Designing and Building a LED Fixture

[sunfire99]

Congrats Hoppy. I bet it's a relief to have it working, and the fact that it does what you hoped has got to be rewarding for you.

[Hoppy]

Originally Posted by sunfire99

Congrats Hoppy. I bet it's a relief to have it working, and the fact that it does what you hoped has got to be rewarding for you.

Yes, it is a relief, especially since it has been on for 6 hours now and is still working

I now think I can predict what light will result from using these LEDs, for a range of conditions. Of course, using a different LED would make predicting just as hard as it was this time. I'm trying to work out an "instruction sheet" for designing a fixture for almost any tank using this LED. That should take only a couple or three days.

[691175002]

Nice work, its good that it finally looks like its working. Even better, with that much heatsinking and only 400mA those LEDs should last nearly forever without degradation.

I've got my LED light finally running. 10 3W leds at 750mA and about 6W of assorted other LEDs. The entire fixture gets pretty hot without a fan since I thermal epoxied all of the linear regulators and some transistors to the case as well but I'm seeing pearling on the plants and its only a few hours in.

My LEDs are probably going to go downhill in a year or so but the microcontroller gives some nice sunrise/sunset effects.

[Hoppy]

The light fixture is still working fine, going on when the timer tells it to, and off when it tells it to do that. No burned out LEDs either. The shimmering shadows don't bother me at all. But, fish with reds don't show up as brilliantly as with the GE9325K bulb, leading me to wonder if a few weak red LEDs would be a good addition. (I wont try it due to the excessive complexity that would be needed.)

[691175002]

Originally Posted by Hoppy

The light fixture is still working fine, going on when the timer tells it to, and off when it tells it to do that. No burned out LEDs either. The shimmering shadows don't bother me at all. But, fish with reds don't show up as brilliantly as with the GE9325K bulb, leading me to wonder if a few weak red LEDs would be a good addition. (I wont try it due to the excessive complexity that would be needed.)

I know what you mean, I actually disliked the spectrum of the cool white LEDs so much that I have about a hundred 5mm leds in yellow, red and green to even out the colors.

You can see the reflection of the light in the water:


If I were to do this again I would buy a random selection of cool, neutral and warm LEDs to have as varied a spectrum as possible.

[evilc66]

Glad things are doing well Hoppy. I think I will try my setup with neutral whites seeing as you are not getting reds to pop as much. They have pretty much the same blue output with a bit more red than the cool whites, but less than the warm whites. I have a bunch of both laying around, so I could mix and match if the neutrals don't work out.

[redman88]

hows that instruction sheet coming along

[Hoppy]

Originally Posted by redman88

hows that instruction sheet coming along

Here is a start:
Designing a LED light fixture
to produce 100 micromols per square meter per second PAR.

Using my data plus the data given by Cree for their LEDs, I made these charts:



Notice that as you get closer than about 20 inches from the LEDs, the inverse square relationship between intensity and distance switches to much closer to a linear relationship (intensity = 1/ distance instead of intensity = 1/ distance squared). The pink line is for the first setup of the LEDs when they were running at some much higher current than 400 mA, and quickly burned out the power supply. The black line is for the current 400 mA LED setup, plus some data from my AHSupply fixture.





To use this for designing a fixture to give 100 micromols PAR, first, decide how far from the substrate you want the LEDs to be? Let’s assume you want that to be 22 inches.

How much LED current do you want to use? At 700 mA or less, the LEDs should last for at least a few years. Higher current will reduce the life of the LEDs. Let’s assume you want to use 700 mA current.

My LED array gives about 55 micromols of PAR at about 22 inches, with 700 mA current. (Determined by studying those charts) You will need 100/55 or 1.8 times more light. To get that you need to space the LEDs closer together to get more of them illuminating each square inch of the substrate.

My LEDs are spaced at 3 inches on centers, or .11 LEDs per square inch of the fixture. Increasing that to 1.8 x .11 or .20 LEDs per square inch, will give the needed PAR. With my tank, that would require 4 rows of 11 LEDs instead of 3 rows of 8 LEDs. And the spacing would be 2.25 inches.



A good heat sink, fully capable of handling this array of LEDs can be made from 2 inch width aluminum channel, http://tinyurl.com/nuf5op, or equivalent purchased locally. Four 24 inch lengths would be needed, which from the ebay source, would cost about $50. These could be bolted together, side by side, separated with 1/8” thick spacers, to make about a 8 3/4 inch wide heat sink.

For different foot print tanks you can adjust the number of LEDs to cover the foot print. For a 55 gallon tank, for example, 48” x 12.5”, the heat sink could be the same 8 3/4 inches wide, but 42 inches long, requiring 4 rows of 18 LEDs, 72 total.

