Controling the wavelength of an LED through voltage?


by hsherwood68
Tags: controling, voltage, wavelength
hsherwood68
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#1
Jul27-13, 12:19 PM
P: 2
I was reading some spec sheets for some leds, and it says that, for example, at 1.4V the light is at 650nm and that at 1.7V the light is at 670nm.

This is a feature that could be potentially quite useful to me, so I was wondering: is this a common feature of leds? Is it dangerous to change their voltages to get a range of wavelengths?

Thanks!
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Averagesupernova
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#2
Jul27-13, 01:30 PM
P: 2,452
You don't really change the voltage of an LED. An LED will drop a relative constant voltage (although not perfectly constant) regardless of the current through it. Of course there is a usable range before damage occurs.
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I guess I had never looked at this part of the LED datasheet. It could mean that variations between LEDs due to manufacturing tolerances and etc. cause LEDs that drop 1.4 volts emit a certain wavelength and LEDs that drop 1.7 volts emit a slightly different wavelength. It may not mean that increasing or decreasing the current to get a slight change in voltage will change the wavelength. This is something new to me and although admittedly not particularly useful to me at this point in time I will watch this thread for any other input.
Baluncore
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#3
Jul27-13, 10:14 PM
Thanks
P: 1,298
Colour is a fixed function of the chemistry. Planks constant comes into it. E = h * u .
The voltage needed to start generating light from an LED is
LED voltage = 1239.5 / wavelength in nm
More voltage is needed to overcome series resistance with higher currents.

Mike_In_Plano
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#4
Jul29-13, 10:26 AM
P: 557

Controling the wavelength of an LED through voltage?


Temperature and excitation are two common ways of setting the wavelength for semiconductor lasers. Generally, it's safer to use current as the control parameter as attempting a fixed voltage may lead to a thermal run away condition.

- Mike
NascentOxygen
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#5
Aug1-13, 08:30 AM
HW Helper
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Hi hsherwood68 ....


Quote Quote by hsherwood68 View Post
I was reading some spec sheets for some leds, and it says that, for example, at 1.4V the light is at 650nm and that at 1.7V the light is at 670nm.
Alas, I think you may have misinterpreted what the data was saying. Did your datasheet look something like this one?




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Baluncore
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#6
Aug1-13, 10:05 AM
Thanks
P: 1,298
I think the confusion came from the false belief that the forward voltage determined the colour, when in fact it is the band-gap chemistry that determines the forward voltage and so the colour.
Mike_In_Plano
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#7
Aug1-13, 03:33 PM
P: 557
Both temperature and voltage / current affect wavelength slightly. In the early 90's, we played with putting Infra Red Lasers in liquid nitrogen to see them produce a red beam.
Now, it is very common to regulate laser temperature with a TEC to tune them for fiber optic use.
Sweeping the excitation is more common for sweeping the output for use in analytic equipment, and Thor labs offers equipment for this.
Bobbywhy
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#8
Aug1-13, 07:39 PM
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Here’s an excerpt from an article regarding the output of LEDs:

“In addition to affecting light output, temperature also has an effect on the dominant and peak wavelength. LED die wavelength characteristics are commonly reported at 25C junction temperatures. With increasing LED die junction temperatures resulting from higher drive currents or ambient conditions, wavelengths typically increase in from 0.03 to 0.13 nm/C, depending on die type.”

The article is here: http://ledsmagazine.com/features/4/8/1
NascentOxygen
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#9
Aug2-13, 01:53 AM
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Quote Quote by Bobbywhy View Post
wavelengths typically increase in from 0.03 to 0.13 nm/C, depending on die type.
Thanks for ferreting out that reference, Bobbywhy.

LED colour drift is a curious phenomenon, λ increasing with temperature. So the λ figures that OP cites ("for example, at 1.4V the light is at 650nm and that at 1.7V the light is at 670nm") would correspond to a die temperature increase of at least 150, maybe 666, if we apply the values you provide.

OP's data also relates λ to forward voltage, the higher λ corresponding to higher forward voltage. This is exactly the opposite to what one would expect, for a LED's forward voltage falls with temperature, with fixed drive current.

If in deriving that data the LED current was not fixed, then it seems pointless and misleading for OP's datasheet to relate λ to a forward voltage instead of to current or temperature.

My money's still on OP having misread the data sheet's ranges.

Waiting to hear back from hsherwood68 ......


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