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[Help-gnucap] diodes - determining IS.
From: |
al davis |
Subject: |
[Help-gnucap] diodes - determining IS. |
Date: |
Sat, 30 Aug 2008 23:25:36 -0400 |
Here's a way to find the saturation current of a diode.
(repost from gEDA-user)
FAQ......
> Where can I find some instructions on obtaining and adding
> new models?
As you probably already figured out, you need "model" statement
to parameterize the diode ...
D2 (a k) 1n4004
Now you need a model statement ...
.model 1n4004 d (a whole bunch of parameters)
You could do a web search for a detailed model, but usually you
can just do some experimenting..
If you are at the stage where you are asking about this, the
detail will only confuse you, most likely of the worst kind of
confusion, where you don't realize it.
Most of the time, the only parameter you need for a diode is
saturation current, which of course isn't in the data sheet.
ok .. so how do I figure that out??
Well ... Look at the specs, or measure it. Let's say we found
that the forward voltage at 1 amp is .7 volts.
Use the simulator to build a test circuit ..
I1 (0 1) dc 1
D1 (1 0) my_diode
You need a model statement ..
.model my_diode d (is=something)
but I don't know what the value of something is.
With gnucap .. try this.......
gnucap> build
>i1 (0 1) dc 1
>d1 (1 0) my_diode
>.model my_diode d is=something
> ******** blank line to exit build mode
gnucap> probe op v(1)
parameter something has no value
gnucap> op
# v(1)
27. 0.83376 **** using default IS
gnucap> param something=1u
gnucap> op
# v(1)
27. 0.35733
gnucap> param something=1n
gnucap> op
# v(1)
27. 0.53599
gnucap> param something=1p
gnucap> op
# v(1)
27. 0.71465
gnucap> param something=2p
gnucap> op
# v(1)
27. 0.69673
gnucap> param something=1.5p
gnucap> op
# v(1)
27. 0.70417
gnucap> param something=1.8p
gnucap> op
# v(1)
27. 0.69945
gnucap> param something=1.7p
gnucap> op
# v(1)
27. 0.70093
gnucap> param something=1.75p
gnucap> op
# v(1)
27. 0.70018
gnucap> exit
There .. close enough ,,,
Now use the value 1.75p for IS.
Here's your model statement:
.model my_diode d is=1.75p
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