Build LED Under Counter Lighting That ROCKS!

Background:
I admit it, I am a geek when it comes to LED's and LED lighting. The fluorescent under counter lighting that came with my hose wasn't cutting it. Time for an upgrade! With a background in physics and surgical lighting, I knew I needed a good Color Temperature and Color Rendering Index (CRI). So what are Color Temperature and Color Rendering Index? They are two measures of what light looks like and how it makes things look when it illuminates them. Color Temperature can be read about here, and Color Rendering Index here. Color Temperature is the main “color” of the light based on the temperature of a black body radiator. It is measured in degrees Kelvin (Which is “zero” equals absolute zero and the same per degree increase as Centigrade.) A lower Color Temperature, say around 3700-3500K is considered “warm” light and a hotter one such as 7000K is considered “cool” because it is bluer. Yes, it is backwards from what you would think. There is a lot of debate going on about Color Rendering Index as it is more subjective than people realize. It also depends on Color Temperature, so a CRI of 90 at 3700K will look different than a CRI of 90 at 6700K. Why is this important and why do I care??? Well, when it was just incandescent lighting vs. fluorescent you kinda knew what you were getting into.

For me to achieve success with my kitchen counter lighting project I needed one key thing: wife approval factor. As much as I liked LED's my wife has a different opinion. I first had to prototype a 4 foot section and do a side by side comparison. The LED's won!

There are a lot of power LED's available these days from Nichia, Cree, and Seoul Semiconductor. All are available in warm or cool Color Temperatures. The LED's I used, I lucked into. I got a bunch of 1 watt Nichia emitters that have a color temp of 4400K and a high CRI. Unfortunately for me, they were bare emitters, not mounted on a star PC board.

Another piece of the puzzle I needed to figure out was: How much light do I need? Light gets measured in “Lux” which relates to “Lumens” by means of one lux = one lumen per square meter. Check here for more info: Interestingly, there is a push to have light bulbs listed in lumens instead of watts as that measurement is more relevant to what you see. There are also recommended levels for the amount of light that should be present in living and working spaces. Check that out here  OK, with all this background info I was ready to start... 

Step 1: Design Phase

I really wanted to improve on the current light output and provide a nicer quality light. To answer this I needed to figure out how much light was there to begin with. First I researched the light output of typical fixtures and came across this great info: 
So I looked under my counter and found a random assortment of fluorescent lights. I have a lot of counter space including two five foot stretches of cabinets on a 24 foot counter that includes the sink and stove. The five foot stretches were different, one had a four foot fluorescent and one had a two foot and a one foot. I guess they were “what ever was on the back of the truck” designed... I now had the chance to even things out. I measured that available space and came up with 16.5 feet of total space broken down into sections of 5, 3, 1.5 an 1 foot. Based on the light output I wanted I came up with a need for about 400 lumens per foot. My LED's were 70 lumens per watt driven with 350ma (About 1 watt). At 440ma I determined I could get 90-100 lumens. After modeling this in excel I figured I needed an LED every 2.7 inches. I rounded down to 2.5 inches.

How to mount these was my next thought process. I came up with using 1.5” angle aluminum that would also act as a heat sink. There are many IC's designed to drive LED's and LED strings with boost convertors etc but I went old school. White LED's have a voltage drop across them of about 3.5 volts. If I put 8 of those in series I would need 28 volts. This worked for what I was going to use as a power supply. I had several 24VAC transformers from other unfinished projects that could supply 2 amps and I wanted to be able to use these. They are also readily available. One of these could run 3 strings of 8 LED's with a margin of safety. Check out schematic 1, because the 24 VAC is an RMS value the peak value of the voltage is 1.4 times larger. So after rectification and charging a filter capacitor there is about 35VDC available at no load. Under load of about 1.5 amps we drop to just over 30 volts. I planned on using a simple current regulator based on the LM317. They are well known, robust, can supply 1.5 amps and, are not expensive. Because the LED's I had were not mounted on a PC “star” board I had to source those and figure out how to solder them to the boards. The following is my experience with all of this with a very good outcome. You can use as much of this as you need to build your own LED lighting system. Lets start building!

Materials:
LED's

• http://ledz.com/?p=led.power
• mouser search for LED, power, white
• Digikey search for LED, power, white
• ebay search 1 watt led
• Star PCB for the emitters (you need a bunch of these unless you are lucky to have your LED already on a Star) http://www.dealextreme.com/p/star-connection-board-for-cree-led-emiters-5-pack-4512

Aluminum Angle:

• (Prototype) Lowes
• Final Source Onlinemetals.com

Mounting Hardware:

• 4-40 nylon screws Digikey H544-nd 
• 4-40 nuts Digikey H216-nd
• Transformer, 24VAC Jameco
• Wire 22 gauge, white and black. I used Black for ground and white for +DC
• Wire nuts Lowes
• 5 Dual gang J-box 2 1/2” height to contain the power supply and connect to LED strings Lowes
• Covers for J-boxes (should be right next the the J-boxes)  Lowes 

Step 2: Construction: Soldering the LED's to the Stars...

