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
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!
- 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
- (Prototype) Lowes
- Final Source Onlinemetals.com
- 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