Re: [Help-gnucap] Model for led diode

[al davis]

On Thursday 28 August 2008, Jesus Genicio wrote:
> Vcc ( 1 0 )  DC  5.
> Vin ( 5 0 ) PULSE iv= 0. pv= 5. delay= 1. rise= 0. fall= 0.
> width= 2. period= 4.
> Rc ( 1 8 )  122.
> Rb ( 5 2 )  12.K
> Rled ( 8 3 )  121.
> Q1 ( 3 2 0 )  Q2N2219A  area= 1.
>
> The result is correct:
>
> gnucap> tran 0 2 0.1
> #Time       V(5)       V(1)       V(Rled)
>.........
>  1.3        5.         5.         2.4256

> Rled must be one led.
>
> If i change Rled for a diode 1N4004, example. The results is
>
> gnucap> tran 0 2 0.1
> #Time       V(5)       V(1)       V(D1)
>.............
>  1.3        5.         5.         0.74569
>  1.4        5.         5.         0.74569
>  1.5        5.         5.         0.74569
>  1.6        5.         5.         0.74569
>  1.7        5.         5.         0.74569
>  1.8        5.         5.         0.74569
>  1.9        5.         5.         0.74569
>  2.         5.         5.         0.74569
>
> But V(D1) must be 2.42 for light diode. If change Rc, de
> V(D1) is equal but vce change...

That result is correct for a 1N4004.

The key parameter is "IS" (saturation current).  To make a model 
for your LED, make a ".model" where you specify only "IS".  
Choose the value so you get the correct forward voltage.