A little problem with CREE XR-E Q5 Leds I can't figure out..Need Help!

[LittoDeviL]

Hey guys, i've been reading the forum for quite a long time but never posted much... before I go out and re do my entire led array setup i wanted to pick the brains of some of you guys and see what the problem could be.

First off I design crazy custom lighting technology for my individual cilents in the automotive world for a living except sometimes the things I do with lighting gets so crazy I can't figure out what broke them at the end =P.

On two different vehicle, a camry hybrid and a smart car... I took 6 CREE XR-E Q5 led's and I made licenses plate lights out of them. They are in a set of 3 mounted on heat sink and a fan blowing across it for active cooling.

The LED I used was bought from dealextreme http://www.dealextreme.com/details.dx/sku.11023

Powering them is an led current regulator, the 1000ma buckpuck http://ledsupply.com/03023-d-n-1000.php

For a few months both cars with different heat sink and fan setup had ... lets just say a very bright white led licenses plate light =P...

Then afterwards both array on the smart car and one array on the camry hybrid became noticably dimmer.. dim to the point where it looks just a tad brighter then the factory led licenses plate light on the newer BMW and Lexus vehicles.

I went to measure the current coming out of the regulator to the led's and it still measures .95 amps so I don't think the led current regulator is going bad.

I know the led's are dimmer because now I can actually stare into the led and see the die lit up without being blinded... they seem a little bluer or maybe that's just my imagination.

So my question would be... what do you think caused them to become dimmer? Did the heat kill them? Or did they initially become bright and then it levels off to a lower light output?

Could it be that the raw cree led mounted on those dealextreme pcb have poor heat transfer quality that's why over time they kinda burnt up?

By the way here's a picture of the licenses plate light when they were bright.. around... x-mas time obviously =P...

Thanks guys!
Cheers!
Larry

 

[saabluster]

What was your "heatsink"?

Hey guys, i've been reading the forum for quite a long time but never posted much...

BTW that is a bit of an understatement. :welcome:

 

[LittoDeviL]

Hmmm my heat sink.. hard to describe, which I had a picture to show. The 3 led array was mounted on a heat sink the size of.. uh.. 1.5" wide by 6" long by maybe... the fins are 3/8" tall with two fans blowing directly on top of them? The heat sink never got hot to the touch even though at times when I put my finger on top of the glass dome it would be hot.

It makes me think maybe the heat isn't getting to the heat sink well enough?

And... not sure what you meant by understatement but I did join earlier this year... not sure was it to post heh but been reading for quite a while. And thanks for the welcome always nice to read thru all the information on here!

 

[mds82]

I know i did something similar with my licence plate, but i am using 2 XR-e's powered by the 350ma version of that driver. Personally i think its bright enough.

With your problem, i would say heat is the issue. I know with the seoul P4's LED's, when they overheat they turn an angry blue color, so i guess that could also happen with the XR-E's.

I would take the whole unit out, let it run for 10-15 minutes out of the car and see how hot it gets.

 

[dom]

Welcome to CPF
Definitely a heat transfer problem - the heatsink should have been cooking within 5mins.

How were the LED stars attached?

Should have had thermal grease under them and screwed on or used Arctic Alumina or similar adhesive.
If you did that already - then it may have been the LED attachment to the star ,though i think DX do a pretty good job on them these days.

EDIT:
I also remember reading about sometimes needing a capacitor (or something) for the input voltage on your driver -as a guy who used them in a similar setup as you
was blowing drivers. Though not sure if that would affect your LEDs or not as you checked that the driver is still working OK.
EDIT 2:
Found the thread -though may have nothing to do with your problem,could be of interest.
http://www.candlepowerforums.com/vb/showthread.php?t=208756

Cheers
Dom

 

[evilc66]

I'd be willing to bet that it's the driver, not a heat transfer issue. There will be many times where the source voltage may dip below the forward voltage of the LEDs. The Buckpucks have a tendancy to burn up if the internal capacitor is being overworked by trying to compensate for the lack of voltage. Even if the source voltage gets close to the total Vf it can cause issues. You're now taking away the 2 volts that the driver requires to operate above the total Vf. A buck/boost driver would do better in this situation, as it will compensate for whatever changes in source voltgae.

And why do you need 6 LEDs at 1000mA to light up a license plate? Two would be more than enough. Are you running the LEDs in series, or two groups of three in parallel?

 

[saabluster]

And... not sure what you meant by understatement but I did join earlier this year... not sure was it to post heh but been reading for quite a while. And thanks for the welcome always nice to read thru all the information on here!

