Designing and Building a LED Fixture
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Old 03-14-2009, 10:29 PM   #1
Hoppy
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Designing and Building a LED Fixture


This is getting to be an obsession with me! Hours spent on the computer, searching for parts and data, sketching out design layouts, doing calculations, etc. So, I decided to share what I'm doing, hoping that cooler, more experienced heads will speak up where appropriate.

I have a 45 gallon tank, with about a 30 inch by 15 inch footprint, about 20 inches tall, with a PC light fixture mounted about 24 inches above the substrate level. My goal is to replace the PC guts with a LED array that won't break my bank account wide open, and will give me around 50-100 micromols per sq m per sec of PAR at the substrate level.

One approach is to use relatively low output LED's and many of them to get that intensity. Another is to use the fewest number of LED's that will do the job, but using high output units. The first discovery I made is that lower power LED's don't cost 1/10th of what higher power ones cost, so every arrangement I looked at, using many LED's ended up with a higher material cost than using just 3 high output LED's. So, here is the layout I decided to pursue further:


You can get this LED, http://www.luxeonstar.com/endor-rebe...aoe1im80dot2b2 with a lens to concentrate the light, for about $24 each.

Next I figured out the approximate angle of the cone of light I would need to get good coverage of the substrate with these. It turns out to be about 45 degrees. (The LED emits light in about a 160 degree cone, but the intensity drops pretty fast as you move from the centerline - the lens will partially correct that.)

To verify that this would give me enough light I found this chart to use to convert the LED lumens output to a PAR intensity at the substrate:
From this it looks like a good number to use is 70 lux per micromol per sq m per sec, where one lux is one lumen per meter squared. Since I will have most of the substrate getting light from 2 LED's I used 25 PAR units to calculate from: 25 x 70 = 1750 lumens per sq. meter. And, the area each LED will primarily illuminate is about a 20 inch diameter circle, or .3 meters squared. 0.3 x 1750 = 525 lumens. The LED I chose gives about 435 lumens output at 700 milliamps of current, but can be driven to 1000 mA, so this comes close to what I want.
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Old 03-14-2009, 10:42 PM   #2
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I realized that some of the spillover light can be caught by reflection off the inside surface of the glass, and by reflecting from the light fixture reflective surfaces. So, I sketched this to see how that might work:


It looks promising, to me, at least. I then priced the heat sink required to keep the LED's cool. That would be a one foot long piece of 5 3/8 wide heat sink, cut along the centerline to make two 2 5/8 wide pieces, placed end to end. A single 120 mm cooling fan mounted above this should keep the heat sink cooled. That adds another $35 for the heat sink and about $20 for the fan.

To power the LED's, drive them, I found a 40 volt DC unit - it takes about 29 volts for the LED's in series and 700+ mAmps of current. This is the unit I found: http://www.cdiweb.com/PortalProductD...x?ProdId=23479 for about $20.

So, the total cost of parts would be about $150. So far, so good.
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Old 03-14-2009, 10:53 PM   #3
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With my experience with LEDs, you're going to need more than 3 to get an even spread of light at the bottom. If you don't mind sharp shadows and a "spotlight" effect, 3 may be enough.
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Old 03-14-2009, 11:31 PM   #4
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One of the attributes of MH lights is the shimmering effect the surface ripples give you, from the shadowing effect of the small light source. I figure I will get something similar to that effect this way. But, I'm still a bit nervous about whether or not this will be an adequate amount of light, as well as the less than perfect distribution of light.
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Old 03-15-2009, 01:06 AM   #5
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I don't have personal experience, so take my comments for what they are worth...

I agree with Epic, 3 LEDs will lead to very focused areas of light. I understand the ripple thing, but keep in mind you are comparing 150W HQI bulbs to tiny 3W LED emitters.

When you balance between the output and number of LEDs, lots of low power LEDs are no good, but too few spotlight might not be good either. Rather than 3 triple LEDs, I'd do 9 (or 12) single emitters. When you go from 1 Watt to 3 Watt you can see the brightness triple, but going from 3 Watt to 3x3 Watt doesn't do that.

Check out Cree/Seoul Semiconductor LEDs. LED technology is advancing rapidly, and what was the best yesterday could be replaced by less expensive, more efficient things today.

Adding a lens to a LED to bundle the light sounds weird to me. Add a reflector, sure, but LEDs have a directional light output already, and adding a lens IMO just reduces light output further.

If you add distance between the LEDs and the water surface, keep in mind that the inverse square law darkens things rather quickly.

And finally, for $150 you can build yourself the awesomest, brightest T5 setup you can imagine. But of course that isn't as exciting.

(Just subscribing to your thread).
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Old 03-15-2009, 02:27 AM   #6
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Hoppy,

this might be a subtle effect, but in the schematic in your second post you neglected to take refraction on the water surface into account.

Refraction happens at the interface of two mediums with different optical indices (sp?). This should slightly reduce the amount of "spill over"

If you are interested in correcting for refraction just search on wikipedia if you don't already know how to do that.
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Old 03-15-2009, 02:51 AM   #7
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My 4 gallon build has cost me about $300. I am going with 12 3 watt Crees. The buckpucks that are running the LEDs will be able to be dimmed. I am going with 12 so they don't give off a spot light effect and will blend well. I think the rule to good blending is 2'' or less between leds, anything more and they spotlight. I know 12 leds is overkill but i will dim them down to a good level of light.
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Old 03-15-2009, 04:28 AM   #8
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Merk, there is no way I would even waste time on this if I felt it would cost me $300 - my wife would kill me! So, while I agree with what you are saying, I'm still looking for a way to get down to around $150.

