LED die afterglow

Flashlight Modding and DIY Parts
1

I just noticed that some LEDs have an afterglow on the die for a few seconds after power being switched off (hard off). So far, it is obvious on the XPL-HI 3A of the Eagle Eye X6R and the XML2 4C of the Convoy C8 but missing on the XML2 5A?/7A? of the Fenix PD35 2014 Edition and all other lights I have (all CW Fenix’s). Interestingly, the few lights I have with the fake Q5 LatticeBright do have this faint afterglow despite being purple. :stuck_out_tongue:

Is this fluorescent material on the die surface? If so, why do some LEDs have it and some don’t?

2

I could be totally wrong regarding this, but I don’t think the afterglow is due to the Led itself, but something in the driver that has residual energy in it.

However, I only have one light that exhibits this clearly, and it’s my JetII PRO with a XML hi, though my ST25 baton has a tiny bit with I think an XML T6.

3

My nitecore tm16gt does this when turning it off. I just assumed it was the coil or caps on the driver bleeding down through the led.

4

Isn’t it due to the phosphor of the LED glowing for a short time?

5

I think it is the driver as well. CFL light bulbs do the same thing because of their built-in ballast.

6

Take out the battery, and then try lighting the LED up with an external “royal blue” or UV light source
(careful of your eyes)
then see whether the glow persists when you remove the external illumination.
If so it’s the persistence of one or more components of the phosphor that you’re seeing.

White LEDs use a blue emitter under “a phosphor” — which actually is a mix of different chemicals.
Some use coarse enough chunks you can see the color variation over the zoomed image of the die
Some have them finely divided and well mixed.

7

Doh! Didn’t think it could be the driver!

Hank, what you describe is interesting, too. Don’t have any UV at the moment but that just means am gonna have to get one. :smiley:

8

Doesn’t have to be UV. Try with any cool white LED, just shine it onto the emitter through the flashlight’s lens.
There’s plenty of blue leaking past the phosphor.
The blue spike is the fundamental driving frequency; the broader curve is the phosphor re-emitting that energy.

PS, on almost any power supply, you can see the power light takes a second or two to fade out when it’s switched off or unplugged.
That’s definitely energy in the power supply, probably a capacitor that takes a bit of time to discharge.
Those (usually blue or green) LEDs are typically not the kind made using phosphors, and won’t show any glow when illuminated while unplugged.

9

The most noticeable light I have that does this is the BTU PK26 w/XHP-35 HI. I'd suspect the driver electronics - caps most likely. This isn't my favorite driver, and has the turn on bright flash as well.

I mean this afterglow fades slooooowly. You clearly have enough time to turn the light off, flip it around so you are looking at the LED, and watch it fade for a couple seconds.

10

My SC600w mkIII does the same, it’s an XHP-35 too, maybe it’s the normal operation of this LED?

11

I think it’s the “normal operation” of the boost drivers used to power a 12v emitter from a single cell…

12

I think that ‘afterglow’ is just the soul leaving the emitter after it ‘dies’. :stuck_out_tongue:

13

Some leds do have an afterglow albeit quite faint. You can check this with a blue or uv led.
I used a 405nm laser, the only blue light source I had. The afterglow is weak but noticeable,
it fades away in 5 seconds or so. I think it may be made brighter with a more powerful light source.

14

:laughing: :smiley: