Designing and Building a LED Fixture

[Hoppy]

Originally Posted by PRESTON4479

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.

The trouble is, each time I get the price nailed down, I learn something more, and the price goes up substantially. Now, I'm back to $200+ again, after learning that there is no way I can handle LEDs unless they are preassembled onto "star" mountings, which costs as much as the bare LEDs cost. I don' t recommend selling the wife on a $150 project only to find you are spending $250 on it - I'm not quite ready to sleep out on the deck.

[Dalban]

Originally Posted by Hoppy

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.

I'm curious, why do you replace your bulb so often? Typical T5HO bulbs are rated at 20,000-30,000 hours with end-of-life lumen maintenance rated at about 95%.

I'm always keen to find factual, quantitative evidence that backs up the notion that T5 bulbs are significantly dimmer one year into their life. So far I have trouble convincing myself that this isn't a relic of the days when T12 were in vogue.

[Hoppy]

Originally Posted by Dalban

I'm curious, why do you replace your bulb so often? Typical T5HO bulbs are rated at 20,000-30,000 hours with end-of-life lumen maintenance rated at about 95%.

I'm always keen to find factual, quantitative evidence that backs up the notion that T5 bulbs are significantly dimmer one year into their life. So far I have trouble convincing myself that this isn't a relic of the days when T12 were in vogue.

I use GE9325K PC bulbs, not T5. They probably don't require replacing once a year, but I have seen some data that supports a 18 month to 24 month change cycle. I figured on annual replacements to fudge the savings up a bit.

[Dalban]

Originally Posted by Hoppy

I figured on annual replacements to fudge the savings up a bit.

Ahh. I guess that also explains the high electricity rate you're using (i.e., Alameda PG&E rates are about 25% less the rate you've used). And, for others who are following along, California rates are almost twice as much as many other states.

Fudging the numbers is a great way to fool wives, etc -- but doesn't help advance the hobby. Perhaps we should calculate an honest budget for people who are seriously considering the cost benefit equation.

[Hoppy]

Sacramento uses a multiple tier electricity rate, so, since we are always in the second tier, I used that rate - that number isn't fudged at all. And, I actually have replaced my bulbs in the PC fixture at about one year, because I was disappointed with the bulbs I was using, so I'm not sure that my once a year number was fudged that much. Some folks do it more often. Then there is the intangible "cost benefit" from using less electricity and helping to stop global warming. Plus, as I said, this is a hobby, something to do for fun, and much of the fun I get from the hobby is making new things for my setup, trying them out, and trying to quantify the performance of them. I don't do this as a business.

[evilc66]

Originally Posted by Hoppy

The trouble is, each time I get the price nailed down, I learn something more, and the price goes up substantially. Now, I'm back to $200+ again, after learning that there is no way I can handle LEDs unless they are preassembled onto "star" mountings, which costs as much as the bare LEDs cost. I don' t recommend selling the wife on a $150 project only to find you are spending $250 on it - I'm not quite ready to sleep out on the deck.

If you buy the LEDs from ETG or LED Supply, you can get them pre mounted usually for only a dollar more (thats what ETG charges).

[Hoppy]

I just discovered http://www.plantedtank.net/forums/di...d-fixture.html today. This is so close to what I wanted to attempt that I am going to wait for the results of malaybiswas's project before attempting my own.

[evilc66]

Didn't get the test done for you this weekend Hoppy. I was fighting with a bum drive on my file server that consumed all my time in between plumbing my reef tank. I'll try and get the numbers for you this week.

[Hoppy]

Originally Posted by evilc66

Didn't get the test done for you this weekend Hoppy. I was fighting with a bum drive on my file server that consumed all my time in between plumbing my reef tank. I'll try and get the numbers for you this week.

I am very slowly learning more about LEDs and how to calculate light "intensity" from them. The more I learn the more I think this really is "rocket science". Some data will be very, very welcome!

[Hoppy]

Will this power supply, http://tinyurl.com/cpk3a8 work to drive two parallel strings of 12 each LEDs with 3.7 volt, 700mA rating? Would this require load resistors in each string to limit the current?