Regloh, I agree about the refraction, but most of the light will enter the water close enough to vertical for refraction to make little difference, especially given the overall inaccuracy involved with this.

Wasserpest, I agree about the T5's too, but if I go ahead with this, it will be in part to avoid having to buy new bulbs, and it is an opportunity to study up on a new subject for me. As usual, the more I learn the less I find I know! The problem with using more LED's with lower power is that the cost doesn't drop as fast as the power, so I end up again with a $300 fixture. Tonight I started studying the Cree website, and I'm still at the utterly confused stage. A couple of good resources I found were http://www.highend.com/support/training/lightingfaq.asp and http://www.theledlight.com/technical.html A couple more weeks and I may understand the subject a little better.
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Old 03-15-2009, 09:44 PM   #9
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After studying the CREE LED data, and the reference materials I found, it looks like an array of 12 of the 100 lumen CREE LEDs will do what I want, without lenses, with the fixture lowered back down to the top of the tank. Here is the layout:



I think it takes about 4 overlapping LED light cones to get to the 50 micromol PAR that I want, but that is a very crude guesstimate. Twelve LEDs may be overkill, but at about $3.50 each, the price is right. This whole set up should get close to my $150 goal too.
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Old 03-15-2009, 09:51 PM   #10
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Quote:
Originally Posted by Hoppy View Post
The problem with using more LED's with lower power is that the cost doesn't drop as fast as the power, so I end up again with a $300 fixture.
Nowadays LEDs are available for all sorts of prices.

http://www.dealextreme.com/details.dx/sku.1445

These guys also have the SSC P7 for $19. Might be good in single bulb config over a 3gal cube.

Not saying this would be the best in terms of light color and such, just keep looking around and you find things cheaper and cheaper. Same applies to the other parts of the system, like fans, heatsink and such.

Keep in mind that LEDs don't have a very high efficiency - most of the wattage is converted into heat. As you go up in wattage (like 12W for something like the P7) you will have to deal with lotsa heat in a very small spot.

edit - ninja'd big time. Need to type faster.
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Old 03-15-2009, 10:24 PM   #11
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Quote:
Originally Posted by Wasserpest View Post
Nowadays LEDs are available for all sorts of prices.

http://www.dealextreme.com/details.dx/sku.1445

These guys also have the SSC P7 for $19. Might be good in single bulb config over a 3gal cube.

Not saying this would be the best in terms of light color and such, just keep looking around and you find things cheaper and cheaper. Same applies to the other parts of the system, like fans, heatsink and such.

Keep in mind that LEDs don't have a very high efficiency - most of the wattage is converted into heat. As you go up in wattage (like 12W for something like the P7) you will have to deal with lotsa heat in a very small spot.
Actually, I'm using a SSC P7 over an 8" cube. It works beautifully!
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Old 03-15-2009, 10:25 PM   #12
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Quote:
Originally Posted by Hoppy View Post
After studying the CREE LED data, and the reference materials I found, it looks like an array of 12 of the 100 lumen CREE LEDs will do what I want, without lenses, with the fixture lowered back down to the top of the tank. Here is the layout:



I think it takes about 4 overlapping LED light cones to get to the 50 micromol PAR that I want, but that is a very crude guesstimate. Twelve LEDs may be overkill, but at about $3.50 each, the price is right. This whole set up should get close to my $150 goal too.
There we go! I think that setup is much better. Although lower power LEDs aren't proportionally cheaper, the balance between spread, power, and cost in this new array looks pretty good.
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Old 03-15-2009, 11:21 PM   #13
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I suppose it would be reasonably easy to determine what percentage of the output is in the middle 20 degrees of the cone of light, but I didn't even try that. So, I'm not at all sure if this will give the intensity I want. The rest of the cone also emits light, but at a lesser intensity, and that adds to the intensity where it overlaps too. (Just random thoughts as I have been thinking about this.) If nothing else this is an interesting subject to study.
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Old 03-16-2009, 03:55 AM   #14
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Here is what this last setup would cost me: approximately
12 LED's @$3.50 each----------$42 + $10 shipping (guess)
Heat Sink-------------------------$40 + $10 shipping (guess)
48 VDC Power Supply------------$30 + $10 shipping (guess)
Cooling Fan and 12VDC Power---$20 + $10 shipping (guess)
Miscellaneous----------------------$20 (guess)
---------------------Total---------$192

My current light, running 10 hours a day, is a single 55 watt PC
This LED fixture would use about 33 watts, saving me about 220 watt hours per day, or about 80.3 KW hours per year, at $0.17 per KW hour, saving me $14 per year. My bulb needs replacing about once a year at about $30 each time, with the shipping, for a total annual savings of $44 per year.

My enjoyment from making this is worth??? Priceless? Now, will that sell the wife?
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Old 03-16-2009, 04:07 AM   #15
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Quote:
Originally Posted by Hoppy View Post
My enjoyment from making this is worth??? Priceless? Now, will that sell the wife?
That may depend on whether or not that savings of $44 dollars a year is going into her pocket or another DIY project.

Let us know if it does convince her so I can start breaking it down like that.
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