This Instructable details how to easily retrofit a low voltage (12V) bi-pin halogen fixture with a high power LED "bulb" that will use less power (<10W), last longer (50,000 hrs), and give approximately the same light output (~300 lumens). This type of fixture is most often used as an accent light or focused task or down light such as display cases, reading lights, desk lamps, and over-island pendants.
This Instructable is similar to some of my others (see links below), but represents the latest efforts to increase ease of integration, low cost, and practical use of ever cheaper high power LEDs. With each iteration, the roadblocks to using LEDs in real residential applications are reduced.
http://www.instructables.com/id/Practical-LED-Lighting-for-Fun-and-Profit/
http://www.instructables.com/id/High_power_LED_bike_head_light_with_integrated_hea/
This Instructable is similar to some of my others (see links below), but represents the latest efforts to increase ease of integration, low cost, and practical use of ever cheaper high power LEDs. With each iteration, the roadblocks to using LEDs in real residential applications are reduced.
http://www.instructables.com/id/Practical-LED-Lighting-for-Fun-and-Profit/
http://www.instructables.com/id/High_power_LED_bike_head_light_with_integrated_hea/
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Signing UpStep 1: Tools and Materials
The materials that go into an LED lamp are the key to its performance, long life, and ultimate successful replacement of a traditional incandescent or halogen bulb. The most important item is the heat sink and has proven to be the hardest part to find in a format that is compatible with the candidate light fixture. Many heatsinks are out there but few have been designed with a circular art glass shade in mind. Recently I came across a part on Digikey that was designed for powerLEDs, had the size and flexibility to integrate into a conventional light fixture, and was cheap enough to consider.
The other key components are obviously the LED itself and the drive circuit. There are a lot of high output LEDs on the market, but for residential lighting, pure output is not the most important factor. The highest efficiency and highest output LEDs are "cool" in that their output is very blue and not appealing for general illumination in your home. This is often indicated by their color rating, given in degrees Kelvin. Cool white is in the 6500K range, with neutral white in the 4500K and warm white in the 3700K range. The problem for LEDs is that the mix of phosphors used to get warmer and thus more appealing light output become less and less efficient. So a top of the line cool LED might output 100 lumens per Watt while the best warm white LEDs would be in the 60 lumens per Watt range. Bummer.
After endless hours searching and purchasing various LED components I used the following parts to build a practical and relatively cheap halogen replacement for my over-sink kitchen pendant. I used a Philips Rebel 3-LED star. Many people prefer the Cree XR-E line of LEDs, and some of the Cree LEDs do have higher specs. However, the size of the Rebel allows 3 of them to be placed in close proximity which is critical for replacing a small bi-pin halogen. I used a driver circuit from DealExtreme, which ships direct from China.
Tools and Materials:
Heat Sink, $3
Bi-Pin Drive Circuit, $2
Rebel 3x LED star, $15
Thermal Compound, $7
A scrap of wood
Hot glue
3 Small screws (e.g. 4-40)
Drill bit and tap to go with screws
Solder and wire and the will to use them
Total cost is about $20 bucks if you have the thermal compound and screws on hand. This is far cheaper than in the past. Woot!
The other key components are obviously the LED itself and the drive circuit. There are a lot of high output LEDs on the market, but for residential lighting, pure output is not the most important factor. The highest efficiency and highest output LEDs are "cool" in that their output is very blue and not appealing for general illumination in your home. This is often indicated by their color rating, given in degrees Kelvin. Cool white is in the 6500K range, with neutral white in the 4500K and warm white in the 3700K range. The problem for LEDs is that the mix of phosphors used to get warmer and thus more appealing light output become less and less efficient. So a top of the line cool LED might output 100 lumens per Watt while the best warm white LEDs would be in the 60 lumens per Watt range. Bummer.
After endless hours searching and purchasing various LED components I used the following parts to build a practical and relatively cheap halogen replacement for my over-sink kitchen pendant. I used a Philips Rebel 3-LED star. Many people prefer the Cree XR-E line of LEDs, and some of the Cree LEDs do have higher specs. However, the size of the Rebel allows 3 of them to be placed in close proximity which is critical for replacing a small bi-pin halogen. I used a driver circuit from DealExtreme, which ships direct from China.
