LEDs have finally begun to surpass compact fluorescents (CFLs) in effeciency (lumens per watt) signaling the perfect time to move beyond crappy LED throwie projects to real, practical LED home lighting. This instructable presents a method to make an LED replacement for the low voltage halogen bulb pendant you currently have over your kitchen countertop or island just like everyone else who has watched trading spaces or extreme makeover home edition in the past 10 years.
Step 1: Gather Your Materials
LEDs have been making big strides in terms of luminous efficiency and cost recently. The current champion in terms of efficiency is the the Philips Luxeon Rebel line of power LEDs. The Rebel is not only tiny at about 3mm by 5mm, it is available in models with >100 lumens per watt. For comparison, typical incandescent bulbs hit the 15 lumens per watt and fluorescents the 75 lumens per watt range. The best source for power LEDs is www.ledsupply.com which sells a variety of LED products for all your LED needs. The item we are interested in here is their "Endor Star" which is a thermally conductive but electrically insulated package with up to 3 Rebel LEDs mounted on it. I used the 3x90 lumens/watt star with 480 lumens of output at 700mA drive. This is pretty much equivalent to a typical 50W halogen bulb that puts out 600 lumens that you might have in your low voltage lighting system. This LED array will cost in the neighborhood of $30 but will last as long as $25 worth of halogen bulbs while using 7W versus 50W for a $50 lifetime operating cost versus $375 operating cost for the halogen.
Besides the LED you will also need a driver circuit. Since the plan is to build an LED replacement for a low voltage lighting bulb, we need a driver that can handle an AC input. Low voltage lighting uses a transformer to reduce the typical 120VAC power to a "low" 12VAC level. www.ledsupply.com conveniently sells an AC capable "buckpuck" that will accept 12VAC input and output a regulated 0-700mA output current for driving strings of LEDs.
So go ahead and shop at LEDsupply or equivalent to get the electronics parts. If you want to be able to dim your LED bulb you will also need a potentiometer. The Endor Star will need to be mounted on a heat sink, such as a section of aluminum angle or channel extrusion. This will require holes to be drilled and tapped as well as screws, heat sink compound, and some wire.
1. Power led such as a 3-up Endor Star with 3 Luxeon Rebel 90 lumen LEDs
2. Constant current LED driver with AC input ability such as BuckPuck 03021-A-E-700
3. 1.25" section of aluminum extrusion for heatsink
4. 4-6 screws fro mounting LED to extrusion, 6-32 works great
5. Tap for threading holes for screws, 6-32 tap works great
6. Heat sink compound for mounting LED to heat sink. Since the LED is isolated, you can use a metallic based heat sink paste like molybdenum anti-sieze
7. Some wire to hook everything up
8. Soldering tools and a clue
Step 2: Build the Heat Sink
The chosen 3 piece Endor Star will dissipate in the neighborhood of 7W of electrical power. Since the Rebel LED is about 15% efficient, 6W of that input power will be dissipated in the form of heat. In addition, the output of the LEDs will decrease with increasing temperature and their lifetime will decease with increased operating temperature. So it is in our best interest to keep them as cool as possible. To do this, we mount the LEDs to a aluminum heat sink.
The pics below show how you should drill a series of holes in your chosen heat sink to match the layout of the LED array detail in the datasheet. Then you will need to tap them to accept mounting screws and test fit screws into them. In my case I used 6-32 screws which I had handy and a tap for. Your mileage may vary.
Prior to mounting your LED you may also want to sand and polish the aluminum surface to improve heat transfer. To do this, start with 400 grit and move up to 600 grit sandpaper before moving to a polishing compound like Mother's aluminum polish. Since these LEDs don't generate that much heat this might be overkill, but it is easy overkill.
Step 3: Mount the LED and Wire the Driver
The next step is to mount your LED to the heat sink. As you can see from the pic, I only used 4 of the six holes for mounting and used the other two to run the power wires from the driver circuit. This has worked fine since all 6 screws are not needed to maintain good thermal contact with the heat sink. Before screwing down your LED, put a dollop of heat sink compound under the LED star. Since the Endor Star is isolated you can use a metal or dielectric based heat sink compound. I used a nickel-moly based anti-seize grease that I had handy since I couldn't find my syringe of heat sink grease. I have not had problems.
Also you will want to solder wires to your LED driver and LED array. In my case I did not want a dimming option so I left those pins open as directed in thedatasheet. Since this project will be part of a low voltage lighting system with its own light switch I left out any switching capability as well.
With the LED screwed down and everything wired as needed, you can glue the driver circuit to the heat sink as well since it is so small. There is no cooling requirement for the driver but this will make mounting the system easier. See pic.
Step 4: Replace That Halogen Bulb
With the LED and driver wired up and mounted to the heat sink, you will want to test the bulb. Since the AC capable BuckPuck driver can handle AC and DC inputs, apply a suitable input voltage and observe the brightness. Since you have 3 LEDs in series you will need at least 12V of input to achieve full brightness at 700mA of output since the LEDs will drop 10.2V and the driver needs 2V of headroom. If you have a lower input the output will just be dimmer. If your LED doesn't light, read over the datasheets and double check your wiring.
At this point it would be a good idea to double check that your light fixture is really low voltage. Get out your multimeter and set it for AC and measure the voltage across the lamp terminals with the power on. You should see in the 12-15 VAC range. If you see 120VAC then you will need a supplemental step down transformer to bring the voltage down to the BuckPuck's maximum rated input voltage of 36VAC. You can get one at Home Depot or Radio Shack. Since your LED lighting system will only use around 10W you can get by with a pretty small transformer.
Now that the LED is working, the next and final step is to swap it in place of your low voltage halogen bulb. I simply removed the halogen bulb and tied the LED assembly in place with string in my halogen fixture. The wattage of the LED is low enough that the string won't melt. I also used a clamp to hold the glass shade temporarily up and out of my way while I worked. See pic. Feed your power input wires into the fixture as needed and flip the switch for a test drive.
The pics below show the LED lamp lighting up my kitchen. The pics were taken without a tripod, showing the lighting power of the LED lamp. To my eye the light generated was about 75% of that generated by the 60W halogen it replaced. After about 10 minutes of on-time the LED assembly was pretty warm (~40C) but not hot to the touch.