K2 LED Design Help

[Straegen]

I hope I am not in the wrong forum for this, but this seems to be the place I need to go to get some help with this setup. This is an LED array for use over an aquarium. I wanted feedback on what pitfalls I may not be seeing.

First up here is the parts list (and I currently own all this save the optics and slugs which are "in the mail" so I am not talking out my rear here):

9 High Current K2 LEDs - White
9 High Current K2 LEDs - Blue
18 5w 1ohm resistors
15 grams of Arctic Alumina Epoxy
18 15degree optics
40 Copper Heatsink slugs
400w 12v PC powersupply already wired to turn on without MB
100% Silicon Caulk

I also have several (40+) small CPU heatsinks. I have additional K2s as a backup for burning out during testing and I have already smoked one due to poor heatsinking hence the Alumina Epoxy.

The design is simple. Epoxy the LED to the heatsink, solder them together with the resistors, silicon the optic on and screw them into a piece of wood. Each LED/heatsink will have a couple inches of wire between it and the next LED so I can yank burned LEDs out easily.

The circuit is pretty simple with 3 LEDs wired with a 1ohm resistor and then wire those in parallel.

So what am I missing here because I know it is something?

Ohh... my first test which smoked a poorly heatsinked LED or at least I think that was the problem after about 5-10 minutes of running. The other two are fine.

http://www.cameroncole.com/cgi-bin/...quarium_Light_1
Thanks in advance.

 

[brighter]

It depends of the type of k2 you are using. There are k2's with 3.6v Vf (...R00)and those with 3,85 Vf (...U00). Which are your's?

edit: I see now that those are high current k2's

Than, maybe you can try to lower the current with 1.5ohm resistor in series instead of 1ohm mentioned. It will be about 20%-30% less bright but will keep your leds from overheating and will longer lifetime. I would wire it like that:

+12v (+led- +led- +led- 1.5ohm) -12v

You don't have to resistor each led (I assumed that because I see you have 18leds and 18resistors).

Here is the link for series led resistor calculator:
http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

Measure your supply voltage with multimeter while it's working at load.

 

[yellow]

Except for no longer liking (never did) the K2 which main purpose seemed to be to produce heat...

...this will not be a portable device, but an aquarium light with plug in wall power? Is there a need for exactly these wavelenths?
Better use a real bulb (or similar "normal" light) that emits a couple of wavelengths? LED light sucks imho, when unlimited auxilary power is available.

2nd: those heatsinks, while looking fine, will get much too hot with K2, especially when airflow is restricted by the mounting board.
Heat management under these circumstances needs way bigger heatsinks. If the Leds were electrically neutral, it should be sufficient if the mounting board were from aluminium, but ...
As these seem to be long runtime lights (aquarium), the leds will "quickly" dim within one year at least.

Looks like a way too expensive showpiece light (18! K2s), that wont give a good result on the long run.
Sorry if this is not Your opinion

 

[wakibaki]

Er, this is not exactly straightforward. Correct me if I'm wrong, the main purpose is to encourage the growth of aquatic plant life? Plus illuminate the aquarium for viewers.

Are you sure the mix of frequencies you propose is good? Perhaps RGB is a better way to go than white LEDs.

You should arrange your LEDs in strings of 3 (probably) (with this PSU) and employ a constant current driver with each string. You can do this by arranging an LM317 regulator as a current source (google LM317 current source) but this is wasteful, may require you to go to 2 LEDs to get enough voltage overhead, and will require its own heat management. Switching current sources are more efficient and you should really look for some that are suitable.

You almost certainly don't need the optics, I'd be looking at a very simple curved sheet ali reflector/aquarium cover. Very little of the light goes sideways so this 'reflector' could be flat (curved would be more self-supporting). If you keep the LEDs in the centre or on the centreline of the tank pointing down it's unlikely that much of the LEDs natural 'beam' will be aimed outside the tank.

You need to arrange your heatsinking so that heat bleeds into the reflector. The slugs are probably too small for this and it will require some engineering as you need to maintain electrical isolation between each heatsink. While some thermal epoxies are electrically insulating, it might be unwise to depend on this in practice. Paint the reflector black on the outside and leave it natural the other.

If the heatsinking is well done and the current does not exceed the devices spec. then there is every likelihood that that the LEDs will outlast conventional aquarium lighting (this, after all, declines in output too), thus recovering some of the higher initial cost. Your design should not anticipate the replacement of blown LEDs. Just watch the temperature (test with a finger on the heatsink [at your own risk]) and add cooling if necessary.

 

[Straegen]

First thanks for the quick replies.

The design is 3 LEDs and a 1ohm resistor sorry for the misprint. I will try a 1.5ohm to bring down the heat as suggested. Other than that, do I have any other problems with the basic design?

If not the K2, what LED would you suggest? I looked at the Seoul LED, but can't get my hands on them easily.

The optics are important to spread out the light and prevent "pin-holing" on corals which simply means too much light at specific area on the coral. This causes them to change color or bleach out in that area. I was trying to difuse the light a bit.

The white/blue combination is for PAR which is simply a measurement of usable photosynthetic light. This is concentrated in the blue spectrum. The white is there to aid in growth and the overall look. RGB was excluded because red, yellow and green lights do virtually nothing in an aquarium and it is important to use as many lights as possible to focus PAR.

The reason LEDs are used (or at least want to be used) in the aquarium is simple. They are cheap in the long run. A single 250w Metal Halide lamp costs about $300 with another $75 for the bulb that must be replaced annually. They also cost about $25 bucks a month to run from the juice being drawn. Then the heat transfered to the aquarium usually involves a chiller which is another $15-$20 a month + $500 for the unit itself. The only thing cheaper and nearly as bright are flourescent T5 bulbs which are cheaper than MH, but barely. They do cost less to run than MH over time, but don't approach that of an LED.

There is only one product on the market currently and it uses 25 3w LEDs in a bank with 30 degree optics. Each bank consists of 12 whites and 13 blues. A two bank unit costs over $2000 so you can see why I have tried a DIY project as currently I only have $300 in my project.

Thanks again.