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Re: [Help-gnucap] Model for led diode
From: |
al davis |
Subject: |
Re: [Help-gnucap] Model for led diode |
Date: |
Wed, 3 Sep 2008 14:55:46 -0400 |
User-agent: |
KMail/1.9.9 |
On Monday 01 September 2008, Jesus Genicio wrote:
> I make the change in "is" and ok, the diode led run in 2.46
> volts with 5 volts in Vin but de vce with vin=0 is only the
> 2.8 volst.....
> gnucap> tran 0 5 0.2
> #Time v(Vin) v(D1) v(Rc) vce(Q1)
> 0. 3.2074f 2.1327 163.09n 2.8673
> ..............
> 2.6 5. 2.5594 2.3628 0.077836
> .................
>
> Vin=0 => Led off but v(d1)=2.13 Volts and led light. Sorry,
> the "is" ok but there are some parameter more that configure
> to make that diode run correctly?.
>
> One reply is that " you need more study of theory of
> circuits" :)), please be pacient. Thanks.
>
> the only way that i found for this, is:
>
> vin=0, V(d1)=2.13 but ic=163nA and this is off.?
Check the current ... You can probe it, but my calculator says
I(Rc) = 652 uA. The LED won't light up on that. If you think
of it like a resistor, you could substitute a 3.2 giga-ohm
resistor for a similar result.
Below the knee, a diode is essentially open, so there can be
some voltage there with nearly no current. With a little bit
of leakage in the transistor, some extra due to the
simulator's "GMIN", .. There could easily be some volts across
the LED.
If you try it on a real circuit, you would get a similar
situation, until you hook up the meter. Then all of the
voltages change. A typical meter is equivalent to a 10 meg
resistor.
If all you care about is the LED, it is probably ok as is, but
not a good design. You should add a "pull-up" resistor, from
the collector of the transistor to the power supply, or in
parallel with the LED. I think 100k is a good value.