My first challenge was to mount the LED's on a Star PCB. At first I thought I could solder these by hand and all I needed was to find the LED star mounts. I found some from Digikey (Search LED Star PCB) but the price was a little steep. I then found that Deal Extreme had them for much less. I ordered a few to experiment. That is when I learned that hand soldering these was a no go. After doing some sleuthing around I found that people were converting electric skillets into surface mount soldering stations. I ordered some solder paste from digikey:,  read up on the soldering temperature profile for LED's and drove to Walmart to buy a skillet. I found that by turning the skillet to the lowest setting I could get the right rate of heating, then when I reached the part of the graph where the temperature should peak turn it to high until the solder re-flowed then turn the hot plate off and let it cool. That met the cool down part of the graph. I monitored the whole process with a non contact pyrometer I got on sale at Harbor Freight. My son was fascinated with this and still uses it to see how hot or cold things are. It was a manual process but I could do about 50 LED's at once so no problem!

1. Put solder paste on the + and – tabs of the Star PCB.
2. Put some solder paste on the bottom of the LED.
3. Place the LED on the Star Note: be very careful to get the polarity right! + to + and - to -
4. Put the whole thing on the base of the electric skillet. I tried to keep them evenly distributed from the heating elements.
5. Turn it on to low while monitoring the temperature until you get to 150°C.
6. Turn the skillet temperature to High until you see the solder re-flow somewhere around 220°C 
7. When all the LED's have re-flowed, turn the skillet off and let cool

The whole process should take 4-5 minutes or less. See the Pictures

Step 3: Construction: Mounting the Stars

Now how to mount them... To use the angle aluminum I needed to ensure that the mounting hardware did not short anything out. I found non conductive nylon 4-40 screws from Digikey that were the perfect solution. No worries if they touch any of the solder connections. I started by marking off 2.5 inch increments on the angle aluminum. I was going to wire the LED's in series with eight per string. This meant for my 5 foot sections of light (Each 5 foot section was actually one 3 foot and one 2 foot section) I would have to have two extra on one section to connect to six on another section. To accurately and repeatably drill the holes, I used a jig with my drill press. This was simply two pieces of wood clamped so that all the holes were the same distance from the edge of the angle aluminum. I drilled all the wholes at that distance then set the jig to the correct distance for the second set of holes. Add holes for the current regulator every eight LED's. Finally don't forget to drill mounting holes on the other side of the angle to mount to the counter. Use a larger drill bit to clean up the holes when you are finished. I used 400 grit sand paper to smooth the surface that the LED mount too. This is to ensure good thermal contact. When mounting the LED's use the non conductive 4-40 screws and align the LED stars so the they can be daisy chained easily when wring them. Align the + and – connections so that they all go in the same direction. See the pictures. Mount the LM317 regulators IC's. Use a thermal insulator such as this  when mounting them. DO NOT tighten the screws too much or you can strip the threads. When they were hand tight, I added about a ¼ turn with the screw driver.

Step 4: Construction: Build the Regulators

We are using the LM317 regulator it is actually a voltage regulator but it makes a great current regulator by adding one resistor. There is an internal reference between the output and the adjustment terminal of 1.2 VDC. By using a 2.7 ohm resistor we can set it to supply a maximum current of: I=V/R or 1.2/2.7 = 444 ma. I mounted the resistors directly to the 3 terminal regulator by forming the leads into little half loops that I could solder directly to the ADJ and Vout terminals See picture. NOTE: We actually take the output from the ADJ terminal. Because of the voltage drop across the 2.7 ohm resistor, the LM317 will supply a fixed current to ground out of the ADJ pin, which in our case is 444ma. Also note that the current flows through the resistor too, so it needs to rated for a minimum of 1W to be safe.

Step 5: Construction: Wire the LED's

Construction: Wire the LED's
I used 22 gauge wire and connected the LED's in series in sections of eight. For my two five foot sections I had the situation of a three foot piece and a two foot piece. This left me with having two LED's for one string on one piece of aluminum and the other six on the other. This meant I needed to connect the string after I mounted them. Gotta love soldering under counters and upside down.

First step here is to solder a small blob onto the + and = terminals on the Star PCB. This is a little more difficult than you think and turning the temperature up on your soldering iron helps a lot. Then tin each wire and with a clean tip, put a little solder on the tip and press the wire into the blob until it flows in.

