EEVblog #50 - Solid State Cree LED Lighting, and How Thermal Design Sucks.

[PawnozTV]

you remind me of the joker before he was evil/crazy

[wjr123able]

I don't know if someone has already noted this but the maximum possible efficacy of a light source is 683 lm/ W, for the case of monochromatic 555-nm green light. The maximum possible efficacy for an ideal white light source is 240 lm/W because the human eye is less sensitive to wave- lengths on each side of green.

[junk250]

You are wacky lol, but good info in the vid.

I don't have electronics background, but you can get decent led technology cheap these days. Definitely the future of lighting.

I have a couple of led flashlights, with xm-l cree.

They are awesome !

[dasraiser]

awesome shirt :D see you 100 years ago

[LightingShowroom]

Interesting

[shoney]

arctic silver does make pastes.

[frosty1433]

why do I always find myself making fun of the way this guy talks?

[TheBlueCoyote]

my said she doesn't understand most of what he's talking about but likes his accent!

[jasonuscg]

Somehow I watched the whole video... Talk about waste and efficiency this 21 minute video could have been done in 7, not very efficient.

He sounds like Herbert form Family Guy!!!

[EEVblog]

@jasonuscg It's called a conversational off-the-cuff style. If you think it's too long then you go redo it in 7 minutes. I'll give you a week, but remember - no script and no retakes - I'll trust you. Once you are done, post it as a video reply so we can all critique your style.

[AxelTiger]

@jasonuscg

well if you think you can produce a more 'efficient' video then off you go! link it to me when your done! I'll be waiting..

[OldSkull87]

What are ussually good working temeratures for LEDs? Or at least safe? I've built for myself small lamp, geat for working - it's small, portable, with LiIon accu - and rather bright (2 3W 120lumen LEDs - weak and cheap ;) ), but on full power (it's regulated with pot.) it's 6W cooling on maybe 50cm2. When on 50% it'c slightly warm. But on 100% it's 90*C measured on STAR just at the diode - which means ~110*C. I'm using it ~4h/day - how long will it work? Should I lower maximum power?

[EEVblog]

@OldSkull87 Impossible to say unless you have the reliability specs for the LED in question and you can accurately measure the junction temperature. The generic answer is "as low as possible"

[devilofspades]

we americans might not know much, but we do know that aluminum is not an alloy, hence why we don't pronounce it that way...

all kidding aside, very entertaining vblog. most informative and raciest electronics blog on the web! keep it up.

[qmartq]

Couldn't the heatsing calculation be simplified by just taking each LED separately -as it would draw it`s 1.4W power to a heatsink of 1/8 of the total plate surface?

This should give quite accurate result if the placement of the LEDs on the plate is optimal.

[johnmartin1024]

Re: radiant energy from the LEDs. I have definitely felt the heat in front of the headlights on my car. Nothing was mentioned about the "beam width" of the light coming off of the LEDs. If it is like 90 degrees, I would say . . . nothing to worry about. If it is substantially less, say 10 degrees or less, then it would probably be a good idea to measure the temperature at various points on the ceiling to make sure that no fire hazard exists - no spontaneous combustion can occur.

[shodanxx]

it's disingenuous to say XYZ doesn't work and then prove it's not worth doing by doing it several orders of magnitudes wrong !!!

yes thermal design has a medium high learning curve but it's a useful consistent tool when you use it properly you don't HAVE to do trial and errors

if in doubt overdesign , put fins and more mass and more powerful fan when you can

but a good engineer will try to be as lean as possible (I'm NOT saying you're bad)

on the same topic what do you think of those 100W LEDs

[shodanxx]

also you say it's messy and not worth doing and that we shouldn't write because your model is wrong

well.. that's why you think it's messy and full of uncontrollable variables, your model is wrong !!!

I have a very hard time believing that those LEDs are 39% efficient

from what I read LEDs are 22% efficient at best ! eye color response is irrelevant to the calculation of input power to luminous flux also a 39% efficient LED isn't outputing 39% of it's power as heat that would be 100-39=61%!!!

