DIY LEDs - The write-up

[widmer]

LOL could it be the 800 that are in my cart? I donno... Actually I have XREs sitting here on my bench which I haven't had the chance to put into a fixture yet, and I paid more for them than those XPGs cost....to think my fixture could have been 1.5x as bright for a bit less money

 

[maddhatter81]

OK need some advice/help
I have a 180Gal 72X18X31 it is eurobrased and has 3 sections at the top
I currently have 3 X 250W HQI. I would like to make 3 seperate fixtures like I have now with the Metal halides. How many LED's do I need on each fixture. I am also asuming because of the depth of my tank I will need Optics. I was thinking of 40 degree on the outside edge and 60 in the middle.

 

[L98-Z]

This is pretty quality for the XPG's, $7.45 each

http://www.lck-led.com/p673/Cree-XP-G-Emitter-w/-14mm-Base---R5-Bin,-345LM-@-1A/product_info.html

LOL! And XRE blues or the 3 diferent whites for $4.50 each

http://www.lck-led.com/p682/Cree-XR-E-Emitter-w/-16mm-Base---Colour-LEDs/product_info.html

Or a 3-up Cree white with built-in driver to run on your old 12v wall wart for $19.50

http://www.lck-led.com/p340/3xCree-Q5-LED-Module-w/-12v-Constant-Current-Driver/product_info.html

...I have 3 $6 white Crees and a $20 buck puck sitting here waiting to be made into a sump light lol..

Wish I had known before I bought my XRE's lol I haven't even made the fixture yet...

Anyone ordered from this place before?

 

[der_wille_zur_macht]

OK need some advice/help
I have a 180Gal 72X18X31 it is eurobrased and has 3 sections at the top
I currently have 3 X 250W HQI. I would like to make 3 seperate fixtures like I have now with the Metal halides. How many LED's do I need on each fixture. I am also asuming because of the depth of my tank I will need Optics. I was thinking of 40 degree on the outside edge and 60 in the middle.

First, the usual disclaimer that there really aren't hard and fast rules yet, but that said, most people are finding that if you stick with the typical LEDs (XR-E Q5 cool whites or equivalent mixed 50:50 with blues or royal blues, driven at 700mA) you'll need 24 - 36 to equal the power of a 250w MH. I'd go for the high end of that range on your tank, since it's so deep.

Your optics sound well chosen but it'll depend a bit on the height the LEDs are mounted above the tank. If you stick around a foot or so that should be OK.

 

[DFason]

I am looking into getting my heatsink's anodized. Does anyone know if this will alter the preformance of the heatsink?

Thanks in advance!

-Dave

 

[Neogenocide]

I would not worry about it. I've seen many heatsinks in many different applications that are anodized. Most being used in a much more demanding way that cooling LED's, the main one that comes to mind is computers.

Scott

 

[der_wille_zur_macht]

I would believe that, but would wonder about the effect on:

1) Bonding and heat transfer under the LED star
2) Heat transfer to the air

Can anyone speak to those two points for anodizing or other aluminum coatings? I'm wondering too, since I don't think I'll use heatsinkusa for my heatsinks, but want to make sure that the product I get is suitable.

 

[Santoki]

One thing to note:
I think past a certain number of LEDs (yet to be determined), intensity can only be adjusted via:
1. current
2. use of a dimmer
3. use of different optics
4. fixture mount hight
Since the number of corresponding light beam overlaps will be finite, adding more LEDs to the fixture will only add to the total coverage area and will have no affect on intensity (unless LEDs were mounted on a concave surface, where beams converge at a central point).
In my PAR tests, an array using 40˚ optics can easily "out shine" your average 400w MH at distance ranges of <2'.
-Robert

 

[dipan]

I am looking into getting my heatsink's anodized. Does anyone know if this will alter the preformance of the heatsink?

Thanks in advance!

-Dave

Well I don't know the technical aspects (of how the thermal transfer of aluminum oxide is different from bare aluminum) but do know that anodized surfaces are common on computer heatsinks, so would be fine for this application also. I'm sure a resident engineer can chime in on the science ...