Connect the top of the string of LED's (by that I mean the end with the + on it to the ADJ pin of the the LM-317. See the schematic. I had one section that was only on foot long and I only used six LED's. I used a different transformer for this section, an 18VAC one. This gave me less of a voltage drop across the LM317 and thus less heat. I soldered separate ground wires after estimating how long they would need to be to get to the power supplies but daisy chained the positive leads to all the Vin on the LM317's and left enough extra to go to the power supply. Which brings us to the power supplies...

Step 6: Construction: Power Supplies

The power supply is the classic: Transformer, Full Wave Bridge and a Capacitor. Just like Grandpa Jules used to make.  I had a couple bridges lying around but this one is perfect.  Then for filter caps these work great.  I built the supplies into 2.5 inch tall dual gang J-boxes. These are the kind of metal boxes that you can knock out circular openings and put in strain reliefs to bring wire into the box. I knocked out one of the cutouts and put in a ½” strain relief. This is to bring in the wire I used to connect the AC power. Then mount the transformer to the bottom of the J-box using a 6-32 machine screw and nut. I soldered the capacitor to the bridge rectifier ensuring that the + and – were correct. Then connect the 24VAC wires to the AC input on the bridge. If you are not sure about which wires are which, the 115VAC side will have a higher DC resistance that you can measure with a DVM. My transformer read about 18 ohms on the 115VAC side and about 2 on the 24VAC side. I used heat shrink tubing to cover the AC leads and then soldered about 2 feet of the black and white low voltage wire. Then I wrapped the whole rectifier and capacitor package in electrical tape to insulate it. I did a 8 hour test run to make sure it did not get to hot. For the 115VAC side, I used about 2 feet of SOJ 14-gauge rubber insulated wire from Lowes. Based on my specifics this worked for me. You may need to do this differently depending on your use. Strip back a couple inches of the outer insulation and then using wire nuts connect the line and neutral to the 115VAC side of the transformer. I connected the ground wire to the case of the J-box to be safe. Strip about the same length from the other end and expose enough copper wire to use wire nuts on the AC power coming out of the wall. Tighten the strain relief and you are ready to install!

Step 7: Installation

Open the AC breaker supplying the existing lights and remove the old fluorescent lights (if you have them).

OK, I first mounted the angle aluminum up against the back of my cabinet. This posed two problems. First is the AC power wires came out of the wall where I wanted to mount the lights. Second, after mounting a set I found that butted up agains the wall, there were spots in my kitchen, specifically while seated at my kitchen island, you are in direct line of sight of the LEDs and they were bright! This would not do! I did a little experimentation and determined that moving them forward just a few inches made all the difference. To allow me to permanently mount them this way I bought a 4 foot piece of 3/8” PVC pipe from lowes and cut a bunch of 2-1/2” pieces. Then I mounted them using 3” drywall screws. This gave me space behind the lights to tuck away the AC power leads and get them flush agains the cabinet bottoms.
After mounting the Lights, take the J-Box and mount it on the bottom of the cabinet using a short wood screw or two and the holes already in the bottom of the J-box. Route the AC power around the LED lights and connect it to the existing AC wires Black to Black (Hot) and White to White (Neutral) DO NOT FORGET THE GROUND WIRE! For the DC connections, cut the wires to appropriate length so you can tie wrap them back or tuck them in and solder to the LED strips see the schematic for details. After double checking everything, shut the circuit breaker and turn on the power. You should have a beautiful light! If not it is trouble shooting time. If everything works, turn off the power and carefully put the metal cover on the J-box. Continue for each power supply and light section. I used one power supply for every three sections of 8 LED's this gave me five total for my project. I had one small section of a cabinet that only had 12 inches of space. I used a string of 6 LED's and an 18VAC secondary transformer.  

Step 8: Conclusion and Taking It Further

This project gave me exactly what I wanted. Like most projects, it cost a little more than I anticipated but the results are fantastic. Things I intend to do are make it dimmable via PWM. I plan on experimenting with a MOSFET to control the supply of DC to the LM317. If that works, it will be very simple to control the MOSFET via an AVR micro controller or even an Arduino. The challenge for me will be how to get the PWM signal distributed to each light section. I had AC power everywhere I needed it but the wiring was already there. I also plan on covering each section with a diffuser using sheets from Lowes that are the ones you would put in a ceiling tile based fluorescent light fixture. I also plan on building a few other sets of these for other lighting. One transformer will power 24 LED's and that puts out about as much light as a 40 watt 4 foot fluorescent light. A friend of mine already wants me to build him an aquarium light.

Jules