Introduction: Low Profile Linear LED Array

Got a dark closet with no room for a light? This Instructable presents one way to solve the problem: a low profile array of high intensity LEDs mounted in aluminum channel. Simple, effective, cheaper and better than what you can buy out in the market currently. I wanted a 60 inch span outputting a total of around 1000 lumens. There were a few options out there, but none fit the bill. Also, none allowed me to design and build my own light and live to tell the tale.

I looked into buying Elara strips but since they are $20 apiece I would need to spend $100 to span my 5 foot wide closet and still need to buy and mount to a heatsink and get a power supply. Additionally, the warm white Elara strips are about 130 lumens per foot which would give only 650 lumens for the whole array and at pretty poor efficiency (<50 lumens per Watt before power supply losses). The more powerful Luxstrip II strips output in the neighborhood of 500 lumens per foot at 350mA which is too much for my application and at $45 per foot would be even more painful on the wallet. A heatsink and power supply are also required.

After careful consideration, I determined that purchasing single high efficiency Cree XPG warm white LEDs on star boards and mounting them to a heatsink and providing a constant current driver would be more cost effective and yield superior results.

Step 1: Materials


This build is pretty basic. The light fixture is the heat sink, and the rest is just LEDs, mounting, driver, and wiring. I'm still working on a diffuser but after having the light installed for a while I kind of like the look of the bare LEDs. Total cost was about $100. A note on the driver, this is a small and cheap driver but it is not UL listed, is not dimmable, and generates some interference in my experience. So if you are trying to listen to a far off radio station, turn this light off.

Materials:
8 feet of 3/4" wide by 1/8" thick aluminum channel from McMaster Carr ($13.70)
11 Indus star boards with warm white Cree XPG LED ($6.05 x 11 = $66.55)
16W 320mA constant current LED driver ($7.60)
22 4-40 screws
Suitable wire
Thermal compound
Heat shrink tubing

Tools:
Drill
Drill bit and tap for 4-40 threads
Soldering iron
Solder
Screwdriver
Hot glue gun
Aviation snips
File
Multimeter

Step 2: Cut LEDs to Fit Channel


This is a very simple light. Rather than try to retrofit LEDs into an existing fixture, I built a fixture optimized for the LEDs and application. The application was a closet with a sloped ceiling with no room for traditional lighting, with the added complication of needing to clear bifold doors. A tight fit indeed.

The solution was to simply mount a length of aluminum channel directly to the top of the closet door jamb at the back of the closet as close to the sloped ceiling as practical.

The tight fit required a narrow channel, such that the LED star boards need to be trimmed to fit inside the channel. The channel internal width is 1/2" so a large portion of the boards needs to be cut away with aviation snips and filed to smooth and fit. The star boards are aluminum with the traces and pads laminated on top and protected by a solder mask coating. When cutting the boards and filing you will inevitably encounter the traces and may even remove parts of them. This is OK, but be sure to check that the traces and pads are not electrically shorted to the board before installing. Also test fit the cut down boards in the aluminum channel. The result should look like the pictures.

Step 3: Prepare Channel


Now that the LEDs are cut to fit inside the channel, prepare the channel by drilling and tapping a pair of holes for each LED. I planned to mount 11 LEDs spaced 5" apart and 5" from each end of the 60" channel. The holes for the 4-40 screws need to be about 20mm apart for each star. Use a #43 indexed drill bit (0.089" or 2.26mm) and tap the holes with a hand tap. Once tapped, you'll want to deburr the entry and exit of the holes by hand with a larger drill bit to ease threading. I also drilled and counter sunk larger holes to accept #10 wood screws for mounting the channel to the door jamb. See the picture for an example of the holes needed.

Step 4: Solder Wires to Stars and Screw


The LEDs are wired in series, so you need to run the first wire to + on the first LED and then from - on the first LED to the + on the second LED and so on down the line until the last - pad on the last LED will have a long wire all the way back to the beginning for connecting to the driver. You should have a short wire to the + wire on the LED driver from the + pad on the first LED and a long wire from the last LED - pad to the - wire on the driver. Soldering to the stars can be a challenge since they are pretty good heat sinks. You are going to need to solder the LEDs prior to screwing them to the aluminum channel. I found that soldering to one LED, and then applying thermal compound to the back of that LED and screwing it to the heat sink and then starting to solder to the next LED was a good way to keep the wiring to a low level snarl. You should try to keep the wires as short as possible and still bridge the 5" spacing. I used all white wire to minimize aesthetic impact. If you do the same, take care to keep track of which wire is + and which is -. As you go along soldering and screwing the LEDs into the heat sink, use a multimeter to check for shorts to the heat sink. If you find that any of the + or - LED pads or wires are shorted to the heat sink, track down the problem and correct it. You don't want to damage your LEDs or the driver and you don't want the heat sink to be electrically "hot". In normal use the voltage out of the driver is less than 36VDC and not too dangerous but if the driver fails there is no telling if it can fail with the mains shorted to the DC output, which would bring the heatsink to 120VAC if it is carelessly connected to one of the output lines.

