LED Desk Lamp

AnAppleSnail

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Here's how to make an LED lamp, assuming you've done the research and some planning. If not, feel free to put questions in. The Cree LED Luminaire design guide gets you through LED selection and fixture planning. Once you know the electrical design, it's usually easy to put those in an appropriate housing. Consider heatsinking and electrical safety (and maybe appearance - it's no good building a light you want to hide in the closet!). I retrofitted a fluorescent desk light when the ballast failed. Once I had the heatsink settled, it didn't take long to get the electronics installed and wired. Just don't do what I did and wire the CC driver wrong! That's an expensive habit :(

I wanted about 140 lumens of neutral white light, so I picked a Q4 neutral white XR-E from Cutter. I planned two settings, a low mode for ambient desk light and a high mode for desk working or ambient room light. Some tooling limitations: I have flux-core solder, a 15-watt ratshack soldering pencil, sheet copper, thermal goo, file/hacksaw/tin snips, a brazing torch, and old laptop parts. This is easier with thermal epoxy, flux, a hot plate, and a more powerful soldering tool. With the things I have I can reflow-solder an LED to a copper plate, then use epoxy and thermal goo to attach that to a larger heatsink. With flux I could attach the LED to aluminum. As a side note, the bottom pads on Cree LEDs stop being electrical contacts if you sand off the ends of those pads (bottom corner of the LED)- as noted in evan9162's thread. This would have been easier if I'd know that a few months ago! Here's Evan's picture:
xre_q4-2.jpg


I like the LM317 circuit from ratshack because it's common and I can easily change the constant current it provides. I use a circuit similar to this one, but note that LED power comes from Vadj, not Vout. With a switch changing the control resistor, I can make high/low modes. Once I attach a wall-wart power supply for the LM317 and its voltage drop, all the electronics are settled.

Wall wires to switch and wall wart AC prongs.
Wall wart(-) to LED(-), Wall wart (+) to LM317(Vin).
LM317(Vadj) to LED(+)
LM317(Vout) to resistors, then to LED(+)
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That's the electronics, all of them.

Next, the LED+heatsink assembly. I don't have flux, a hot plate, or thermal epoxy, so I reflow-soldered an LED to copper, then used thermal compound and JBweld to attach that to a laptop heatsink plate. Using an IR thermometer, the LED itself can get up to about 110 degrees with a room temperature of 77 degrees (43C, so I'm losing about 6% of my rated lumens through heat. There is a small sheet copper spacer between the LED and the plate to keep the contacts from short-circuiting - I can't measure its temperature. The copper plate the LED is soldered to is 86 degrees and the laptop plate is 81 degrees. 'High' is about .8 amps, low is much lower.

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Here are pictures of the LED lamp, and its assembly. In use, it's plenty bright enough, though not as bright as the 27-watt fluorescent it replaced.

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saabluster

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Hey I recognize that copper! You didn't happen to get that off of ebay did you? I bought some a while back that had that patina created from the protective mask that had been left on it for many many years.

Nice work BTW. I would say however that measuring an LED using an IR thermometer is a very imprecise way to measure. I wouldn't draw too many conclusions from it. ;)
 

AnAppleSnail

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Aug 21, 2009
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South Hill, VA
Hey I recognize that copper! You didn't happen to get that off of ebay did you? I bought some a while back that had that patina created from the protective mask that had been left on it for many many years.

Nice work BTW. I would say however that measuring an LED using an IR thermometer is a very imprecise way to measure. I wouldn't draw too many conclusions from it. ;)

I don't know where I got the copper. The piece I had was a long strip of that width, but it was a closed loop pinched into an airfoil shape. Perhaps it was once a pipe, and it's quite thick copper. I know that an IR thermometer isn't accurate for measuring an LED, I just hope it's overestimating. The LED dome barely gets warm to the touch, and I've always been a fan of running electronics cool. The IR thermometer is not the best for heatsink temperature either, really. But my only other thermometer is an alcohol one for cooking - my projects involve lots of "If I only had xxxx piece of kit." It was a fun one, and I thought I'd mention the weird-but-easyness of using LM317 as a constant-current driver.
 
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