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I Am Fascinated By Physics

The voltage drop across an LED seem to be directly proportional to the colour (energy) of the emitted light. e.g. red LEDs (wavelength c.660nm)have a voltage drop of about 1.8v and blue LEDs (wavelength c.450nm) about 3.3v.

If a LED needs 3.3v to emit blue light, how can a white LED (which must have a blue component) operate on only 3v?

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Faust76 · 46-50, M

Tables of real diodes seem to show blue and white leds having same forward voltage drop, as one would expect. The voltage varies a bit though, surely there's inefficiencies and impurities in different leds, which has me think that one reason for the apparent difference might be many blue leds are older technology/cheaper make.

rob19 · M

@Faust76 The tables I've seen show blue LEDs having a higher voltage drop the white. Efficiency could be a factor. The drop across white LED is very precises, allowing them to be connected in parallel without individual current limiting resistors. I don't know if that's relevant.

Faust76 · 46-50, M

@rob19 Here's a popular table for specific series of leds:

And Aliexpress for some that they are selling:

There's couple of product tables that list several off-blue colors with different voltages, however, like "Super Blue", "Ultra Blue" and "Ultra Super Blue". So as said it's possible the blue's aren't from same series/manufacture, or they're not same blue.

rob19 · M

@Faust76 Thanks, that's really helpful. It's now looking like the differences could simply be due to different manufacturing techniques. Comparing the voltage drop of blue LEDs from the first table with white LEDs from the second would make it appear blue LEDs had a higher voltage drop. :(

Tastyfrzz · 61-69, M

Higher frequencies /shorter wavelengths require more energy. I suspect an x-ray emitting LED would require a lot of voltage drop. Basically then you are just arcing across a gap.

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rob19 · M

@Tastyfrzz The brightness can be considered the number of photons per unit time, not the energy of the individual photon.

Tastyfrzz · 61-69, M

But you can plot the intensity by frquency. E=hv where h is plancks constant and v = frequency.

rob19 · M

@Tastyfrzz That's the energy (colour) of the individual photons. The intensity is the number of photons.

SW-User

white LEDs are blue LEDs with a phosphor, so they [i]should [/i]have the same forward voltage as a blue LED

rob19 · M

@SW-User That's what I would think but it seems they don't. I suppose it could be related to efficiency.

KayraJordyn · F

Haha... Wut?

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