how many LEd on a 4" X 4.25" heatsink?

[Devaji108]

talked to the guys @ Rapidled and they thought 5 led was to much for a 4X 4.25??
what is the size of a Cree star?

http://i1103.photobucket.com/albums/g480/devaji108/LDsumplight.png

also if I went with there moon driver and ran 4 @ I think was the equivalent of running them at 1W would that work for a refuguim 13l"x16w" 12h" to grow cheto mangroves and other macro? I am thinking not...

once again thanks for all the info & help!

[ghellin]

If you put a fan on it I doubt there would be an issue I am running 24 on a 5.9 x 9 heatsink in my aquapod for a year now.

[Devaji108]

nope no fan it's going in my stand to light my refugium. I think i will have trouble heating my tank as is, so don't need extra stuff cooling it down.

anyone know a "rule of thumb" on how much heat sink per LED you need?

[kcress]

If you have a fan on it you could have all the LEDs touching - that's how many you could have...

[Devaji108]

it was my understanding that 1" spacing per LED was the standard that ppl was using.
did i miss understand that? It happens haha :)

[kcress]

Cross post. There's no rule of thumb. Most designs are crazily conservative and can take more LEDs than used. Horizontal sinks aren't as effective as vertical so fans can greatly leverage them. I'm not sure why you think a fan on a heatsink has any thermal bearing on a tank?

Without a fan you could run five at 350mA no sweat. Probably you could even run 5 @ 700mA on that size sink with no fan.

[Gorgok]

Stars are 20mm by the way. I have 30 on a 7x7" heat sink (1.63 sq. in. per LED) while 5 on yours is 3.6 sq. in. per LED. I also have my driver dump its heat into the same heat sink...

I ran it vertical, without a fan, and the temperature stabilized well below any sort of dangerous heat, but it wasn't something you would like to hold onto for long. A fan on it and it never hit 31 C over a full light cycle.

[kcress]

Hey Gorgok, are you using it vertically?? ATS?

[Gorgok]

Was testing, thats all.

[der_wille_zur_macht]

I've had eight LEDs on a small (maybe 3" cubed, counting the fins) at 700mA, with a tiny wimpy fan, and the heatsink was barely warm. I managed to fit 8 on the heatsink by using two 3-up stars.

I agree with kcress that, in general, most LED designers and reef-oriented vendors WAY over-design their thermal management. I don't see a problem with your proposed design.

[nstiesi]

Two suggestions:

1.) if you are going fan-less, paint, powder coat, or anodize the heat sink. Since you are not relying on convection, radiation will be your most dominant form of heat transfer. Bare aluminum has a low emissivity and thus will radiate poorly. Coating will raise the emissivity substantially.

2.) look in to getting a thermocouple that you can solder on to an led to monitor the case temperature. compare it to the data sheet for the led, and that will take the guesswork out of the excercise, and you will KNOW how your light is performing thermally.

Oh, and I am running that same heatsink, uncoated with a 60mm pc fan on top, and 24W of leds, and it barely gets warm. Just on gut, you should be fine with the 15W worth you have pictured.

[Devaji108]

as always thanks guys. I think i am gonna try it, i feel that i'll be fine. I just might try the small PC fan. I was thinking big fans like the clip on ones, that's way I was thinking it might lower the temp in the DT.

also will look in to the thermocouple that nstiesi is talking about. must admit i have no idea what that is...haha google here i come.

oh also I read some where that painting the heatsink help like nstiesi suggested, what type of paint?

[Devaji108]

another Q? do you pain the whole heatsink or just the fins and side? I thought you "should" clean with alcohol before attaching LEDs.

[der_wille_zur_macht]

A thermocouple is just a probe you can use to measure temperature. The implication is that you need something to connect it to - i.e. a DIY controller or other instrument. Otherwise it's not useful.

Some coatings can help, but honestly, I don't think you'll need it and if you don't have the means right at hand it's probably more effort than it would be worth for you. Try it on it's own. If it's too hot for your comfort zone, add a small PC fan.

[der_wille_zur_macht]

another Q? do you pain the whole heatsink or just the fins and side? I thought you "should" clean with alcohol before attaching LEDs.

If you did chose to apply a finish to the heatsink, you'd want the LED to contact bare aluminum, not a finished surface. So you'd either mask off the surface that the LEDs will be adhered to, or you'd remove the finish after it was applied.

Cleaning the surface with rubbing alcohol is really intended to remove oils from your skin, or other contaminants.

[nstiesi]

Yes, thank you for mentioning that, I had forgotten to. You absolutely want to mount the leds to unpainted surface, so masking will be necessary. Anywhere that an LED is not mounted, can be painted. The more surface area that is finished, the more surface that will radiate efficiently.

If you did chose to apply a finish to the heatsink, you'd want the LED to contact bare aluminum, not a finished surface. So you'd either mask off the surface that the LEDs will be adhered to, or you'd remove the finish after it was applied.

Cleaning the surface with rubbing alcohol is really intended to remove oils from your skin, or other contaminants.

[Devaji108]

ok I ordered my stuff from RapidLED
I got 1 osram Red
2 ww with 65 degree lens
2 CW with " "
the Mean Well LPC-35-700 constant current driver
and the heat sink. I am very excited to start this project. if needed i'll ad the PC fan for cooling.

as far as painting the heat sink goes I have some black indoor/outdoor acrylic paint i used for the back of the DT would that work?

[nstiesi]

A quick google search of "acrylic paint" yielded few results, but for example, DuPont Acrylic Lacquer reported and emissivity around .9. Considering that unfinished aluminum generally sits at .1 or below, it should definitely help.

[Devaji108]

thanks every bit helps. i'll get back to ya with the type of paint.