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Old 12th January 2004, 03:19 PM   #1
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Default spice models for led

Hi,

I was not able to find any spice
model for L.E.D. on the web

can someone help me?

Thanks in advance

Federico
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Old 12th January 2004, 03:24 PM   #2
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You could use a diode with 2.4V forward drop, if there is no other solution.

Jan Didden
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Old 12th January 2004, 03:26 PM   #3
Pedja is offline Pedja  Serbia
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*Typ RED GaAs LED: Vf=1.7V Vr=4V If=40mA trr=3uS
.MODEL LED1 D (IS=93.2P RS=42M N=3.73 BV=4 IBV=10U
+ CJO=2.97P VJ=.75 M=.333 TT=4.32U)

*Typ RED,GREEN,YELLOW,AMBER GaAs LED: Vf=2.1V Vr=4V If=40mA trr=3uS
.MODEL LED2 D (IS=93.1P RS=42M N=4.61 BV=4 IBV=10U
+ CJO=2.97P VJ=.75 M=.333 TT=4.32U)

*Typ BLUE SiC LED: Vf=3.4V Vr=5V If=40mA trr=3uS
.MODEL LED3 D (IS=93.1P RS=42M N=7.47 BV=5 IBV=30U
+ CJO=2.97P VJ=.75 M=.333 TT=4.32U)
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Old 12th January 2004, 03:31 PM   #4
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These two LED models come with the library for LTSPice. I don't
quite know what type of LEDs they are, though. The QTLP has
about the same voltage drop as an ordinary red LED, although
somewhat lowish, perhaps. The other one has a much higher
voltage drop. Haven't checked what types they really are since
they are nothing I can buy locally anyway.



.model NSPW500BS D(Is=0.27n Rs=5.65 N=6.79 Cjo=42p Iave=30m Vpk=5 mfg=Nichia type=LED)
.model QTLP690C D(Is=1e-22 Rs=6 N=1.5 Cjo=50p Iave=160m Vpk=5 mfg=Fairchild type=LED)

Edit: I saw Pedja had posted some models in parallel with me.
Maybe they are better. Although obviously intended as generic
models, we at least know what they are supposed to model and
since they are modelled in a different way, they may be easier
to modify.
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Old 12th January 2004, 04:58 PM   #5
sam9 is offline sam9  United States
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I'm a little down on LEDs for bias setting at the moment. A couple of months back, I took a few "standard" red LEDs from different sources and use a 9V battery, a resistor and a DMM to measure voltage drop. The same part no fro the same manufacurer was pretty consistent. Across manufacturers there was enough difference that I'm not so sure the idea of a "standard" LED (choose a color) is to be depended on. ( I was careful, I thought to avoid "extra bright" or other specialy LEDs.) Thus I think it a little uncertain to treat LEDs as "standard" like can treat 1N4148s as pretty much the same regardless of source. On the otherhand, I don't think the differences are show stoppers, just something to be aware of.

This isn't to say there is anything wrong with LEDs for setting bias, but that it may be a good idea specify manufacurer and part number. Also, I don't find LED data sheets to have the info I want as they are more oriented toward the otical properties - hardly surprising. Anyway, I'm still willing to use them but I measure the drop and make sure they all have the same part number. I'm also thinking of making a survey of the various specilaty colors and types -- maybe someday and extra bright blue will be just what I need.
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Old 13th January 2004, 09:03 AM   #6
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Thanks for your useful replies,

Do you think one can use zener diodes instead of LEDs in real world applications or there are some drawbacks, e.g. noise , etc.?


Federico
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Old 13th January 2004, 10:23 AM   #7
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I'm not a device physicist, but as far as I know, zener diodes with low voltage values have real zener breakthrough, which doesn't generate much noise (just a bit of shot noise, I believe), while "zener" diodes with higher values have avalanche breakthrough, which generates enormous noise. Zeners around 5V have a bit of both types of breakthrough.

So yes you can use zener diodes, provided that they have low voltage values (well below 5V; I ususally don't use anything above 3.9V, 3.6V if noise is critical).

Low-voltage zeners have the disadvantage that their differential resistance is rather high. When you bias them at 2mA, the voltage can be hundreds of millivolts below the specified value, which is usually tested at 5mA for low-voltage 400mW zeners.
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Old 13th January 2004, 10:30 AM   #8
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Quote:
Originally posted by sam9
Across manufacturers there was enough difference that I'm not so sure the idea of a "standard" LED (choose a color) is to be depended on.
I've seen a tendency to use the Red LEDs. The graphic below is from a Fairchild LED datasheet. From it, one can see that the red LED has a steeper If v Vf curve. This is desirable because the voltage drop is less susceptible to changes in current. (In the case below, forward current is rather high: >10mA, but I think this phenomena holds true for the lower current varieties as well.)

Quote:
Originally posted by fscarpa58
Do you think one can use zener diodes instead of LEDs in real world applications or there are some drawbacks, e.g. noise , etc.?
FWIW: I've read (but not seen data) that LEDs are quieter than zener diodes. I would like to find actual noise data for LEDs.


JF
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Old 13th January 2004, 01:48 PM   #9
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Thank you again,

F.
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Old 13th January 2004, 02:09 PM   #10
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Quote:
Originally posted by johnferrier


I've seen a tendency to use the Red LEDs. The graphic below is from a Fairchild LED datasheet. From it, one can see that the red LED has a steeper If v Vf curve. This is desirable because the voltage drop is less susceptible to changes in current. (In the case below, forward current is rather high: >10mA, but I think this phenomena holds true for the lower current varieties as well.)



FWIW: I've read (but not seen data) that LEDs are quieter than zener diodes. I would like to find actual noise data for LEDs.


JF
John,

please forgive the ignorance of a non-native speaker, but
what colour is "HER"???

Regarding noise, I think there once was a claim posted on
the fourm that green LEDs are the lest noisy ones. There
was no motivation or reference, though, as far as I can
remember.
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