# Homework Help: Thevenin equivalent of an LED circuit

1. Apr 26, 2012

### Marshillboy

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. Apr 26, 2012

### rude man

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
where
q = electronic charge
k = Boltzmann's constant
T = Kelvin temoperature.
At room temperature, kT/q ~ 26mV.