[shodanxx]

thermal conductivity of aluminium is 237 W·m-1·K-1 according to wikipedia

it's high, that's why you didn't get "hot spot" or large differences between your half quarter and under button

if you'd used "angle iron" aluminium you would have gotten better convection cooling and even better with a couple vertical fins

a thicker aluminium strip would ramp up slower but the plateau would be the same temp because the choke point is how much convection you're getting so only higher surface would cool it

[zker666]

Lol, aluminium strip, or Aluminum strip for you US people, lol that made me laugh

[teflondon91]

You are correct with the observation that aluminum base on the LED has very low loss (probably negligable) but I liked how you made it a factor. Do you know what the lifespam is of these driven at max current vs. 700ma?

[teflondon91]

Really liked your video -- going to sub and watch the others.

[teflondon91]

Artic Silver is good stuff for CPU's too... good idea to use that.

[nwimpney]

Another issue with efficacy to efficiency calculations is that chromaticity of your light can make a really huge difference.

Most likely the efficiency of your leds is actually a lot lower than you calculated, because they tend to be fairly high in the green/yellow region in order to get those amazingly good numbers. Also, they're probably measuring the efficacy at 350mA, and not 700.

Regardless, they're pretty amazing, and unlike a lot of the other power leds they look reasonably natural.

[michael10725]

Seeing that these are so powerful... Could they sustain them self with soralr panels... And If not could on magnefigh the lifgt to do so... No matter what I Loved the show... I learned a bit... Thanks for taken the time to share your knowledge Peace!!!

[tobsmonster2]

He sounds and looks so surprised with everything he says lol.

[TheMetalPerson]

Luminous efficacy is the correct term for lumens/outputwatts.

Efficiency would be a measure of watts/watt, not lm/watt.

Lumens is an arbitrary unit based on the luminous efficacy of the human sight.

That means that a green led that outputs 1W will produce a significantly higher lumen rating than a UVA LED that outputs 1W (i don't mean consuming 1W, but emitting 1W).

[TheMetalPerson]

cont.

This means that if i wanted to sell a light with maximum lumins/watt consumed, I would it would make more sense to use phosphors as close to the green range as possible.

This makes it really easy to trick users in many other applications that don't involve simply making things as "bright as possible".

A good example of this is 660nm vs 630nm. Its obvious the 630nm leds will produce more lumens simply because humans see 630nm more.

Lumens don't matter if your target isn't a human!

[ashotvedi]

Such a great video :)

thank you for sharing it with us

you just saved me $250

Thanks again

[kooookky]

Great video!!!

Hi Dave,

You may be interested in LED flashlights using CREE such as Fenix, Jetbeam, Nitecore, Eagletac, Olight, Surefire and etc. I wonder why they can shoot up to 1000 lumens using only one CREE. Wow!! Their price are very high too. Can you describe about them?

Thanks :D

[nickjacket]

I use a Fluke 61 infrared thermometer to keep my fingers burn free. Your LED's toward the center of the aluminum bar will get hotter than the ones at the ends. As long as you don't have to replace LED's every year then you've done well.

Since the black art of thermal science can be daunting, I would have used heat sinks between each LED which would have caused a convective flow of air that competes well with the advective air flow that your overnight storm gave you at random.

[pdtnc]

Totally mad, I can't say I understood it all, but it was interesting what I did learn and most entertaining watching your excitement :D

[mattnourse]

Hahahaha just watched your video which is very good by the way.

I actually design heatsinks for High Power LED Arrays and remember when i first started this about 2 years ago.

I called CREE and spoke to them about how efficient the X-RE is and was stunned when they said 25% at best. Calculation is always the first port of call but QFIN 4.0 is a very easy CFD software. I use a mix of this and SolidWorks FloWorks which i admit is a black art.

Glad you feel my pain.

M

[woodlanduk]

Dave love the ending...:0) after playing with LEDs since 1976 i work on is it hot to the touch YES? bigger heatsink or forced air cooling if NO, no worries . Same goes with any Semiconductor that requires thermal management .

