help deciding what led to use

Hey all help a rookie out.

Recently I have been bitten by the led bug. My latest project is for an underwater syatem which will incorporate multiple leds. I will be making several different color options and that leads to my first issue.

When starting this I didnt realize that blue leds produce far less lumens than say a white led. I didnt even think about that when starting so my first issue is trying to make a fixture which produces blue light at roughly 700 lumens. Yeah I could just smack a bunch of 3 or 5 w blue luxeon stars in a fixture however the amount needed would require a fixture which is too big. So my first question is do you all know or recommend a specific led for producing blue light.

My next question is about the new rebel leds. I was reviewing the specs on their web site and it appears that they have an led which produces blue light at 48 lm with 700 ma. As im sure you know the rebels are extremly small so that would solve my space issue im just not sure of the their durability. Has anyone had any experience with installing these in a circuit. Do they have any issues with heat. I am toying with the idea of etching some flexible circut boards which i could use to power the rebels but it will be a first for me.

any thoughts

thanks

I cant recommend any specific blue led. I have only used a 3W luxeon.

When it comes to reliability it is the same for all leds. The hotter you run the led, the shorter is its lifetime.
There is not much difference in driving a blue rebel 700mA than a white one. (as it has similar die under a phosphor layer)
At 700mA you get 2-2,5W of heat from one led. So you really need to keep them well heatsinked. Use for example an aluminium plate that is in good contact whith the outer housing and you will be fine.
Only problem is the lenses for the rebels. The small 10mm carclo lenses are quite wide: 18 degrees FWHM.
If you need more narrow beam you can look for the Khatod 10 degrees which are sold as 4 lenses in a round 25mm package. (don't remmember part number, sorry)

Why do you want to have a blue light? It will not work well other than in blue or green water, the same as the cool white works well in. But maybe you want it for the looks, not the lumens.

gillestugan said:

I cant recommend any specific blue led. I have only used a 3W luxeon.

When it comes to reliability it is the same for all leds. The hotter you run the led, the shorter is its lifetime.
There is not much difference in driving a blue rebel 700mA than a white one. (as it has similar die under a phosphor layer)
At 700mA you get 2-2,5W of heat from one led. So you really need to keep them well heatsinked. Use for example an aluminium plate that is in good contact whith the outer housing and you will be fine.
Only problem is the lenses for the rebels. The small 10mm carclo lenses are quite wide: 18 degrees FWHM.
If you need more narrow beam you can look for the Khatod 10 degrees which are sold as 4 lenses in a round 25mm package. (don't remmember part number, sorry)

Why do you want to have a blue light? It will not work well other than in blue or green water, the same as the cool white works well in. But maybe you want it for the looks, not the lumens.

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Yes these lights are only for astetics or I would be using a white led. I plan on offering a blue red green and white version. Im really trying to make all of them realitively the same as far as light outpt goes hence the reason im worried about the lumens. it would suck to buy a blue version only to find out its alot less bright than say a green version.

Im not really worried about the heat. I have done exactly what you said and am using an aluminum heatsink to dissipate all the heat.

I havent yet experimented with lenses. It may very well be possible for me to not need a focusing lens however im not ruling it out.

Now that im been researching a little I like the rebels due to the size however that in itself may be a problem. They sure are small. I have also been looking into the cree leds and have read good things about them. . The issue I see is that they are only rated at 30 lms at 350 ma. Now what I wonder is since the cree leds have a max rating of 1000ms could I run them continuously at 750ms and if so what light output would I get. im guessing it would be somewhere around 40- 50 lumens but im not sure...


anyone have ideas...

You can usually find a optical output/current graph in the data sheet of the led. The efficiency gets lower the higher the current is. I'd guess you would get about 55 lm at 750mA.
You can easily drive them at maximum current when you have them well heatsinked. Especially if they are water cooled

I first thought you meant some kind of diving light, thats why I worried about the lenses as you usually want as narrow beam as possible in a diving light.

gillestugan said:

You can usually find a optical output/current graph in the data sheet of the led. The efficiency gets lower the higher the current is. I'd guess you would get about 55 lm at 750mA.
You can easily drive them at maximum current when you have them well heatsinked. Especially if they are water cooled

I first thought you meant some kind of diving light, thats why I worried about the lenses as you usually want as narrow beam as possible in a diving light.

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well 55 lm at 750 ma would be pretty good. Thats one of the highest I have heard about.....

Any preference between the cree and rebel emitters

also have you all had any success using the electrically conductive adheasive insted of solder.

and can someone tell me where I can find such an adheasive as well as thermally conductive adheasive.


thanks

My next question is about the new rebel leds. I was reviewing the specs on their web site and it appears that they have an led which produces blue light at 48 lm with 700 ma.

Click to expand...

I asked this very question about a month ago and didn't get any useable responses regarding first hand experience regarding the rebels.

The 48Lm spec seems impressive on paper, but the small size of the rebel and relative lack of a lot of people having used them eventually turned me away.

Since Cree isn't really known for making mindblowing blue LEDs, unlike their obvious dominance with more 'white flavored LEDs', I fell back on 3-watt blue Pro Lights from Best Hong Kong to save money:

http://www.besthongkong.com/product_info.php?manufacturers_id=&products_id=195

They say 30Lm, but the Pro Lights were brighter than any 3-watt blue LED I've used, and actually ended up being overkill for the eventual project being obnoxiously bright. They are now mounted on the ceiling of a night club with narrow reflectors and custom heads I built, and with some smoke in the air they project as darn near solid beams at less than 350ma. I should have some pictures of this in the Fixed lighting forum soon.

If you end up not being able to decide between blue LEDs, I can at least give a solid reference to the blue Pro Lights - if you need them on stars at least.

I use thermal epoxy for everything, including mounting K2 emitters to heatsinks which I was able to drive to the point of being too hot to touch without failure.

I prefer cree, maily because of availability and bad experiences with the luxeon 1W and 3W. But the rebels are modern and probably just as good.
The XR-E is actually possible to solder from above. The cree XP-E and the luxeon rebel has to be sodered from under.

I doubt electrically conductive adhesive is a good idea for high currents. Probably too high resistance, but I'm just guessing.

Dealextreme has some termal glue I'm very happy with. It's cheap and pretty strong. (bent a star when trying to pry it loose.) sku.4579

The also sell solder paste if you want to try to reflow solder the leds to stars. sku.7952 Search for reflow in the forum for instructions.

Can you recommend a couple sites where I can buy the cree and ebel emitters. I was onto ledsupply however you can only buy them on star boards. I will most likely make my own heatsink and circuitry.

thanks

Depends on where you live.. You may look at retailers like digikey.com, Conrad.de, Farnell.com, elfa.com.