 

[der_wille_zur_macht]

In my PAR tests, an array using 40˚ optics can easily "out shine" your average 400w MH at distance ranges of <2'.
-Robert

Can you remind us again what the spacing was between your LEDs?

 

[Santoki]

spacing from emitter center to center was 1.5".
-R

 

[Neogenocide]

I would believe that, but would wonder about the effect on:

1) Bonding and heat transfer under the LED star
2) Heat transfer to the air

Can anyone speak to those two points for anodizing or other aluminum coatings? I'm wondering too, since I don't think I'll use heatsinkusa for my heatsinks, but want to make sure that the product I get is suitable.

I'm no professor on this, but I look at it from real world experience. The die on the older Pentium 3 and AMD line of processors didn't incorporate any sort of heat spreader and I've seen many anodized heatsinks. The die was about half the size of the LED's we are using, and I never saw any overheating problems caused by the coating. All of those processors put out much more heat into a much smaller heatsink, albeit the active cooling "fan" was taken much more aggressively. I personally wouldn't have any reservations on doing it.

Scott

 

[Santoki]

FWIW,
I recall someone posting in one of these LED threads that anodizing aluminum results in a more efficient heatsink.

 

[stugray]

You'll get better emissivity to the air with anodized Al., but worse thermal conductivity to the LED.

A good radiator is aluminum that is the texture of sandpaper.
Sandblasting would help get the heat away from the Al.

Best of both worlds would be to anodize the AL everywhere EXCEPT where the Stars attach.


On another note: I just got notification of a new LED from CREE.

It is the MPL-EZW.

700-800 lm @ 150mA 2700-2500K color temp and a forward voltage of 25.0-27.5 Volts :-0

They can do 20 Watts at 250mA!! Viewing angle is 120 deg.

Stu

 

[dipan]

Interesting finds on a web search. Again, I'm no engineer, but ...

Thermal conductivity of aluminum oxide sucks compared to aluminum alone, BUT, the aluminum oxide layer of your typical anodizing process is very thin, so I doubt it has much of an effect. Look at the table on this website and it lists thermal conductivity of aluminum at 250 while aluminum oxide is 30 ...

Conductivity Coefficients

Thermal emmisivity, however, at the fin-air interface in our DIY setups, is a different story. Aluminum oxide has a much higher emmisivity coefficient than bare aluminum. Aluminum oxide is listed at 0.77 while aluminum plate is 0.09 ...

Emissivity Coefficients

I suppose it would be best to anodize just the emmisive (fin part of the heatsink) and not the base (where you mount the LED's), based on the above info.

 

[widmer]

On another note: I just got notification of a new LED from CREE.

It is the MPL-EZW.

700-800 lm @ 150mA 2700-2500K color temp and a forward voltage of 25.0-27.5 Volts :-0

They can do 20 Watts at 250mA!! Viewing angle is 120 deg.

Stu

:inlove: 200 lumens/watt WOW. How much better could it get?? These things must be guaranteed to replace all the lights in our homes at some point...

What is the upper limit in theory anyways?

 

[DFason]

Ok so I was planning on sandblasting the heatsink first and then getting it anodized black or silver.

To make it the most effective I would need the led spots not anodized?

Thanks for all the quick reply's!

-Dave

 

[dipan]

Ok so I was planning on sandblasting the heatsink first and then getting it anodized black or silver.

To make it the most effective I would need the led spots not anodized?

Thanks for all the quick reply's!

-Dave

Yup

 

[stugray]

" 200 lumens/watt WOW. How much better could it get??"

Be careful. At 150mA it is running 700-800 lm and consuming ~12 Watts.

So that would be ~67 lm/W. Not 200.


I cant even find the datasheet on the CREE website.

The copy I have is labeled "preliminary" If anyone wants to see it, PM me an email address.

Also these LEDs have three strings in one device so for a forward voltage drop of 26, there must be something like 6-7 XRE dies in series.

Stu

 

[widmer]

Maybe I'm calculating wrong?

~26V * 0.15 A = ~3.9W

then ~800 lumens / ~3.9 W = ~200 lumens/watt?


Also though, where does a person find a 26 volt power supply? Too bad it's not sub-24V