Fair warning, this is the longest step! This is basically my thought process on designing the setup. Skip this step to see the materials list and build instructions...
Under cabinet lights can make or break a kitchen. They can add instant and real appeal to a space, but they have to meet certain criteria. They have to be effective task lights. They have to add the right "ambiance". They have to match up with your current lighting scheme, and finally they have to work well and last a long time (due to the fact that installing lights under your cabinets often requires some modifications - it's a pain to have to re-do it or constantly fix things!).
In designing my setup I was able to cross off the typical halogen puck lights almost immediately. They are bright and beautiful, but they have many weaknesses. They are too big, too hot, and as a result they don't last very long (plastic cracks, glass falls out, and bulbs burn out quickly). Probably the worst part about them is the horrible amount of wire needed to hook them up!
For me the obvious choice was LED.
Scouring the internet for project ideas turned up very few truly "DIY" LED options. Most DIY projects were related to installing a commercial product. I checked with local lighting stores and home improvement stores and found solutions that were either woefully inadequate or ridiculously expensive. I found some modular systems that came close to what I was envisioning, but I quickly came to the conclusion that I could build it to look and perform better, for cheaper.
I have some basic LED knowledge from building a light for my reef aquarium. Oddly enough I think that the reefing hobby has given a monumental push to high-power LED lighting in recent years. I've also messed around with some normal 5mm LEDs and such while experimenting with my arduino and other electronic gadgets. I am still by no means an expert...
With LEDs you need to keep a few things in mind. Namely, LED type & placement, power, thermal management, and color.
LED Type & Placement:
LED under cabinet lighting can be divided into 2 groups, strip lights and individual lights. The strip lights typically provide more even light throughout the surface (like a fluorescent bulb), while individual, or "puck" lights offer a more dramatic lighting source with varying intensities that start out really high when you're right under the light fading out as you move further away from the light.
I went through several designs for both and found that typically strip lights use smaller SMD LEDs mounted on a long, thin PCB or flex tape. These are nice, low-profile options, however, I found that they aren't nearly as intense as single lights. If I were to do a strip light application using LEDs I would use 2 rows to get enough light. Using 2 rows increased the cost significantly though.
I ended up settling on high power 3W LEDs, just like what are commonly used in reef lighting, specifically the CREE XT-E LED
. They are very versatile, they put out a lot of light and there are several drivers that are perfect for powering this type of LED, especially if you want to get fancy with dimming (many support 0-10v dimming as well as PWM dimming). The important part is getting the spacing right to avoid shadows and to have the right thermal setup. I experimented quite a bit and decided that the best light was when the LEDs were spaced evenly apart under the cabinets about 12" on center
. More LEDs than that and I would probably be wasting efficiency (because I would end up dimming it most of the time). Less LEDs than that I may be sacrificing some of the practical task lighting.
For power I went with a dimmable constant current driver. The LEDs I used have a 3v forward voltage @ 700mA, to wire them in series you basically just add up the total forward voltage (I used 11 LEDs so 3x11=33v) and make sure the driver you buy supports that voltage at whatever current you want. 700mA is a good amount of current because it has a good efficiency but the LEDs won't get as hot. The LEDs are rated to much higher than that, and while they do get brighter the more current you feed them, they get a lot hotter and the efficiency drops as well. I decided to use a reliable inventronics 40W driver
A nice thing about this driver (and some others too) is that it's scalable. According to the datasheet @ 700mA it outputs a minimum of 18v and a maximum of 54v. This means that if you have 3v LEDs you can safely use a minimum of 6 LEDs and a maximum of 17 LEDs or so (you want a little wiggle room at the top range). By using the spacing I described above you could light anywhere from 6 to 17 linear feet of counter top! If you still need more LEDs than that, don't worry. Just look for a constant current driver that supports the voltage range you require. Just take your LED voltage at the current you want and multiply it by the # of LEDs you want to get the voltage requirement. Meanwell, Inventronics, and Phillips Xitanium are just a few. A LED driver takes your homes 120v power and converts it into DC power for the LEDs.
Thermal management is going to be important in a high power LED array, and while I thought about just using aluminum channel or flat bar from home depot I ended up with a much more elegant (and more effective) solution that didn't cost any more. I spent a lot of time searching for heatsinks and while I found a bunch, they mostly came from China or they were too tall for my application (I only have 3/4" under my cabinets). I ended up deciding to use a really nifty looking circular heatsink that was designed to be used with LEDs. A typical CPU style heatsink wouldn't work in this application because the heatsink has to be up against wood, so this design is perfect to get enough airflow. Best of all, you can get this heatsink in several different heights, and no drilling is required to mount the LED or the heatsink to the underside of the cabinet! It's the Ohmite model SA-LED-113E
Let's not forget about color! This is probably the most important... I would deal with those crappy halogen pucks before I chose a fluorescent light for this exact reason. The color temperature is going to dictate the mood of the lighting as well as how good or bad things look underneath them. Imagine you're preparing some food on the counter and the broccoli looks brown... You're not going to want to eat that. Now imaging looking at broccoli that looks clean and bright green, as if you just harvested it. That's the power of selecting the right color light.
Warm white is the color most often chosen, and the color I desired for my kitchen. The kelvin range for "warm white" is between 2700k and 3500k. Warm white has the highest CRI (color rendering index) and IMO things look most true to life under this color lighting. I decided to stay on the slightly cooler end of the spectrum though, since I don't have many windows. I chose 3250k LEDs which I found correlate quite well to the "soft white" compact fluorescent bulbs that I use in the ceiling lights. On that note you need to try to match the color of your under cabinet lights to the rest of the lights in your kitchen or it will look funny. So you would either need to find the right color LEDs or you'll need to change out the other lights in your kitchen.
So those are essentially the principles I used to design the system. Depending on your space you may need to tweak some things, but I what I put together has worked out really REALLY well in my opinion and for my purposes.
Sorry that was a lot of reading, but it's important to spend the time to do it right and enjoy the results! I'm not going to upload all of my sketches, but the excel diagram is what I ultimately ended up with (1 square is 6 inches):