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Alioth's Journal: Luxeon Rebel...colour me impressed!

Journal by Alioth

This evening, things got off to a bit of an inauspicious start after I made the PCB for the power supply for my new Luxeon LEDs, which I'm experimenting with to see whether they make for good lighting for my Dad's old boat.

The driver IC is a tiny little SOT23-5 sized device, i.e. it's roughly the size of a grain of rice. They really aren't that hard to solder - only 5 pins, and I was using solder paste and hot air.

But you try and find the one you dropped on the floor. I got one out of the packaging, and then it suddenly...disappeared. And has not been seen since.

Fortunately, I had bought 10 of these ICs. So the next one gets assembled with the rest of the components. The circuit is pretty simple - all the hard work has been done by the engineers who designed the chip, all I had to add was an inductor (it's a buck converter), a 0.1 ohm current sense resistor, a couple of capacitors for decoupling, and some connectors for power etc. And lo, there was light.

Well, that's until I decided to do a test with a lead acid battery powering it all, and I must have momentarily touched the flying leads I'm using to power the breakout board possibly in reverse bias. I don't actually know what happened, well, except the chip promptly failed and started working as a resistor, with the LED no longer being particularly bright, and the PCB getting pretty warm. No magic smoke was observed, but my oscilloscope very quickly confirmed that there was a distinct lack of switch mode power supply like activity going on...

So I desoldered that chip, and soldered on the third. Well, after dropping the thing twice while trying to turn it over the right way, and nearly losing another one. (So much for antistatic precautions when it falls on the carpet).

But it worked when I got the board powered up again, off the lead acid battery. The tiny LED (it's the size of a 3mm indicator LED!) is now adequately lighting my room, all by itself. It doesn't fill the room with light quite like the room lights do (a pair of 60 watt incandescents), but this tiny LED is putting out an impressive amount of light. If I put it in an Anglepoise lamp enclosure it'd make a great work light for use when soldering boards.

My Dad's boat is quite small, so I think half a dozen of them over the cooking area will provide excellent lighting. What's more the PCB that I soldered the LED to (it's in a leadless package) provides an adequate heatsink (I didn't even etch the LED's PCB, I just cut out pads with a dremel cutting wheel, and soldered the LED package to it - it has a thermal pad under the LED, plus the anode and cathode). The PCB even right next to the LED is only warm, not hot.

LEDs have come along in just a year, the previous white LEDs I tried with this level of brightness got much, much hotter. This is a "cool white" LED (the most efficient colour that Lumileds produce in white). I was concerned that the light might have been a bit too blueish, like my bike headlight, but it's not - it really is proper white, rather than the very pale violet of other "white" LEDs that I have tried.

Next will be to make some smaller PCBs custom shaped to fit the boat roof liner. The data sheet provides a pattern for making a PCB that acts as a better heatsink than the one I made, so hopefully I can get the size of each module pretty small (which will keep the cost down, since I intend to get the PCBs made up by someone like Olimex).

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Luxeon Rebel...colour me impressed!

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