Does heat turn a white LED into a dim blue LED?

[audioguru]

I made a night light with 36 red Luxeon SuperFlux LEDs each at 53mA.
They dissipate a total of 5.5W.
I packaged them in the plastic case for a compact cassette tape and even with many air holes it gets pretty darn hot.

I have some solar garden lights with a fading RGB colour LED and a white LED. I always have them set for colours so the white LEDs were wasted.

I removed two white LEDs from my solar garden lights and installed them in my night light at 23mA each. They were a bright white for a while then they changed to a dim blue.
The yellow phosphor is gone! The heat killed the phosphor?

 

[Hero999]

Did it change suddenly or was this a gradual process?

This doesn't surprise me.

 

[audioguru]

The white LEDs changed into dim blue in a few days.
The red LEDs are fine.

Oh, the red LEDs are high quality American.
The white LEDs are cheap Chinese.
I'll replace the bad white LEDs with high quality blue ones.

 

[RODALCO]

That has to be a very bright nite light audioguru.

I made some with 4 and 8 standard blue 5000 MCAD LED's and they run at 10 mA and give ample of light to find your way around, in the dark.

 

[duffy]

Did they get wet? We had a problem with moisture killing the phosphor in an ELM display one time, might be related.

 

[knaaphix]

voltage problem might be the reason

 

[duffy]

Audio McDuck will be smarter than that.

 

[audioguru]

That has to be a very bright nite light audioguru.

It lights a long hallway and can be seen from outside through windows that are fairly far away. Yes, it is very bright but it is red so I thought a little white would make it better.
I also made a smaller one with 16 red LEDs and it is also bright and hot.

voltage problem might be the reason

Voltage is fine. The white LEDs used to have a visible yellow phosphor but now it is gone.

 

[mneary]

Most "white" LEDs are bright blue LEDs with one or more phosphors that turn some of the blue energy into other wavelengths. Either the heat destroyed the blue LEDs making them too weak to activate the phosphors or it destroyed the phosphors. Maybe the bright blue bleached out the phosphors and then later the heat destroyed the blue LEDs. Either way I guess you need new white LEDs and a way to keep them cooler.

Did the white LEDs ever receive 23 mA continuous while in the garden lights? Maybe the garden light maker knew something that they didn't tell you about those LEDs.

 

[Hero999]

I doubt moisture was the cause because the phosphor would be sealed in the LED's epoxy package.

Red is supposed to be a good colour for a night light because it doesn't destroy your night vision. The disadvantage is that it has a lower luminious efficiency than white or blue at low light levels.

 

[audioguru]

I think the Chinese white LED manufacturer used piss or mustard as a yellow phosphor. A half decent LED would survive the temperature of my night light.
The solar garden lights get pretty hot in the sun all day long but then the LEDs were not lighted.

 

[Hero999]

Why not just use pink LEDs rather than separate red and white LEDs?

 

[audioguru]

I used the red LEDs because a nice guy on another forum sent hundreds of them to me for helping him with a project.
I have never seen a pink LED. Does it look sexy?

 

[Hero999]

I've got some in my junk box which I bought for curiosity's sake.

They look like this:
**broken link removed**

They're just a blue LEDs with a phosphor which makes them pink.

I bought them from **broken link removed** stock no: 55-1822

**broken link removed**the datasheet.

 

[duffy]

I doubt moisture was the cause because the phosphor would be sealed in the LED's epoxy package.

That's what we thought on the ELM project. Turns out water will go through plastic - not much and not fast, but enough to kill the phosphor.

 

[Hero999]

I would have thought that it would take much longer for water to penetrate a thick epoxy LED case than a thin ELM film.

I routinely put white and pink LEDs in my mouth and I've nver had any problems with my spit killing the phosphor.

 

[duffy]

I'm sure it does take longer, but AuDuckGuru mentioned they were in the garden. So I'm figuring that means left out in the rain, condensation, possibly quite wet for quite a while.

And most of these higher-power white led's are not in T 1 3/4 cases, they are flat with a thinner layer of some rubbery plastic with the phosphor embedded in it.

 

[audioguru]

My cheap Chinese white LEDs from solar garden lights are the same size as an ordinary LED (T 1 3/4) but they have a fairly flat top so that the viewing angle is very wide, about 140 degrees. They turned into dim blue LEDs only after spending a few days in my hot night light.

A 3rd one was also outside for a couple of years but was never in my hot night light and is still bright white at 23mA. It still has a visible yellow phosphor when it is not lighted.

 

[duffy]

Be careful when you are looking into that dim blue light, Mr. $crooge.

"White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium-based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu, Al). This is a method analogous to the way fluorescent lamps work. However, the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness. Another concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin."​

- Wikipedia

 

[audioguru]

I replaced the defective white LEDs with one that sticks out of the case so it doesn't get too hot. It has been lighted for one hour and it is still as bright as it began.

The yellow phosphor on the defective LEDs is gone. Before the chip was covered with phosphor and now I can see the bare chip.