Replace Low Voltage Bi-Pin Halogens With LEDs





Introduction: 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.

Step 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!

Step 2: Drill the Heat Sink

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.

Step 3: Mount Driver in Wood Block

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.

Step 4: Solder the Drive to the LED and Mount to Heatsink

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.

Step 5: Mount Driver Circuit

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.

Step 6: Test

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!



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    Do you need a heatsink that large? I thinking about using these for cabinet lighting and something that large would be hard to hide.

    17 replies

    A good heatsink is really important to maintain as low a junction temperature as possible. Airflow helps a lot but in a passive cooled heatsink size and surface area are the only ways to improve cooling. You could find a larger area but shorter height heatsink that might be easier to hide. Let us know if you find anything that works.

    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.

    Where do you get your warm white LED strips? I'm confused by the enormous price range and am never sure if the high-priced strips that claim to be better, are really only better at marketing.

    Oops. Just realized you linked to the LED strip.

    what about using a old cpu heatsink

    Sure, if you can find one with the right form factor to fit. The circular Zalman heatsinks might be a good but pricey choice. The traditional extruded, square with lots of fins, heatsinks are not suited for integration in residential lighting fixtures. For making a bright light that you hook up on your benchtop to see it light up they work fine, but replacing an incandescent, halogen, or CFL bulb is another story.

    Those heatsinks might not work as a typical bulb replacement because of the shape, but how much surface area do the leds need at this current? I went to the data sheets, but all they said was you should use something and there will be more info here later.

    It will be the heatsink rating that will tell you the performance of the heatsink and its suitability for use in the LED lamp. The number we are interested in is the passive degree C per Watt rating (from here on out I will abbreviate this as C/W), the rise in temperature for a given input power in still air. In general, the bigger the heatsink and the more fins, the lower the C per Watt rating and the better the heatsink. If you add a fan, then you will need CFM airflow numbers and commensurate heatsink numbers at that flow.

    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.

    You might actually be able to use one of those mini 12v blowers instead of a heatsink, or along with a TO-220 chip heatsink stuck to the back of the star.

    You can certainly use a fan, it just increases the size, complexity, and noise. If your fan fails without you noticing, the LED will soon follow.

    Size? You can find very quiet, tiny fans for pretty cheap.

    Your best bet is a fan from a laptop. They are tiny, and generally have integrated heatsinks.

    Sounds ideal if you can fit all the parts in. Prolly not difficult if you dremel away some fins in the middle.

    well i was thinking more along the lines of OLD intel pentium oem sinks, or something found on a amdk6/2 or penitum 3, where the footprint is ~ 2 inches square and ~an inch tall

    something like

    not a huge modern one

    That should be even easier.

    Anything that fits where you need it will be OK as long as the thermal performance is good enough. See my answer to e_lectro above for more discussion on heatsink rating.


    If anything, you need a heatsink larger IMHO. The cooler they are, the better LED's run (and 50,000 hours is only when kept cool- overheated, you'll be lucky to get 1000 hours). Passive heatsinking (no fan) will require a very large heatsink. If you can find a small, quiet fan, then you could use an active heatsink- the fan rigged to the smallest CPU heatsink would be sufficient to keep the LED's running nice and cool. The cool thing about LED's is what they can if integrated properly- i.e, water cooled with the water that's entering the water heater. Now that'd be pretty swish : )

    I agree, I need a bigger heatsink for the 3-up rebel star. My initial reason for purchasing these heatsinks was to use a single Cree (or Rebel) on each sink and to build a linear array fixture for over a sink. That project is still underway. Stay tuned. For the above halogen replacement with 3-up star, the goal is to run it and see how long it lasts. This fixture serves as our "night light" in the house and is on more than any other in the house so I hope to get data from the experiment more quickly while reducing our energy usage by replacing the 50W bulb with LEDs. Water cooled? I can see it now, special 14/2 ROMEX wiring with integrated PEX 1/8" water tubing for cooling. Just don't kink or nick it. Nightmare on my street. Fans are a tough thing to integrate into an existing fixture, and no one wants to hear their lights. In a fixture designed from the ground up to use LEDs it would be much easier to build in fans, but if building from a clean slate then I would design a fixture that leveraged the distributed light capabilities of small LEDs rather than the single point source of bulbs. For example, spreading a large number of small LEDs over a long strip or plate rather than a linear array of 3 bulbs. You could build the structure to serve as a heatsink and cool it passively while using a large number of LEDs spread out to reduce local heat load.

    I'd like to do this for a light that runs GY8.6 75W 110-130V hallogen bulbs. Do you think it's possible or practical? What would I need to do in addition to this protocol?