SparkFun Forums

[angelsix]

OK simple question to save me the job of days more looking, but I am after some 0603 LEDs with lower forward voltage drops to use on existing systems with lower voltages available. Are all LEDs based on their wavelength going to have the same Vf?

For example most red are around 2V, presumably due to the methods used to create the light. Some with higher Vf like White, UV, Pink etc... are they always going to have a 3V+ Vf, or is it techincally possible to get them at lower Vfs?

I suppose the real question is what is the lowest voltage each wavelength can have, and is there a definitive list anywhere?

My main reason is I would like some UV lights working from a system with 2.5V so I am a good 1V out of range at the moment.

[phalanx]

Forward voltage is linked to wavelength of light produced. Wavelength is dictated by the band gap energy of the semiconductors being used.

Without getting over technical (ie this is very simplified), materials have energy bands their electrons can reside in. The Valence band is the lowest energy state which you can think of as the at rest condition. The conduction band is the highest energy state where electrons are actively being transported. In a conductor, the valence and conduction bands overlap so very little energy is needed to move electrons across the material. In an LED, there is a gap between the valence and conduction bands. For electrons to make it over the gap, their energy needs to be increased (voltage) by an amount that increases with an increase in band gap. This is why typical diodes have a 0.7V forward voltage since it takes 0.7V for the electrons to break over the gap into the conduction zone.

With an LED, the gaps are tuned to produces specific wavelengths of light. Larger wavelengths which we perceive as red are lower in energy and have a smaller band gap and smaller forward voltage. Short wavelenght LEDs (blues and ultra-violets) are higher in energy and have a correspondingly larger band gap and forward voltage.

This means that you are very unlikely to find any ultrviolet LEDs with a considerably lower forward voltage.

-Bill

[skimask]

Vf depends mainly on color, true, but it also depends on temperature, manufacturing techniques, etc.
A specific manufacturing technique will yield an LED of a specific color and Vf.
Another technique might have the same color, but a different Vf.
Another technique might have the same Vf, but a different color.
So, in general, no, at the present time you won't find any widely available blue (or whatever) LEDs with a Vf of 1.5v, and by the same token, probably won't find any widely available red LEDs with a Vf of 4v either.

[leon_heller]

If you need a higher voltage to drive the LEDs you want to use, you could use a boost converter.

[angelsix]

Thanks for the highly detailed replies its always good to read up on and learn from. Explains it very well.

Regarding boosting the voltage, its sort of possible but I am trying to make use of existings pads as they are in specific locations also, but I may just have to do that.