You can also use brighter LEDs. For example you could use the Q5 version of that Cree LED and get 22% more light per LED, or use the best Luxeon Star and get about double the light per LED. Using these brighter LEDs lets you use fewer of them, but I suggest not getting them farther apart than 3 inches, to avoid any spotlighting effects.

Still to come: mounting the LEDs and determining how to power them.

[JDowns]

I would think that if you used the Q5's at the same configuration or more per square inch with a dimmer to bring down the mA value to achieve the same par should extend the life of the lamps and reduce heat (heat shouldn't be a concern though).

Great thread Hoppy. Really makes me want to do a LED build, looks like fun.

[Hoppy]

Originally Posted by JDowns

I would think that if you used the Q5's at the same configuration or more per square inch with a dimmer to bring down the mA value to achieve the same par should extend the life of the lamps and reduce heat (heat shouldn't be a concern though).

Great thread Hoppy. Really makes me want to do a LED build, looks like fun.

Thank you, it is a lot of fun. I had about 4 months of enjoyment from spending around $250 on this, and I spent several hours almost every day of that time on it. If I were to have used that money to eat out with the wife, all I would have to show for it would be a bigger belly! (Save this argument, it might win you one some day )

Using Q5 LEDs at a lower current would give the same result, but I think once you get below 700 mA or so you don't gain much more in life. My current of about 400 mA is already way below that. I don't think we need the higher power, higher efficiency LEDs for planted tanks, unless they are very high ones, perhaps 30+ inches. Otherwise we just have too much light, or we get spotlight effects. Reef tanks are another subject, and I'm sure the higher output LEDs are very useful for that.

[Minsc]

Originally Posted by Hoppy

If I were to have used that money to eat out with the wife, all I would have to show for it would be a bigger belly! (Save this argument, it might win you one some day )

I'm not sure about this argument Hoppy. It sounds like, "I could have taken you on 5 nice dates, but instead I have some glowing doodads above my fishtank!"
Might not be the best go to justification

Thanks for the charts, this thread is getting bookmarked for sure!

[Hoppy]

Originally Posted by Minsc

I'm not sure about this argument Hoppy. It sounds like, "I could have taken you on 5 nice dates, but instead I have some glowing doodads above my fishtank!"
Might not be the best go to justification

Thanks for the charts, this thread is getting bookmarked for sure!

Good point! I think I will rewrite my script before trying this again.

[691175002]

A breif scan of the cree reliability document says that if you keep the LED cooler than 80C it will last 50,000 hours before dropping to 70% its original output. That comes to around 14 years of 10 hours a day light.

From what I remember, the thermal resistance of the LEDs are around 8C/W which means that if you run it at a full 2.4W (3.4V*0.7A) it will be at around 20C hotter than its heatsink. In extreme conditions, you can have your heatsink at nearly 60C and have the LEDs last virtually indefinitely even at higher current levels.

LEDs running at 400mA with an ambient temperature of 30C probably have 20 years or more in them of 80%+ output.

When it comes to powering LEDs I have just discovered the many driver ICs manufacturers have available. I used a generic LM317 circuit but if I were to do it again I would use some of the LED specific components.

Most of them boast 90%+ efficiency, a whole slew of protective features for short/open circuits and overheating, built in dimming and more.

[Hoppy]

Powering LED Light Fixtures

LEDs require a DC power supply, and an electronic device to provide a constant current to the LEDs from that power supply. Automobiles use 12 volt DC power for everything, with a socket or sockets available for plugging in 12 VDC appliances, from radios and cell phones to camping equipment like mini-refrigerators. To make those appliances usable on standard 110 VAC circuits there are many AC to DC adapters available, including this one, http://tinyurl.com/mfn7vo , which costs only about $25, and which will provide 5 amps (5000mA) of 12DC current. This is a good source of power for multi LED lights.

Each Cree LED drops 3.5 volts across it when operating at 700 mA current. A 12 volt source will, therefore, power only 3 of those LEDs in series, but strings of 3 LEDs in series can be wired in parallel up to a total of 5000 mA/700 mA = 7 strings in parallel. That means one of those 12 VDC power supplies can power 3 x 7 = 21 LEDs.

The 100 micromol PAR fixture we are designing requires 44 LEDs, but reducing that to 42 will have negligible effect, so we can use two circuits, each with 21 LEDs running at 700 mA for the fixture.

LED drivers are the electronic devices that connect the DC power to the LEDs to run them at a chosen constant current. There are many commercial LED drivers available, but, for 700 mA current, at a cost of roughly $5 per LED, more than doubling the cost of the LEDs for a light fixture. This suggests that a DIY driver is a better option.

