Replace Low Voltage Bi-Pin Halogens With LEDs

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.

https://www.instructables.com/id/Practical-LED-Lighting-for-Fun-and-Profit/
https://www.instructables.com/id/High_power_LED_bike_head_light_with_integrated_hea/

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 first step is to drill three holes in the heat sink that match up with three of the six slots on the LED star board. The star board is made of a special thermal sandwich that allows heat the move out of the LED die and into the board and then heat sink with minimum resistance (and thus delta T). The back of the star is metal but not electrically connected to the LEDs and must make solid thermal contact with the heat sink. So take your star and place it on the heat sink and eyeball its location and then make three marks in the notches around the star to mark where you will drill your holes. Then remove the star and drill your holes and then tap them so that you can thread the screws directly into the heat sink. If you don't have the tools or wherewithal to tap the holes, just drill them large enough to let the screws slide through the heat sink and use nuts on the backside to hold the star down. See the pics for clarification.

The key to getting this LED "bulb" to fit in the halogen fixture is to compactly mount the driver within the heat sink. This is accomplished by cutting a scrap of wood that fits between the heatsink legs and that has a drilled out center where we can hot glue the drive circuit securely. You can also drill holes in the heat sink legs to add screws if you don't trust the hot glue. This joint will take any insertion forces when installing or removing the bulb and will also hold the whole assy up against merciless gravity. I used a 3/8 inch drill bit to drill two holes side by side and joined them by wiggling the bit while under power. Crude yet effective. The result was a slot that the drive circuit fit into nicely. With the driver in the scrap wood, I filled the holes with hot glue to hold everything in place. Pay attention to how the wires exit the wood so that don't impede the pins interfacing with the light fixture.

Before you attach the LED to the heatsink it is a good idea to solder the wires from the drive circuit to the star. Once the LED is mounted to the heatsink, the heatsink will do its job and suck the heat away from the star, making soldering difficult. So take a look at the drive circuit and identify the positive and negative wires. You may want to replace the ones that come with the driver with longer more clearly identifiable wire as the Chinese drivers often come with strange color wire and short, poorly soldered leads. You get what you pay for.

Solder the wires to the star, making sure the + and - driver and LED wires match. You should briefly test the LED now with a power supply or battery. You'll need 12VDC or greater, less than 30VDC or so. Check the specs if in doubt. Don't leave the LED lit for too long without it being mounted on the heat sink or you can damage or destroy it. It gets hot fast and you know what they say about a race car in the red. The bi-pin driver is capable of operating regardless of the input polarity so don't waste your time looking for which pin is which.

Satisfied that everything is working, apply some thermal compound to the heat sink under the LED and then apply the LED. Insert and tighten the 3 screws, taking care that they do not short out on any of the wires or pads on the star. Once fully seated, use your multimeter to test for shorts.

The next step is to wedge the wood scrap with driver circuit into the back legs of the heatsink. You may need to file, sand or carve some of the wood away to get it to fit. Be patient and don't cut any of the wires. Try to push it in until the prongs from the bi-pin driver are just barely extending beyond the back legs of the heatsink. You might also want to take some measurements or test fit the assy in your intended light fixture to ensure that it will fit and adjust your height accordingly. With that all squared away get out the glue gun and glue the block in place. Once this cools you can drill some holes in the side to add screws if you desire or just use as is. The heat sink may get warm enough under prolonged use that the glue softens, depending on your LEDs, the current used, and the glue formulation. This heatsink is rated for 10W dissipation and a 5 degree C per Watt temperature rise in still air. So since we are running the LEDs at 600mA, this is about 9W and we can expect the heat sink to get 45 degrees hotter than ambient at steady state without active cooling. That is pretty hot, so some screws might not be a bad idea. Or you can dial back the current by replacing the current set resistor on the driver board. I believe it is R1 on the board and is either 1.5 or 3.0 Ohms depending on which version you buy.

With the driver mounted, you are ready to test. You can do this at your bench or installed in the fixture or choice. If you see a lot of flicker then your fixture may not be low voltage DC but rather low voltage AC. You need to modify the circuit to handle the AC by using a full wave rectifier and some capacitors, not too bad but not ideal. Enjoy, and be safe!