What does dimming and bluing of the LED actually mean?

Hey all

My ST90 flashlight has been running fine for a couple of weeks now, but all of a sudden with my last battery change (all four 18650s were around the 4.10 Volt mark) the light starts to visibly dim after 8 - 10 secs, and the light colour possibly start “bluing” … well at least becomes cooler.

Also I see wisps of smoke coming out of the LED module. If I turn it off then try again in a little while, it will repeat the same pattern.

LED looks ok as far as I can tell.

So, is this dimming / bluing, along with a little smoke, caused by:

- too muck voltage ?

- too much current ?

  • poor heat sinking ?

Just wanting some ideas so that I can check things before going back to the manufacturer.

Cheers everyone


It is burning up. sounds like it is getting to much current, or is not heatsinked properly.

I would put my money on something has come loose and so its now not heat sinked properly.

I agree.

Well, when I have time this evening I’ll remove the LED board from the large heatsink (its bolted down by allen bolts), then clean the contact areas between the board and heatsink and reapply a fresh thin layer of thermal grease.

Fingers crossed that this is the cause … otherwise I’m not skilled enough with the soldering iron to do much else!

Cheers for now

Message Electrolumens, my guess is something came loose
(I have no electronic skills) the led is definitely cooking, but whether its a loose wire or loose board, I don’t know.

Cheers Carpentry Hero

Yep I contacted Elektrolumens, and I tightened the two allen bolts on the LED board … problem fixed! The board already had thermal grease underneath it, which was sticking tight, so I didn’t bother prying it up and changing the grease. I’ve got the allen bolts pretty firm, but I’m thinking that with use the heat and subsequent cooling may loosen them a bit (through expansion / contraction) … so this may be a possible design flaw.

I just hope the few periods where the light dimmed and turned cooler in tint as not damaged the LED, which “looks” perfectly fine at any rate.

Thanks for the suggestions everyone.

Right on, if tightening the bolts is all it took, next time you check them add some lockTite ?

Excellent idea mate! I might check them before further use and try the locktite … I’m sure they’ll loosen in time otherwise.


Bridgelux says to mount their arrays with a split lockwasher and a flat washer (order: screw head, lock washer, flat washer, array, thermal grease, heatsink).

Don’t want to jump in on the thread, but I am having the same issue with my Dorcy 220Lumen Rechargeable Cree XRE LED flashlight. When I got it, it shot like a laser, but now its very dim and the color of the LED changed.

I think it happened after I ran it until it was completely dead doing some car repair at night. After I recharged it, it worked for a while, but now its like it doesnt work anymore.

Even when I’m running it directly off the 6V AC adapter, its nowhere as bright as it was before. Right now its maybe 10% of what it originally was.

Good thing I have other lights now because its become useless.

Any ideas on how to fix it?

I think the OP caught his problem early enough that his LED wasn't permanently damaged. It sounds like yours is permanently damaged, and you'll have to replace the LED, and then attach it to the heatsink better to make sure that it doesn't overheat.

Thanks Texaspyro - this method must really get everything snug tight onto the heatsink!

Elektrolumens only uses two small allen bolts and thermal grease to secure the SST-90 onto the heatsink … no wonder mine turned blue so easily as the allen bolts weren’t as tight as they could be. I’ll be loctiting these in when I’ve got some time.

Hi Rojos

Yes I “think” I saved my LED in time … it looks perfectly fine and lights up, but it is possibly a little dimmer than it was before. :oO

It’s hard to be sure though, as there was some smoke in the head module as well so this smoke could have coated the reflector a little.

Heatsinking is vital with all higher powered LEDs … just surprised that my ST90 had this issue so quickly, especially being only 2 weeks old.



I think your issue is probably the battery. Sounds like you may have run the battery down and it couldn’t get back to full charge.

Check the voltage on the battery.

You really need the split lock washer to handle the thermal cycling. Without it the bolts can loosen…