[Darrengb]

another great video

[stridermt2k]

As often happens I went kind of cross-eyed during the calculations but as often happens I hung in there because it all comes across in the wrap-up.

Beautiful work, and as always, thank you for sharing your experiences.

Yeah, overkill does it for me too. :)

[DanFrederiksen]

great video. no idea LED had leaped ahead like that. last I checked it was like 60lm/watt or less

[oehrlein49]

Nice video! What are you using to drive the LEDs? do you have a current source? or are you just applying a constant voltage and using a current limiting resistor in series?

[EEVblog]

700mA constant current. See my website for the links.

[FenderGibsonWashburn]

I think LEDs are fascinating. I believe LED lighting will become more popular as people start to look into other alternatives to incandescent lighting besides CFLs. Lumens per watt are going up and prices are coming down, very exciting.

As far as Thermal Design goes, overkill has always worked for me.

[danorambo]

LOL, man your funny with ure voice.

[pikuorguk]

My server kept warning me that the CPU was over temperature and that the world was going to end, etc.

So I opened the PC up, stuck my finger on the heatsink and after noticing it was mildly warm, put the lid back on and decided to ignore the temperature warnings.

[EEVblog]

Did you check the heatsink compound undernearth? If that's not in good nick then you might indeed have a problem.

[pikuorguk]

Yeah, it was fresh compound. The machine had also been running for a few months quite happily before. I blame an uncalibrated sensor in the motherboard after upgrading the software it runs.

[Forssa1]

Loved this video!

[philbx1]

Hey Dave, nice work again.

I've been stuffing around with LEDs for many years and haven't gone into thermal calculations , instead just used PC heatsinks.

It's amazing at what extents people go to in trying to get an 'optimal' cooling setup (candlepowerforums). Got a torch from dealextreme which doesn't seem to have much control circuitry. (SSC-P7) but the damn thing works great, maybe due to good Chinese engineering and much aluminium OR more likely just the "guess it'll work OK" factor.

[pikuorguk]

I've got a Tesco brand 3W Cree LED torch which runs from two C cell batteries. It seems to use the torch body as a heatsink, and after a few hours running it gets "a bit warm". I'm guessing its batteries will run out before the torch heats up enough to damage the LED.

[TheCynicalAtheist]

Might have to check out my local Tesco, i have a LED LENSER at home that has a Cree in it, runs on three AAA batteries and its the brightest mofo torch i have ever had at that size, its hardly bigger than a AA Maglight (although a fair bit chunkier) and its brigher than the 5D Cell maglight.

[kchida1980]

Another awesome vid!! You're awesome! Are those LEDs waterproof? Have you thought of using the calorimetric approach? We know the total power dissipation from V and I. Assume "efficiency" is the ratio of radiant power of visible light over total power. It takes tens of meters of water to absorb visible light. The absorbance of EM radiation outside of the visible range is several orders of magnitude higher. Put some de-ionized water (non-conductive) in clear plastic container.

[kchida1980]

Solder bare copper wire to LED, light it up and put into bottle. Stir around vigorously to ensure that excessive heat is not conducting up copper wire (coiling wire might help to transfer heat to water). Most of visible light should escape system, while everything else should be absorbed by water. Take the temperature delta after predetermined amount of time and plug temp value into Q=m*c*dT. calc avg absorbed power and subtract from known total pwr. Crude, but would it match your estimate?

[kchida1980]

To make thermal mass of LED assembly semi-negligible, you might need to use more water and submerge for longer - e.g., 0.5L bottle of water for several minutes. If the container is too large, the heat lost through the surface of the bottle might be too much for the overall rate of heat pumped into system. If LED efficiency changes with temp, maybe preheat the water to match equilibrium temp of LED in ambient air?

[kolrabi]

In the U.S. some engineers didn't bother to do thermal calculations either and put LEDs into traffic lights. Now they don't generate enough heat anymore to melt any snow blocking them.

Google for 'led traffic light snow' for more information. ;)

[mdmde]

I agree. Try it and see how hot it gets

[jaspers22]

XD!!!! the conclusion is great!