what's so bad about 5mm LEDs?

[Rob]

This comes up a lot, we get a decent number of people in here who buy a sack of 5mm LEDs on popular auction sites, and then come around looking for help doing interesting things with them. Frequently, we give out advice that boils down to getting some other package because of thermal management considerations.

Then today I ran into this picture that neatly summarizes why this is:
chart

Looking at this in terms of thermal resistance of the package is instructive. The units K/W are equivalent to C/W if you've seen that--C/W indicates the amount of temperature rise to expect for each watt of power dissipation, and measure the ability of the package to dissipate that heat.

So looking at a 5mm LED as 50 times less able to dissipate heat as a typical modern high-brightness package is a real eye-opener. I hadn't seen quantitative data behind that before I stumbled into this slide.

I'm gonna also attach the image to this post as insurance against that source moving, but credit goes to  Dr. Schubert for the original.

Edit: cleaned up the local picture some

[BVnursery]

Valid points !!    Graphically shows why a design needs to incorporate so many aspects.

Heat in electronics is always an issue.  Power consumption is another major issue also.
For my particular application both are critical and the power side led me to work with 5mm type units.  All running in series, 5 strings totaling 264 leds has a power supply load of 100 ma.  Heat is addressed by a swiss cheese type pcb.

[Saint Kosmo]

sorry guys i need to disagree... we have developed a new heat sink style chipset for our products and even the 5m Led’s don’t seem to have any issue... keep your eyes peeled the new units are coming onto the market...

[Rob]

sorry guys i need to disagree... we have developed a new heat sink style chipset for our products and even the 5m Led’s don’t seem to have any issue... keep your eyes peeled the new units are coming onto the market...

Welcome to the boards.

I'm curious about what you're saying--do you have thermal resistivity data you can share on the new units? How much better are they than current generation?

[Saint Kosmo]

data has not been made public... but papers have been know to fly off desk.... 1w aprox 4.5 deg at core..

[Rob]

That would be a significant advance in a 5mm package. I look forward to hearing more about that. Make sure you update us when it's public.

[kinnza]

It seems very interesting. 5mm packages with low thermal resistances would be very useful. If they dont get packed too tighly, allowing good thermal dissipation of the whole module, using 20-50ma 5mm packed led would be a very good option for many applications.

I avoid 5mm packages trying to obtain very low Tj values, but if they improve their thermal resistance, i rethink some designs.

BTW, there was an excelent article in Led Magazine, issue4, december 2005, pag 21, titled "The 5 mm package versus the power LED".

[Saint Kosmo]

forget it we got them 3 weeks ago i was out east to test them not a single one passed the test.... lots of hype but no cohonez...

[minster9]

 :chug:I got 16 18,000 mcd cool whights mounted on a board that's mounted inside a spraypaint can cap. No heatsinks,just hotglued at the corners. Been using array for a couple yrs. for desk lamp. Used dc ps w/resistors on each pair. No heat issues(drilled holes in top of plastic cap),but it's like 11 lumens. Which is fine for it is Nothing wrong w/5mm individual leds if you only want a small area illuminated dimly. I drilled a block of aluminum w/holes equally spaced to put the leds into w/leads looking skyward & threaded the leads through a pc board & soldered  the resistors on the top side. The block of aluminum kept all leds pointed in same direction. Kinda like a die. But for the $ stars give more light per sq. in.  :chug:Upon  the introduction of the Luxeon Rebel,I really began to rethink the heat issue. They say it'll tolerate 150C.  I'm kind of inclined to ask the same question:if they can make 18,000 5mms,why not 20k or 25k mcd? IS 18,000 THE LIMIT?

[justDIY]

how did you come up with your 11 lumen figure?

something to consider, beyond simple heat dissipation, is the lumens per dollar rating of indicator grade (3mm, 5mm, etc) leds is also poor.

for example 7mm superflux white cost $0.85 cents each and offer 3 lumens, where as the luxeon iii offers 80 lumens for roughly $5.  maybe I don't need 80 lumens, but even running the luxeon at a fraction of its rated current yields more efficiency (and thereby more light and less heat) than the 5-6 superflux I could buy for the same amount of $

[BVnursery]

White 18K mcd is not the limit,   I have some 55k mcd whites here Im working with.

[minster9]

 ???BVnursery,where did the 55k mcd units come from?  And justDIY, I used the calculator on this site. 16 5mm leds,18k mcd,15 degree,30ma. How many "stars" can one fit inside a spray paint can cap with it's heat sink? Heck if I can get my hands on some 55k mcd 5mm's & they don't cost a mint,I might assemble an array. Providing,of course,they don't have their own heat issues.

[BVnursery]

55k mcd white and much higher.... ~140,000k mcd,  10mm  are always available on ebay. 
The 55's are 5mm and cost me about 30 cents delivered each.

[justDIY]

the thing with "mcd" is it's very easy to get a huge number, by measuring a single point of peak intensity in the beam output.

lets say you shine your 10 deg 5mm led on the wall and get a typical ring pattern... now you crank the operating current up to the absolute max and move the probe around sniffing around for the brightest patch of light.  just before the junction burns out you record some 'good' numbers.  you'll notice, none of these ultra bright leds detail the measurement procedures and conditions which were used to collect the data.  Rob also points out his mcd to lumen calculator that it's just a best guess and is probably wrong.

minister,

if it were me, I would fit 0 luxeon iii's in a plastic paint cap, but i imagine you could get 5-7 in there - I don't know what you'd do with the 15-20 watts of heat.  a project recently circulated the intertubes about a bike light that comprised three luxeon iii stars in 3/4" copper pipe caps, combined with a finned RC motor heatsinks. 

http://www.racedaynutrition.com/features/bikelight.aspx

[minster9]

 :)Very informative link,justDIY! Thanks! Those guys are quite resourceful (racedaynutrition). I'm steve at MINSTER 9,as in Minster Machinery,Ohio. The 9 is assigned to my punch press @ work. The Minster press  I used to run busted its frame a couple years ago(stamping & cutting .062 stainless w/progressive dies ) killed that 45 ton press built in the 70's. Now I'm runnin' a 60 ton Bliss originally built in 1947. Been overhauled,of coarse;doesn't jump back so much when stamping.cutting slots,& parting .062 x 1 1/4 wide steel in one hit. MINSTER 9,ok. I'm not a minister. Yeah, you do make a good argument against 5mm leds,at least in their use in an array. Thanks for the link,I'm going to have to explore that crowds lighting ideas a little further. Have a good 'un.