LED colour and current.

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Stolk
 
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LED colour and current.

Post by Stolk »

I understand that red LEDs draw the lowest current of the colours, I think?

What about amber versus pure green?

The Adafruit led matrices comes in many colours including amber and pure green.
I would like to get the one that draws less.
So hopefully, I can power more than one using the 500mA of a USB port.

https://www.adafruit.com/product/2042

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blnkjns
 
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Re: LED colour and current.

Post by blnkjns »

I think it is more a lumen/watt thing. Classic red and green are not the most efficient, blue and pure green are. You can also PWM the LED's to have less power draw. Make sure to include a capacitor to even out the PWM peak current draw.

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adafruit_support_mike
 
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Re: LED colour and current.

Post by adafruit_support_mike »

LED colors aren't related to current.. almost all indicator LEDs are designed for a nominal 25mA operating current, but almost all LEDs made these days are super-brights that will light visibly at 2mA or less.

The difference between colors is the amount of voltage necessary to generate them.

All diodes have a junction between two materials with different electrical potentials. In silicon diodes and LED's it's called a 'pn-junction', where a piece of p-type silicon (doped with empty orbitals for electrons) is placed in contact with a piece of n-type silicon (doped with extra electrons). The excess electrons are strongly attracted to the nearby empty orbitals, and move to fill them.

In the process, they move negative charge to the p-side of the junction and leave positive charge exposed on the n-side of the junction. Those separated charges create an electric field that tries to pull the electrons back to the n-side. The migration of electrons continues until the strength of the electric field pulling them back toward the n-side is the same as the strength of diffusion into empty orbitals pulling them toward the p-side. The result is a zone where charge can't move forward or back, called the 'depletion region'.

That would be the end of the story if it weren't for thermal energy.

In neutral silicon (and other semiconductors) an atom occasionally gets a hard enough jolt from one of its neighbors to knock an electron away from the atom and leave an empty orbital. The process is called 'pair formation', and at any given time there's about one electron-hole pair per 1e15 atoms.

If a pair forms in a depletion region, the forces that maintain the depletion region pull the electron toward the p-side and the hole toward the n-side. From outside the depletion region, it looks like one electron worth of charge has managed to flow through the depletion region. The typical leakage current through most diodes is a few femroamperes (1e-15A) where 1fA is about 6250 electrons per second.

Applying an external electric field to a depletion region changes the balance between the forces that make it exist. If you push excess electrons into the n-side and excess holes into the p-side, you can increase the leakage current exponentially.

Electrons crossing from the n-side to the p-side have to lose energy to inhabit an empty orbital, and that energy leaves the electron in the form of a photon. The difference between a free electron's energy and the energy of an electron in an orbital is called the material's 'band gap'. We define joules of energy per electron as the unit 'Voltage', which gives us the term 'band gap voltage'. Different materials have different band gap voltages depending on where their orbitals live naturally.

Photons have no mass, and always move at the speed of light, so the only way they can carry different amounts of energy is by vibrating at different speeds. Those different speeds are different colors of light.

Since all the electrons that pass through a depletion region lose roughly the same amount of energy (the band gap voltage for the material), all of the emitted photons are nearly the same color.

- The band gap voltage necessary to generate infrared light is about 1.5V
- The band gap voltage necessary to generate red light is about 1.7V
- The band gap voltage necessary to generate green light is about 2.2V
- The band gap voltage necessary to generate blue light is about 3.1V

The excess energy that gets released when electrons cross the depletion region comes from the external electric field applied across the depletion region. Since electrons can't escape their atom until they have at least the band gap voltage worth of additional energy, the voltages listed above are where current starts to flow through diodes of the various colors.

bjinkins is right that a blue photon carries about twice as much energy as an infrared photon, and our eyes are able to recognize that difference in energy. The amount of energy that moves from one place to another is called 'power', and at least one unit of power is the Watt. Lumens or candela measure the number of photons leaving a source, so Watts per lumen (or candela) is a valid way to describe an LED's output.

In practical terms, a blue LED with 3mA of current flowing through it will look brighter than a red LED with 3mA flowing through it because the blue light carries more power. When you use RGB LEDs, you may find it necessary to increase the red current and decrease the blue current to make the LEDs appear to be equally bright.

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Stolk
 
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Re: LED colour and current.

Post by Stolk »

adafruit_support_mike wrote: Thu Mar 09, 2023 6:26 am <in-depth analysis>
Damn! I am seriously impressed by this deep dive. Thank you for taking the time to do this.

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blnkjns
 
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Re: LED colour and current.

Post by blnkjns »

It is nice for physics class, with spectral analyser or a prism with ocular and nm readout, you can calculate the h constant if you have like 3 or 4 LED's with different colours. You will also see that at the minimum voltage the led needs to start emitting light, the peak wavelength is slightly longer than at the nominal voltage.

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