Heat Sink Question

[beolson25]

So ... I previously posted about my layout of LEDs and got some great responses. The problem is I got my heat sink and the area I thought would fit my LEDs isn't quite exactly what I thought it was. So this is not a question about layout and I wanted to open a new thread since this specifically deals with LED placement on the specific heat sink ... and not with what colors should go where ... Hopefully nobody thinks this is a repeat...


So ... Let me state my situation. I have a standard 75 Gallon tank with a center brace. Lets not consider color of LEDs ... But ... I have 50 CREE XT-E LEDs on 20mm Star Boards to place on the heat sink that I am attaching a link at the bottom of this post. My problem is that when looking at the description I had about 12" of width to play with for mounting the LEDs but ... when I got the Heat Sink and looked a bit more at the pictures I see that there is actually a ledge on the heat sink. So ... there is a section that is about 7.1" wide and a section that is about 1.5-2" wide ... So ... there is plenty of space to put LEDs on both the 7" wide section along with the 1.5" section. I will refer to them as the big section and small section going forward since my measurements are not exact anyway. The small section is "lower" into the heat sink ... meaning that, when complete, if I mount LEDs there they will be a small amount further away from the water than the rest. See the picture on the ebay listing to get a general idea. I wish I would have looked closer but I still don't think I would have changed my mind about purchasing it.

So ... The question is ... How should I mount my LEDs? I see the below options:
1) LEDs on both the Large and Small areas
2) LEDs on only the Large area

I'm curious if people think that the difference in height of the LEDs would make any significant difference in the lighting... along with does anyone think that 50 LEDs is too much for the area of ~ 7" x 46" ... Please note that I will only be using about 40" of that 46" to avoid using the very outer inch and the area over the center brace ... so, effectively it is more like 50 LEDs over a 7" x 40" area.

I'd like to avoid fans but if they are 100% needed I can add some to the build.


Any thoughts would be appreciated ... Worst case I will just use the large area but I wanted opinions from experienced folks.




Heat Sink Link:
http://www.ebay.com/itm/251790494165?_trksid=p2060778.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

 

[rwb500]

put the LED's wherever you want. As long as you dont put the LED's right up against the ledge, it wont block any light that was going to make it into the tank. the distance doesnt matter. I would use the ledge to better distribute heat, but it is probably not necessary. I dont think you would have heat problems if you decided to skip using the ledge. Heat will still be conducted to the fins on that portion of the heatsink.

 

[rwb500]

next time you want to spend that much on a heatsink, check out heatsinkusa.

 

[ghellin]

You will be fine. I have 130 LEDs on three 8.5" x 20" heatsinks. If you average it out I think I am at about 3.49 square inches of heat sink per LED. I think yours is about twice that. I don't have any issues with heat.

 

[ghellin]

 

[beolson25]

next time you want to spend that much on a heatsink, check out heatsinkusa.

At HeatSinkUSA the cost would have been about $120 for the 7" wide version. That doesn't include the splash shield, end caps, side moldings, or feet(which I probably won't use) ... So ... in the end I still think I got a better deal for about $105.

Although ... Honestly I would be interested in hearing why the Heatsinkusa heatsinks would be better. Considering I'm on a limited budget(Wife induced). I'm not sure it would have the clean look and how much adding a splash shield would cost extra.

The 12" model would have been FAR MORE expensive. I really thought I got a GREAT deal on this heatsink.

 

[beolson25]

You will be fine. I have 130 LEDs on three 8.5" x 20" heatsinks. If you average it out I think I am at about 3.49 square inches of heat sink per LED. I think yours is about twice that. I don't have any issues with heat.

Ghellin,
Do you use any fans on those fixtures? What Amperage are you running on them? I'll be running 1.05 Amps so it might be hotter than the 700mA strings most people use.

Any thoughts?

 

[mcgyvr]

The description of the heatsink states "capable of handling 50W of LEDs per foot without fans"..
Unless you have means to accurately measure LED junction temps you should simply include a fan and not exceed the sellers recommendations.

 

[ghellin]

My heatsink gets a little warm at the LEDs if I crank them up. But I run my blues about 800mA, my whites about 350-500mA and UV about 500mA. Once my canopy is done I will add a fan but I am comfortable with it as is.

 

[rwb500]

Although ... Honestly I would be interested in hearing why the Heatsinkusa heatsinks would be better. Considering I'm on a limited budget(Wife induced). I'm not sure it would have the clean look and how much adding a splash shield would cost extra.

you could have got the 6" low profile heatsink 4 feet long for $66 + shipping. it would have been plenty.

 

[beolson25]

you could have got the 6" low profile heatsink 4 feet long for $66 + shipping. it would have been plenty.

Ok ... and for that price I probably could have made a splash guard and stayed under budget. I guess I can consider that next time but I still think the one I bought is easier(Less extra work and planning involved) and will have a better finished look once completed. I think I could have gone cheaper and had a fine finished product but I'm still happy with my purchase. Thanks for your input though.

For everyone else ... I guess I'll wait until I get my LEDs in the mail and decide how I think they fit in the large area vs both large and small ... I'll do my best to post some pics ... but I don't expect everything to be done until the middle of February.

Thanks for all of your comments.