A simple, inexpensive and effective LED driver can be made from an IC adjustable voltage regulator, and the most widely available cheap voltage regulator is the LM1084IS-ADJ, sold by Newark Electronics, http://tinyurl.com/lba8b9

This tiny integrated circuit device can be mounted on the heat sink, using an electric isolator package, http://tinyurl.com/lna4zq to allow heat to be conducted to the heat sink, but not electric current. Adding one resistor, and possibly a couple of capacitors will enable this tiny device to act as a constant current controller for about 6 of the LEDs, so 7 of them will be required, but each costs about $3.00, or about $0.50 per LED, one tenth of the cost of a commercial driver. The voltage limiter is connected as shown:


The R1 resistor sets the constant current provided by the voltage regulator, and that current is equal to 1.25 divided by the value of the resistor in ohms. To get 700 mA for each of the two parallel strings, or 1.4 Amps total current would require .89 ohms, which is not a resistor value that is available, but 2-1.8 ohm resistors, http://tinyurl.com/kqe4fn in parallel give .9 ohm, which results in 1.42 Amps or 710 mA current per LED. This simple circuit will work adequately as long as the DC power supply is kept within about 6 inches from the voltage regulator. If the DC power supply is located farther away, a couple of little capacitors are needed to avoid excessive fluctuations in the LED current.

[Hoppy]

Mounting and cooling LEDs

LEDs require a heat sink to keep them from overheating and destroying themselves. For a big light fixture, a heat sink made from aluminum channel extrusions, bolted side by side to give the width needed, works very well, and is much less expensive than finned heat sinks made for that purpose. It is helpful to use about 1/8 inch thick aluminum spacers between the channels to provide space for cooling air to flow around each channel.

After bolting the channels together, use silicon carbide sandpaper wrapped around a wood block to sand the faces flat, and polish them with finer grades of silicon carbide paper. The objective is to get as perfect a contact between the LED “star” mountings and the heat sink as is possible.

The “star” mountings have notches around them to allow holding them in place with #4 size screws, and flat head screws, normally countersunk, are the best type of screw for this purpose, because they cannot accidentally contact the very close by solder pads on the “star” mounts. Drilling and tapping the holes to fit #4 screws is very easy, using a cordless screw driver or drill.

First mark lines along the heat sink where the LEDs will be located. Then mark a spot 3/8” (9.5 mm) on each side of that line on the centerline for each LED. Use a center punch to make a small indentation on each mark. Drill a 3/32 inch diameter hole through the heat sink at each location. (Extreme accuracy is not needed.) Use the cordless screw driver, or cordless drill with speed control, to slowly tap the hole with a #4-40 tap. It works best to turn the tap a few times, back it off to remove the aluminum particles from the tap, then finish tapping the hole. Working with aluminum is much easier than working with steel, so it is very unlikely that you will break the tap.

Use silicon carbide paper, a fine grade, to flatten the areas where the drilling and tapping raised the metal, and polish again with finer grade paper. Clean the heat sink surface with alcohol until no more powdered aluminum rubs off.

To get better heat transfer between the LED “star” mounts and the heat sink, use a thermal compound on both the heat sink surface and the back surface of the LED. There are now many such compounds available, but one of the best for the money is Antec Formula 5, which comes in a big enough tube to mount about 50 or so LEDs. Fry’s Electronics stocks it. Use only a tiny amount on each surface, a dot of the compound, spread a little with the nozzle of the tube. Rotate the LED slightly as you lay it in place to further spread the compound, and attach the LED with 1/2” long #4-40 flat head screws. (The length of the screws isn’t important, but needs to be long enough to handle easily.)

EDIT: It appears that Cree star mounts do not well insulate the conductive core of the circuit board from the mounting notches. This can be a problem, because the flathead screws put a pretty strong force on the corner of the notches, and that may result in electrical contact between the screw, and therefore the heatsink ground, and the LED electric contacts. I recommend not using screws at all, but using an adhesive thermal paste to mount the stars, such as http://www.arcticsilver.com/arctic_a...l_adhesive.htm

The heat sink will slowly get hotter and hotter, with the light fixture on, unless a cooling fan or fans are used to help transfer the heat to the air. Mounting the fans above the heat sink, blowing down on the back of it, works fine. This gentle cooling breeze will also help keep the AC-DC converters and voltage regulators cool, if they are mounted on or above heat sink.

Use short lengths of 22 gage insulated wire to connect the LEDs in series, in groups of 3, for this particular design, and longer lengths to make the parallel connections behind the heat sink. Soldering is easiest using a solder with a small amount of silver in it, and resin core, http://tinyurl.com/nhgvxf plus a small soldering iron, http://tinyurl.com/6dxbsh

You now know just about all that I know about making a relatively cheap LED light fixture. Try it. It's fun!