so if I is to not change, then the right-hand-side of this equation must also not change. If T changes, for example from 20 C to 21 C, what must happen to V in order to keep this expression at the same value?
     
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