If it isn't obvious I am looking at a 24 LED array, using these LEDs, http://www.dealextreme.com/details.dx/sku.2395

[Hoppy]

I have the price of this project down to about $150 now, so I am starting to order parts. I plan to use 24 Cree LEDs, ordered here: http://www.dealextreme.com/details.dx/sku.2395 in an array of 3 rows of 8 each, on a grid of about 24 inches by about 6 to 9 inches. Here is how I arrived at this:

I spent a lot of time on http://www.nano-reef.com/forums/inde...c=188085&st=20 and a couple of others. This particular thread gave me some good data to use to calculate what I would need for my tank and to get the PAR I want. So, I plotted the data on log log paper so I could extrapolate it:


This data is from 11 Cree LED's in an array that is 42.5 square inches, and it would give 240 PAR micromols at the 17 inch distance I will use. I will be illuminating about 450 square inches, so, assuming that the illuminated area that the data is from is the same size as the array, to be conservative, I will need 450/42.5 times that many LEDs, or 116, to get the same PAR. But, I only want about 50 PAR, so I need 50/240 times 116, or 24 LEDs similar to those used in the reference. I chose somewhat less efficient Q2 Cree LEDs for cost savings, and because of the conservative calculation I did above.

More to follow.

[Hoppy]

The layout of the 24 LEDs will be:


This should give me more than 50 PAR in the center of the tank, and pretty close to it around the edges. The yellow circles are the coverage from the center 10 degree half angle of the light - no lenses used. But, the next 10 degrees should theoretically be even higher intensity, due to the focusing effect of the air-water interface.

To power these LEDs I needed to decide how to connect them. If they are all in series, the total voltage required would be about 96 volts, too much for cheap DC converters. So, I decided to split them into two strings of 12 in series, with the strings in parallel. That requires a 48 volt power source with up to 1.4 amps of current. I lucked out because ebay now has just such a DC converter on sale, http://tinyurl.com/cpk3a8 I offered $12 for one, and the offer was accepted. With tax and shipping, that comes to only $23.82, much less than if I were to use "buckpucks", where I would still need a DC supply. But, this also means I need to use current limiting resistors. Here is how I decided what to use:


The two sets of resistors plus the toggle switch lets me toggle between about 700 and 350 mA of current. The numerous resistors are used first to handle the power that has to be dissipated, and because of ebay again! I found these: http://cgi.ebay.com/ws/eBayISAPI.dll...m=180334106900 which are 3 watt 1% 15 ohm resistors, perfect for this use. I plan to put the resistors and switch in a small metal box.

[Hoppy]

Next is the heat sink. Tomorrow I will visit a salvage metals store that is aimed at DIY folks like me. They have a big stock of aluminum extrusions at very low prices. I hope to find 2 or 3 inch channel extrusions, so I can put 8 LEDs on each of three of the channels, then join them, side by side, with aluminum washers separating them for air flow all around each channel. These will give me at least 24 square inches of surface per LED, with enough thickness to also store and easily conduct the heat to the air. At worst, I can buy the channels on ebay, again, for about $30, much less than the cost of a finned heatsink extrusion.


I have spent about $28 so far, not counting the heatsink or LEDs.

[Hoppy]

This morning I visited Blue Collar Supply, with its vast collection of aluminum extrusions. I found they have 8 foot aluminum 3" x 1" x 1/8" thick channel extrusions, for $24.85 each, or $26.78 with tax included. So, now I have spent $55 total. The fun begins!

[Hoppy]

One problem my current limiting scheme was going to give me was if one LED failed open circuit, the increased current through the remaining circuit would possibly desroy those LEDs. So, I redesigned it a bit:



This will let me toggle between 350 mA and 750 mA, and if one LED fails open circuit, the current through the other leg will be limited to less than 1000mA, which shouldn't burn them out.

Today I got the power supply, and it looks very good. And, I got the "heat sink" assembled, but my wife has my camera so I can't get a pic of it yet. I still need to flatten the mounting surfaces a bit and polish it some.

While working on it I decided that "heat sink" isn't the right name for this. It is a heat exchanger, not a heat sink. A heat sink stores heat, but this is supposed to transfer the heat to the air flow going by it. Thinking of it that way makes the design make a lot more sense to me. Spent another $2.24 on nuts and bolts today. Cost is now $30.