I just thought it was funny that you said "never posted much" when you had never posted before at all. Just made me laugh a little. It would seem we still need more info into how you mounted the stars. Do you have any pictures that you can post? That will go a long way to helping us help you.:thumbsup:

 

[bshanahan14rulz]

Hey Larry!

Curious how you wired the LEDs as well. Sounds like either overheating or somehow too much voltage getting to the LEDs. are they dim AND bluer now or just dimmer?

In an automotive application, I'd say stick to 600 or 700mA max. (350mA isn't out of the question either, seeing as you have probably about 6 Crees lighting up a license plate)

Anyways, getting to the point, here are my ideas:

1: check thermal junction between star and heatsink. enough grease/thermal adhesive?
2: check wiring. I doubt you would make a wiring mistake, but you never know. maybe end-user decided wires were in the way and stapled them to the bumper
3: spray for chip weevils. if your LEDs are infected, replace them before the entire car becomes infested. Also, if you have to make another LED array, make sure it is impervious to chip weevils.
4: take a look at the LEDs, see if they LOOK fried. if an LED fries, it generally should show some signs of it, such as burnt spot on the phosphor or broken bond wires.

Oh yeah, and check the datasheet to make sure you are using the correct Vf values for the LEDs at 1A. I've never used a pre-made driver, just crappy home-made stuff, but most of my solutions drop 1 or 2 volts. Make sure that 12v-Vftotal >= required voltage for regulator

 

[CampingLED]

I made a camping light with 3 x Cree Q5s on a large heatsink (also have one with 3 x Rebels). Two modes 350mA and 700mA to LEDs in series with a constant current source. At 350mA the heatsink hardly warmed up. At 700mA it got hot after a few hours. My feeling is that at 1000mA the heat build-up will be much higher and not suitable for extended LED life.

Another possibility is corosion on the contacts or solder joints. In your setup it may be exposed to many elements and therefore could degrade quickly. With LEDs in series it only takes one bad contact OR one bad LED to reduce output.

Try the elimination method:
Clean contacts and check solder joints (even LED to star)
Replace the driver
Replace one LED at a time, etc.

First find the actual problem to the reduced output and then it will be easier to find the actual cause. Good luck.

 

[HarryN]

Hi, how well sealed are you LEDs from the elements? Is there any path for moisture to get to the LED dome? I am not entirely sure that the Cree package is considered water tight, or at least moisture tight.

Dimming in general does sound like a thermal effect. Here are some distributors for Cree. http://www.cree.com/products/xlamp_dist.asp

 

[LittoDeviL]

Wow hey guys thanks for ALL THE GREAT RESPONSE!

Let me try to respond to a few...

The reason why I have 6 led per licenses plate is because... well someone has to do something crazy =P... being someone that does high end lighting modification on cars I even equipped the hybrid with HID reverse lights =P...

http://www.littodevilperformance.com/hid/gallery/toyotacamryhybrid2007plateledreversehid/

The led's are very well sealed from the element as the whole thing is potted with clear epoxy and.. it actually increases the angle that the light shines out.

The thing is, both side of the smart car and only one side of the camry hybrid experienced the same thing, so... either the driver is going bad or... the heat sink... when I measured the power flowing thru it, it was showing 0.95 amps. Knowing how much less output I could get just based on voltages.. it wouldn't be the same with just a 0.05 amp drop right? I assume a 1000ma driver is like +/- 50ma =P...

If the driver did go bad how could I tell? I mean the only way I was assuming the driver is good was the fact that it was still showing that I was pushing 1 amp thru the led's with the car on at 14.4 volts... which.. should give more then enough forward voltage if the whole array of 3 leds requires a little more then 11 volts.

Oh... and there is two drivers on each car... one per 3 sets of led wired up in series.

The contact points are all good as they are soldered and heatshrink.. as far as the contact points to the led's.. I can't check or replace them as they are under the clear epoxy haha but all the led's on all 3 "sets" out of 4 are experiencing the same time over time...

Maybe it is the heat? I hope it's the driver cuz that would be easier to replace =P... maybe I sholud use another driver and just crank up the voltage and see if they become brighter with more amp flowing thru them?

Right now it's at a brightness where it's not getting any dimmer so... does that mean... 1 amp flowing thru it + dimmer output = less heat?.... haha..

Between the DX Cree star and my heatshink, I mounted it with Arctic Silver Premium epoxy thermal adhesive and used a press to squeeze any excessive epoxy out of it.

So guys if the driver still puts out 1 amp or... i mean 0.95 amp, does that mean it's still good?...

