Led panel using cree xre stars?

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krap101

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So... I'm planning on making a panel for my 10 gallon (possibly eventually my 29 gallon) and I'm a little confused on a few things. I've been reading around, and it seems that I need a power supply of some sort, a driver, the leds, and a heatsink.

So far, I found 2 laptop chargers that I'm planning on using for my power supply. One is a 19.5v 4.62A and the other is a 20v 3.5A. The biggest problem right now is the driver. I hear alot of people use buckpucks and xitanium drivers, but I was hoping to diy one. I was looking around, and this one seemed reasonable, but I need some help interpreting the diagram. (haven't taken any electrical courses yet)

http://www.instructables.com/id/E7QH8NG8I1EWOF32HJ/

Right now I'm planning on using 6-10 cree xre's (is this overkill?) and I'm not sure which ones to choose. I've heard good things about the r2's and the q5's, but nothing is final yet.
 
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You can drive 5 LED's in series at say 0.75A using a single series resistor. The voltage required for 5 LED's is about 18V (5x3.6Vf). With the 19.5V power supply, a 2 Ohm resistor will limit the current to 0.75A. The resistor needs a power rating of 1.125W (0.75x0.75*2) or better. Since your PSU can handle up to 4.62A, you can even have multiple strings of LED's - up to 6 strings and a total of 30 LED's.

With other power supply voltages (and LED Vf), you need to adjust the value of the resistor according.
 
I came to the same conclusion RMTEO did, and that it might just be easier to use a 2 ohm resistor (big one) and put it in series with five LEDs.

Use cool whites, and perhaps throw a couple blue LEDs in there (adds some color) to light a fish tank.
 
I was looking through radioshack and can't seem to find the right resistor. Will I have to buy this online?

Also, is 2 strings of 5 cree xree q5's overkill? I'm planning on growing some plants.
 
I'm no gardener, but I do believe you will need more than just white LEDs. Throw some red and blue high power LEDs in there too.
 
but I do believe you will need more than just white LEDs
Click to expand...

I'm not the expert on this, but my understanding is that 'white' LEDs are basically blue LEDs, but with additional phosphors in them to cover the additional spectrum. Correct?

If this is the case, a white LED would work just as good as blue....unless you want just a blue tank.

On the aethestic side a neutral white LED will look very yellow in a fresh water tank, so this is one case where cool white LEDs are a better bet.

I do know that the value of red spectra 'has not' been globally established as being needed by all plants. Some terrestial plants such as shade plants or some types of evergreen do like red light. It's value in aquariums has not been shown because of the rapid absoprtion of red/far red in water.
 
Yeah, from what I've read, these new white leds work just like a fluorescent bulb. The led puts off uv light, it his a phosphor coating, and it puts off white light. The spec sheet says the range is like 5000k to 10000k, which is basically the spectrum of white tinged yellow to almost blueish white, and this is basically what those compact fluorescent lights use. I think I might add a few reds and blues, but I'm afraid it would make combined strings too complicated. Maybe this would be a case where a 12v would be better? I think that using a 3.7v and a 3.4ish or whatever for the blues and 2.5ish for the reds would be screwy with the voltages, not to mention the amps.
 
If you use LED's of the same type, it will not be complicated. For example, take a look at this table from the datasheet:

a_65.jpg


Let's say you want to drive the LED's at 0.7A with a 20V power supply. You want to use 3 warm-whites and 2 reds in a series connected string.

Total Vf = (3x3.4V) + (2x3.6V) = 17.4V

Resistance = V/I = (20-17.4)/0.7 = 3.71 Ohms
Resistor Power = IxIxR = 0.7x0.7x3.71 = 1.82W min.

You can add more strings in parallel using the above method. The total Vf of each string must be LESS than the voltage of your power supply. Also the total current of all the strings added together must be less than what your PSU can supply.
 
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Okay, I think I was getting confused with leds in parallel (rather than in a series) where you go from a low voltage to a high voltage, it would rather go back to the source rather than to the next led (something about resistance)..

So, as for the resistor (I'm going to try all whites to begin with) I was having trouble finding the right resistor on the radioshack website, so how would I go about stringing resistors together? Would 2 1 ohm resistors strungin a series, be a resistance of 2 ohms? and for wattage.. or would two .75w resistors strung in parallel be 1.5w? or does it not work like that? Would I be able to find a 2 ohm 1.5w resistor at radioshack?
 
You can combine resistors in many ways to get the values you need. However, rather than confuse you further, here is a bunch of 6.5W resistor for less than $1 each that will do what you want.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=CWD-3.0CT-ND

Print out the catalog PDF page to see what values are available. Note that they have a $25 minimum order so you may want to get other stuff at the same time.
 
The tolerance is not critical in this application. Any one of those resistors should be fine. A 2 Ohm, 2W resistor will work with your 19.5V supply and 5 LED's (of the appropriate Vf) in series.
 
Here is my 55gal fishtank LED light I made.. uses 9 warm white 350 ma leds running off of a 1000 ma puck I got from ledsupply.com, which is powered by a 18.5v 4.9amp laptop power supply. Lights been running for 2 years now with no problems...

fishtankledlight1.jpg

The knob on the side controls the brightness of the LEDs...
fishtankledlighton.jpg

The plants are fake and the algae on them is from daylight that hits the tank from the side.
 
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