First LED Light Fixture

[JamisonM]

Right now, I have a 10x11 foot room that has a single 10 watt CFL for lighting. It's in a two bulb fixture with a center post where a frosted glass dome hangs. From what I understand, this bulbs puts out around 550 lumens, but I wonder if I can put something together using LEDs that will put out more light with the same amount of power.

My idea is to buy a 350mA 12 watt Xitanium driver, a 1/4 thick piece of 12x12 aluminum and 8 LEDs. I'll just cut off one of the sockets while and leave the other in case there is a driver failure. If there ever is, I can still screw in a bulb and have light. I'll drill a hole in the center of the aluminum plate and put it in place of the glass dome. I'll mount the driver close to that hole and have the LEDs in a hexagon pattern around the aluminum plate. When It's all said and done I figure each LED will be putting out around 100 lumens each. If I use 8, that means 800 lumens.

This brings me to two questions; how much of an impact will will pointing the LEDs at the calling and using the light reflected off it to illuminate the room and should I go for more light output from the LEDs? I'm sure this isn't the proper way, but I used a SST-50 modded maglite to sort of gauge what I'd be looking at with this fixture. This is my highest output light and driven by one of Gearge's hipCC drivers. It puts out quite a bit of light and illuminates the room decently, but I feel using the reflected light from the calling will require at least twice the output from the LEDs if not triple. Something else that comes to mind is just to point the LEDs downward and use the frosted glass dome to scatter the light. Again, I feel like at least twice the light will be necessary.

 

[AnAppleSnail]

Again, I feel like at least twice the light will be necessary.

LED lighting design requires some assumptions and some estimates, but you won't be too far off. This guy used 6 Q3-bin XP-Es running at 350 mA, and compared it to a 60 watt lamp. It looks like he has high, flat white ceilings. Building the LED device this way is VERY simple

Thread here

A 10-watt fluorescent will output about 600 lumens, and a 60-watt incandescent about 800 lumens; so that's a place to aim at. If you don't want to build your light this way, you'll be using different assumptions. If you plan on losing 20% to 40% of your output through ceiling bounce, you won't be disappointed. First, pick a tint bin. I suggest a neutral of some kind; 5A is popular. Next, pick a number of lumens and a type of LED. XR-E is cheap, XP-G is a good choice, but I would avoid the multi-die single-packages because they are point sources of light, which are no fun to look at.

You're aiming for something between 600 and 800 lumens in the room. With the light diffusion losses, that winds up at 720 to 1120 lumens. Let's arbitrarily pick 900 lumens so that you won't be downgrading your lighting setup. You'll want to get about 900 lumens of LED, based on the 350 mA typical luminous flux. This will lead you to the right flux bin to pay for, as higher ones are often more expensive. I got 8 Cree XR-Es for a project, Q4 flux and 5A tint. They output just about 100 lumens per LED at 350 mA, less at higher temperatures. This means 9 of them. If you were using the neutral XP-G in the R4 bin, you'd only need 7 LEDs (130 lumens each). The Cree Datasheets are a big help for this step.

Heat dissipation. You can estimate the heat you'll be dealing with. Current times Vf of the LEDs gives the total power in. A fair percentage of that (15-20%) comes out as photons, but it's better to overbuild a heatsink a bit. You can use the 12W Xitanium driver. It will drive up to 10 LEDs at this drive level - 12W / .35 A = 34v. 34v/Vf = number of LEDs. The Vf of the LED does make some difference here, with a fixed drive level. With a lumen goal, you'll need fewer higher-efficiency LEDs for the same output, giving less heat. It's better not to enclose an LED fixture in glass so that air can circulate to cool it. In still air, radiation is the main cooling method - so a black-painted metal will run cooler than shiny. In moving air, conduction is king, so shiny is the way to go.

If you grind off the ends of the bottom contacts on Cree LEDs, you'll make the whole base of the LED electrically neutral. This makes it easier to reflow-solder or thermal-epoxy the bare LED to a heatsink. If you'd prefer to work with LEDs on PCB stars, that's fine - they'll run a little warmer but are much easier to wire and anchor. You can actually destroy a lightbulb and get an Edison-socket compatible place to solder wires so that you aren't debuilding your light sockets.

I'd suggest that you use sheet metal - copper or aluminum work well - and, using pliers or something to bend it, make a shape to hold the Xitanium and LEDs. With some space between the LEDs and the frosted dome, it should be diffused enough. If you can't manage that, point them at the ceiling and make sure noplace in the room allows you to see them directly - the domes are quite bright!

 

[blasterman]

For the project here, remember that different type of diffusion materials have radically different transmission rates. Standard 1/8 milk plexi, which most of these lights use, eats a huge amount of light. Higher tech acrylics like Acrylite are a much better material to use, but tough to find in small quantity. Frosted glass is actually pretty efficient and a nice diffusion medium, but there's safety considerations and you have to deal with the green tint.

Bouncing off the ceiling is almost best left to reflector CFLs.

Also, the 130 lumens per watt number is for fairly high CCT bins. Not too many people want 5000k in their bedroom.

 

[JamisonM]

That's what I was thinking. To add, this room has an a sort of textured ceiling. It's may be white, but that's really all it has going for it. It can't compete with a white semi-gloss in term of reflectivity. I'd be better to just face the LEDs downward and have their light scattered by the frosted glass dome so there isn't as much light loss. Another question I'd like to ask is will a 12x12 foot plate of 1/8" thick be enough heatsinking for the 8 LEDs and driver? I think drilling and tapping a hole would be better, but I don't have a tap so the only thing I can do is bolt the LEDs and driver down with some arctic silver. Not pretty, but I don't think anybody will ever see it one it installed.

 

[JamisonM]

Little bit of an update. I've been thinking of what LEDs to use and how to position them on the heatsink. Right now, it's going to be 8 rebels in a octagon pattern. Another thing I've been thinking of is the LEDs emitting pattern. The rebels have a 120 emitting pattern. If I use use a flat piece of 12x12 aluminum, even with the frosted glass dome scattering the light, most of the light will go downward. Because of this, I thought why can't I cut and bend the heatsink a little. Doing so, I think more light will be projected sideward and I won't give up much in the way of heatsinking. I think doing this will provide more even room lighting. Below is a quick picture I put together in paint. The blue line is where the bend will be. The second picture is just a simple side view of the bent heatsink.