Tools and Materials:
Heat Sink, $3
Bi-Pin Drive Circuit, $2
Rebel 3x LED star, $15
Thermal Compound, $7
A scrap of wood
Hot glue
3 Small screws (e.g. 4-40)
Drill bit and tap to go with screws
Solder and wire and the will to use them
Total cost is about $20 bucks if you have the thermal compound and screws on hand. This is far cheaper than in the past. Woot!
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http://www.dealextreme.com/details.dx/sku.13556
So, to summarize, you can use a different driver (linked above) modified to increase the set current (change out resistor) and make your own pins for insertion into the fixture and you will then be able to make a LED "bulb" like the one in the Instructable that runs on 120VAC and outputs ~300 warm lumens. If you need more light, you can try a Cree MC-E for another 100 lumens or so, you can switch to cool white for another 300 lumens, or you will need to come up with a better heatsink, a beefier driver, and more LEDs or higher power LEDs (see link below).
http://www.ledssuperbright.com/25w-led-c-22/25w-high-power-led-1400-lumens-p-108
http://en.wikipedia.org/wiki/Luminous_efficacy
I have been working on an undercabinet design for a while trying to come up with a good solution. The best that I have come up with is to use an aluminum extrusion (u-shaped) as a mounting, housing and heatsink and to use lower power LED strips rather than discrete high power emitters like the Rebel. Such as the elara strip. It is more expensive but has a built in driver ready to take low voltage DC and can be chained. Not a completely satisfying solution, but the best I have found so far. You only get about 150 lumens per foot in warm white so more of an accent rather than task light.
For example, the heatsink I used is rated at 5 C/W. The LED thermal pad to thermal backplane on the star is rated at 6.3 C/W. The LED junction to thermal pad on the LED is rated at 10 C/W The star to thermal grease to heatsink transition can be guestimated to have a thermal resistance of .05 degree C square inch per Watt and if we assume 1 square inch contact area between the star and the heatsink then 0.05 C/W shows that this is pretty negligible and will be ignored in the rest of the example. So we have a total thermal resistance of 6.3+5=11.3 C/W from the LED thermal pad to the ambient and another 10 C/W from the thermal pad to each LED junction. The manufacturer lists the max LED junction temp as 150C, but you will want to stay well away from this level in order to maintain high light output and long life. The light output will fall as the junction temp increases; see the datasheet for the Rebel for more info. For reference, the light output drops to 90% at LED thermal pad temps of 60C. In this case if we assume that the heatsink is at the star temp and ambient is 25C, then we can assume that there is a 19 degree drop across the star ((60-25)*6.3/(5+6.3))=19 and the heatsink is at about 45C. This gives a rough guess at power flux of 4W since the heatsink is rated at 5C/W and the temp of the sink is 20C above ambient. This is just a back of the envelope guess, as the sink will not be at uniform temp as well as other factors we will gloss over. At this power level, each LED will be outputting 4/3=1.33W. Based on the LED junction to LED thermal pad data this means the LED junctions will be 13C above the thermal pad (10C/W) and at 73C which is well below the max rating.
If we start with a 9W input power, then the heatsink will be 45C above ambient, at 70C (hot!) and the LED thermal pad at 107.5C and each LED running 3W will have a LED junction temp of 137.5C which is getting close to max. The light output will be down to ~80% as well.
Of course this is just a rough estimate but you can see the problems inherent in designing a good LED bulb and why the commercial fixtures have such big heatsinks. Bigger finned heatsinks (e.g. 4 by 4 inches and 2 inches tall) have natural convection ratings that approach 1C/W. If you add a fan, then everything changes and the heatsink rating drops by an order of magnitude.
Good luck, I hope this info helps.
something like http://content.etilize.com/Large/11395738.jpg
not a huge modern one
The driver circuit as shown does not support dimming. The drive IC can support dimming but uses a PWM approach that would not be compatible with the typical triac dimmers found in most residential applications. I am working on a triac dimmable driver circuit based on the LM3445 reference design provided by NSC.
LM3445