When the christmas decorations went on sale this past year I picked up a pack of 4 LED low voltage lights on closeout from the local SAMS. They were a good metal housing from westinghouse, but had very weak little white LED’s in them. The kind you might find in a child’s flashlight with the very cool white, almost blue, color and very dim, resembling moonlight. They certainly wouldn’t have shown up from inside the house if you were looking out a window. They also had the ability to switch to a couple of red or green LED’s for holiday mode. Very cute, but not terribly useful.

Converting some low voltage outdoor lighting to high power LED’s is something I’ve wanted to play with for a long time. There lots of choices for high power LED’s now, and lots of color temperature choices as well. You’re no longer limited to the very cool white color that is the most common. The very large LED’s however require proper mounting and heat sinking or they drop in brightness very quickly and will eventually become dim or even burn out. Dealing with the thermal issues of the larger 1, 3 and 5 watt LED’s out there was too much to hope for a good solution I could stuff inside these cases, so I decided instead to go with an array of individual LED’s. I chose an 8mm size of warm white. They really weren’t that good at lumen maintenance either if you just let them run at their rated power of 120ma, so I had to have more of them running at lower power.

The other problem working with LED’s is that you really need a current regulated power supply. It is quite practical to run a couple of LED’s from a power source with just a current limiting resister. But as you increase power or build larger arrays this becomes problematic. As an LED heats up it’s internal resistance reduces, allowing more current to flow, causing it to heat up even more, allowing more current to flow. So you’re initial calculations will be wrong once the LED gets hot and you’ll find that it burns brighter, but eventually burns out due to over heating. If you’re running the LED above it’s rated power it’s even possible to get a thermal run away situation that leads to the energetic disassembly of the LED, or at least causes it to melt it’s bond wires and go dark. So a power supply that regulates current as opposed to just voltage is a necessity if you want good output and long life. 2 or 3 years ago when I started looking into doing this the cheapest switching regulators you could buy for this purpose were in excess of $20 a piece! So I just kept playing with things and waited and not you can buy really good ones in the $4 to $5 range.

These little boards are actually meant for a flashlight, much smaller than necessary for a lamp and extremely difficult to solder the connections to the pads. I ruined the first one I tried to hook up, but got the hang of it. They are only $3.10 a piece if you order directly from Hong Kong, I ordered these from Kai’s Domain be prepared to wait if you’re going to order direct though. A month to 6 weeks wait for the stuff to show up at your door is not uncommon. They also appear to have redesigned the board since I ordered so I dont have any experience with the new layout, it might be better or worse. Since they expect DC input, and most outdoor transformers are AC you do have to also add a full wave bridge rectifier or just run from a DC power supply. Even if you run from DC it’s a good idea to put in the rectifier. I did this as that way the connection polarity doesn’t matter. YOu dont have to try to keep positive and negative straight as you pull the wire through the yard. I used this to power an array of 8 8mm warm white LED’s in parallel. Here’s the 8LED’s in the breadboard:

So 750ma into 8 led’s makes it just under 94ma into each LED. Slightly under driven but still very bright and at that level the temperature never went above about 110 degrees F which is no problem for the LED’s at all. If the new driver board really pushes as much as 1000ma I may have to make a larger array to keep it running long term for future ones I build. It is important when running multiple LED’s in parallel to check them as they are not exactly matched for resistance and if one LED in the group has a significantly lower resistance it can cause more current to flow through that LED overheating it and causing it to fail which would then cause the other LED’s to sink more power without it and then they overheat and fail. You can help that by running the LEd’s in series at a higher voltage, or using a small resister on each LED in parallel. In my case I have actually 2 parallel strings of 4 LED’s in series. All the LED’s here were within a degree or 2 of each other after an hour of running so I think they were OK to run together. They have been on for 4 or 5 hours every night for a month now with no signs of dimming yet, so they appear to be OK.

I didn’t even try to do anything fancy with the boards. Just soldered them in 2 parallel strands of 4 series LED’s to a piece of perfboard.

I did give the regulator board and the LED wires a generous dip in epoxy to protect them from moisture and shorting out against the metal case of the lamp. The install into the lamps was a bit of a pain, To hold the boards I just used a nibbler to cut out some supports from the reflector they came with, after removing the mostly useless little board that was in there. And then some silicone to hold them together. Then screwed it all back together.

 

These pull 3 watts at 12 volts. The output is equivalent to a 10 or even a 15 watt incandescent low voltage bulb which makes them very usable for outdoor lamps and considerably brighter than any currently available LED outdoor lighting. And I should never have to change the bulb.

So after all that work of finding an LED and a power supply that worked well together, gave a light color that I liked and had good lumen maintenance over time, now the board has been redesigned and the 8mm LED’s dont appear to be available anymore either. I’ve got some new 10mm LED’s on order and some other power supplies and so the next batch will be different again. I think that instead of a PC board in the next batch I’ll use 2 strips of brass as buss bars to solder to as they will help heat sink too. There are finally some commercially available bulbs that aren’t just 2 or 3 tiny blue/white LED bulbs available. The best I’ve seen so far are from superbrightleds.com but they are just a 1 watt luxeon and so aren’t even as bright as the ones I made. They are smaller though and in a really nice case. When the price of those comes down I can see putting in a bunch of them. Another good source for LED’s directly from China is besthonkong.com but again, since they are mailing to you from far far away it will take weeks at least to get your order. They also have some nice new power supplies and other fun stuff.

Another option to LED’s is compact florescent bulbs. The local Home Depot has started carrying some 12v MR16 style bulbs that pull only 5 watts. They aren’t exactly MR16 sized though and wont fit in every type of lamp made for MR16 bulbs. I did find a pack of not expensive ones at LOWES that they did fit in though. The 50 watt marking on them is almost funny though. They will not, by any means of the imagination replace a 50 watt bulb. Perhaps a 20 watt bulb if you’re not too picky.

There are 2 major problems with them though. Since the tube is right against the front of the bulb the output is a flood. There is no good way to focus them at all so short distance lighting only. The other problem is the color. It’s very cool white light. Some people really like this, and I’m not against it in places like my kitchen and my shop where I need to see under very bright light, but as a single dim lamp the cool white just looks grey to me. If they made a warm white version I’d be all over these, but they do not appear to. Or at least the local HD doesn’t carry them. They run fine on 12v AC or DC so can easily mix into a system of LED based lights, or regular bulb based lights though which is a plus.

Here’s the bulb in the lamp that I tested them in. I haven’t put these outside yet anywhere so i can’t tell you exactly what they look like, but I suspect that it will be just a slightly brighter patch of moonlight effect due to the wide diffusion and the cool color temperature.

I enjoyed building that first set of 4 LED lamps, and can’t wait for the next batch of LED’s and drivers to arrive which have been in the mail for a month now...

Cheers,

James