dealing with resistance of diodes???

[shashir]

hi,
i m intrested to know about the resistance of light emmiting diode.the problem is the formula i m using is not working.

i wonder anyboby can help me with the correct formula.

thank you

 

[keith1200rs]

What formula are you using?

Keith

 

[shashir]

hi,
at first i calcutated the value of current through the led using the formula:
I = (Vs- Vl) / R

Vs-supply voltage
VL = LED voltage (mines is white so 4 v for this)
R-the resistance i have in series with the diode

then i calutaed the resistance of diode using R=V/I

i dont think i m going right!!


thank you
shashir

 

[doraemon]

Hello!

If you want to do math, you have to be accurate.
So you defined R, Vl, Vs, and your first formula is right.
However, what is V in the last formula? Vs? Vl? Vs-Vl?

Now beside this, there is another problem You cannot calculate the
resistance of a diode because… a diode is not a resistor.
If you look at the V/I characteristic of a diode, you will find out that
I = k1 exp(k2(V-V0)) where I is the current crossing the diode, k1 and k2
are 2 coefficients depending on the diode, and V0 is the threshold voltage.
(this formula may not be accurate, but I remember that it was more or
less exponential).

If you use the model of the ideal diode, you have I=0 when V is less than
the threshold, and I = whatever you feed if V is more than the threshold.

So the resistance of a diode is basically meaningless.

Dora.

hi,
at first i calcutated the value of current through the led using the formula:
I = (Vs- Vl) / R

Vs-supply voltage
VL = LED voltage (mines is white so 4 v for this)
R-the resistance i have in series with the diode

then i calutaed the resistance of diode using R=V/I

i dont think i m going right!!


thank you
shashir

 

[shashir]

awhh i missed it!!its VL.....

thanx for the answer..its really cool..i hope u will help me in future too with this..

shashir