Cheers!
Larry

 

[JohnR66]

I have to agree with the others. Back off the current. The absolute maximum rating is 1000ma and your running them at this level. While this might be okay for a flashlight, Higher duty cycle uses can be a problem especially with heat.

I'd run them in 350 to 500 ma range. I would not spend any extra for Q5s for this application either. Too much light for this application as it is.

 

[CampingLED]

I have to agree with the others. Back off the current. The absolute maximum rating is 1000ma and your running them at this level. While this might be okay for a flashlight, Higher duty cycle uses can be a problem especially with heat.

I'd run them in 350 to 500 ma range. I would not spend any extra for Q5s for this application either. Too much light for this application as it is.

Supported.

Let us know if you have done more tests/checks.

 

[HarryN]

The Lumileds K2 s are a bit more heat tolerant at that drive current and are rated for 1.5 amps, so maybe they would be a little more fogiving. Still, if you can easily touch the heat spreader, you should be ok.

Just wondering, did you happen to use any kind of thermal grease or thermal contact material ?

Are the LEDs hard mounted to the Al, or are they on a board, then mounted? Emitters or stars?

I would be surprised if the driver has failed if it really is still putting out about an amp. The most common driver failure mode is to have it disconnected from the LEDs while on, or if they got a spike from the car. The taskled drivers are a bit more robust from a car voltage spike perspective, but the leddynamics ones you have actually are quite good, and I like the potting seal they have.

 

[saabluster]

Your LEDs are dead. Still wish you would post some pics. My guess is that the ambient temp rise is putting to much strain on an already weak thermal system. With the summer heat those heatsinks don't work as well to shed heat even with a fan. Not generally a problem for flashlights because if it is dark where you would need to use one it is most likely also cool. I assume you hooked up to the existing lighting wiring? Most car's lights are on all the time. You need to replace the LEDs and lower the current to them. It would be even better if you use a driver that can pull the power back if it gets too hot.

 

[WeLight]

The blue your seeing through the phosphor indicates a breakdown of the phosphor due to temperature. Key points

• when you test on a heatsink prior to application, the warmer the heatsink the cooler the led, intuatively people think a cool heatsink means its working, its not!
• There is a device I affectionally refer to as TLT, means "the last thing you think off with an automotive application" and that is transient protection, Luxdrive brought out a 2002 bucktoot that could handle 40V spikes which you can get a Auto apps, if you dont have that then use a varistor or other transient suppressor
• 6 XRE at 1Amp, dude if I am following you it must be blinding, 3 leds over plate length with a 350 driver and good heatsink would still be stacks of light

 

[saabluster]

• There is a device I affectionally refer to as TLT, means "the last thing you think off with an automotive application" and that is transient protection, Luxdrive brought out a 2002 bucktoot that could handle 40V spikes which you can get a Auto apps, if you dont have that then use a varistor or other transient suppressor

Very good point. I thought of that and somewhere in between thinking and typing I forgot to put it in there.

 

[saabluster]

Welight I'm actually surprised that you said nothing about the fact that he was encapsulating the LEDs. We have had our disagreements on this issue before but I think it "could" be an issue here. We can only presume since we have gotten limited information from him but the epoxy could in some circumstances cause the MCPCB to lift up off the heatsink due to thermal expansion. Hard to say since we have no pictures. One other note LittoDeviL. In this application you will be far better off ditching the MCPCB and directly solder the leds down to some copper to get the heat spread out from the die as fast as possible.

 

[bshanahan14rulz]

LEDs are dead. as long as you're running 6 LEDs, run 'em conservatively, perhaps put some kind of switch in place to run them at full brightness when you're showing off though ;-)

 

[WeLight]

Welight I'm actually surprised that you said nothing about the fact that he was encapsulating the LEDs. We have had our disagreements on this issue before but I think it "could" be an issue here. We can only presume since we have gotten limited information from him but the epoxy could in some circumstances cause the MCPCB to lift up off the heatsink due to thermal expansion. Hard to say since we have no pictures. One other note LittoDeviL. In this application you will be far better off ditching the MCPCB and directly solder the leds down to some copper to get the heat spread out from the die as fast as possible.

Your correct, the thermal expansion and contraction, exacerbated possibly by the cooling fans may have contributed to that, to be honest however I would be more concerned that the epoxy has in fact sealed the lens(floating Lens) this is a no no as when sealing with epoxy, a gap should always be left above the lens to allow it to continue to float in the metal ring, not doing so will speed the death of the led, but without seeing the details of the design, I can only speculate