When all the LEDs are soldered and mounted and you are sure there are no shorts, temporarily hook up the driver to test the whole circuit. If you see light, good. If you release magic smoke, bad. If everything is OK, use the hot glue gun to adhere the wires to the heat sink to tidy up the install as best as possible. Keep the wires clear of the LED lens and the holes for mounting the channel to the door jamb. Also, be careful when using the glue gun on the wires as depending on the glue gun and the wire insulation, you can melt through the insulation and inadvertently introduce a potential short path by exposing the wires.

Step 5: Wire Up the Driver


Depending on your installation details wiring up the driver may be different than my route. I had planned on this type of light fixture for a while so had installed switched outlets inside the closets for the lights to plug into. Thus, I took a surplus 2-prong plug and cord and soldered and heat shrunk it to the Hot and Neutral input lines on the LED driver. I then simply hot glued the plug to the body of the LED driver to create a sort of "wall wart". The + and - output lines were soldered and heat shrunk to the appropriate wires coming off the LED array and heatsink. See the picture. When replacing a traditional fixture you could mount the driver in the vacated junction box, hardwire it to the supply lines inside, and devise a cover of some sort, potentially similar to that used for track lighting. Any and all of these approaches may or may not adhere to the local electrical standards in your jurisdiction. Be aware, and be safe.

Step 6: Mount and Test It Out


With the driver wired, take your drill and pre-drill holes where desired for mounting to the door jamb. Screw the array in place, plug it in, and turn it on. I found the light output to be just about perfect and see no need for dimming.  My integrating sphere is not quite large enough for the array, but I'd estimate around 1000 lumens of output based on the XPG datasheet and my calibrated retina.  Looks great and Kill-a-watt measurements showed 11W consumption with a miserable 0.64 power factor.  Investing in a better designed driver may be a good idea.  Thank you generous hosts!

Comments

author
Hesperolinon (author)2011-12-25

Great project and great instructions/post! Thanks for all of the detail and clarity. I have two questions for you. I want to adapt this project for use as a livingroom light bar, placed along a mantlepiece, in which the lights shine upwards. Around each LED I'll position a "shade" to capture the light and channel it upwards. Anyhow...was wondering a) how many of these LED's you can wire together and power off of the plug-in adapter/converter you used in this project, and b) whether I could substitute in a higher-power LED? Thanks for your help!

author
jmengel (author)Hesperolinon2011-12-29

a) The linked LED driver from DX is 16W max drive. With a set current of 320mA, and Vf of approximately 3V at 320mA the power per LED is essentially 1W. That means you could wire up 16 LEDs at 1W each.

b) The linked LEDs are Cree XPG, which can run at 1500mA if heat sinked appropriately. Using the driver shown at the 320mA drive current there are no options outside of Cree XML that will give "higher power" at the shown drive current. The XML will just output about 15-20% more light at the given 320mA but cost about double. The reason I went with the low current drive is two-fold. First the U-channel is not an adequate heat sink for high power. You could probably drive the XPGs at 500mA at the spacing shown without damaging the LEDs but I wouldn't go much higher. The second reason is that driving the LEDs at a higher current will increase the light output by each LED. I wanted a linear, more dispersed light output where each LED was not running so bright that it would blind you. With 11 XPG LEDs in series like this I could put out 3000 lumens if I wanted to, but with each LED running at >3W the LEDs would be blinding. If in your application you will not be able to directly see the LEDs then increasing the current may make sense. Just be careful about heat and retina damage. You'll need a different driver though.   This is such a driver, which will drive the LEDs at 670mA.  You'll need a minimum of 10 LEDs in series for this one to work, which will be roughly 30W and more than 2500 lumens.  Non-dimmable too.

Good luck!

author
Hesperolinon (author)jmengel2012-01-01

Thank you, sir, for that response to my questions and explanation of options. I'm an electrical neophyte and so I wanted to ask for clarification before I move forward with my project. The LEDs will be positioned at 3-foot increments along the mantlepiece, shining upward and ultimately positioned behind a shade of some sort, which should prevent retinal pain. I'll be mindful of heat dissipation but from your response I gather that the U-channel will be a sufficient heat sink if the LEDs are spaced this far apart. So to be clear, I can wire up 10 warm white Cree XPG LEDs using the 670mA 36W driver you pointed me to and the instructions in your post, and the array will produce 2500 lumens total. Again, based on your response it sounds like this could be "blinding," but a single 60W incandescent bulb should put out about 800 lumens, so this doesn't seem like a ton of light. Can you clarify? Thanks again so much for your help!

author
jmengel (author)Hesperolinon2012-01-01

The 2500 lumens in an estimate. It will not be blinding if it is spread out over the strip. Each LED will be putting out roughly 250 lumens in this estimate, which due to the small size of the emitter, can be blinding.

If you use the thick U-channel that I suggest rather than the thin channel available at Home Depot the lights should be OK heat wise. 3-foot increments, and 10 lights in a row? That is 27 feet. One heck of a mantlepiece. I would say 12 inch spacing would be OK as well. If the lights are pointed up rather than mounted below the mantelpiece, then even less would be OK since the heat will be removed more efficiently.

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