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Homework Help: Thevenin equivalent of an LED circuit

  1. Apr 26, 2012 #1
    1. The problem statement, all variables and given/known data
    How would I go about determining the Thevenin equivalent of a simple circuit involving a battery and an LED? I suppose there could be a series resistor to prevent excess draw and burning out of the diode.

    This question seems very odd since LEDs by definition don't involve a linear relationship between voltage and current.

    2. Relevant equations
    Thevenin's theorem

    3. The attempt at a solution
    Not sure how to go about starting this.
  2. jcsd
  3. Apr 26, 2012 #2

    rude man

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    Homework Helper
    Gold Member

    The LED is a nonlinear component, as you say. There is no such thing as a Thevenin equivalent for a component or network including a nonlinear component.

    However, if you consider the LED in its ON state, then you can model it as a resistor for small current variations. If the LED is modeled as a standard diode (which I'm not sure is an entirely valid thing to do), then

    i = i0eqV/kT
    so the variational impedance is Rvar= 1/(∂i/∂V) and so
    (∂i/∂V) = qi/kT or Rvar = kT/qi
    q = electronic charge
    k = Boltzmann's constant
    T = Kelvin temoperature.
    At room temperature, kT/q ~ 26mV.
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