Fixed Indoor lighting with Cree XM-L LED's

[arcaneinsomniac]

The efficiency of these new emitters has me considering the building of light fixtures.

For my first build I was thinking of keeping things simple. I would obtain a piece of aluminum or other metal which I can reasonably expect to dissipate the heat that will be generated, thermal-epoxy LED's which are soldered to those star things, and then wire them together in series.

For 6x XM-L's I would have around 20V, so I would get a power supply like this one and wire a voltage regulator or (beefy) resistor to it to get a reasonable current (I'll be targeting 3A...) I also have some BJT transistors and op-amps and other curious electronic components that are suitable for making more complex current source circuits.

Is this more or less how it's done?

Also related. Are LED's a bad idea to use for lighting in bathrooms? When a hot shower is running the relative humidity may get quite high... Reason I ask is because the new bathroom in the new wing of the house has beams going across and a high ceiling - a perfect place to put some bars of LED's across the top.

 

[Steve K]

I've been mulling over the same sort of thing. My personal preference is to throw together a circuit, but I'm an electrical engineer who likes to play around (and has some experience in designing stuff to resist nasty environments).

The simple/safe answer is to go with a proper pre-built power supply designed for this sort of stuff. A popular electronics supply house is Digi-key, and they sell a pretty wide variety of AC to DC converters that provide a constant current (or voltage) to your bank of LEDs. A search for "led driver" will produce this.

Go to the "current output" selector, click on "3A", and click on the "apply filter" button (lower left), you'll get this.
okay... I was expecting more options, but let's look at this device from ExcelSys Technologies. It runs off of either 110vac or 220vac. It puts out a regulated 3 amps. The output voltage can be anything from 6v to 12v, which is only 4 LEDs in series (at best). There is a link to the manufacturer's literature too.

For humid/wet environments, some sort of protection would be advisable. Maybe mount it remotely, i.e. not near the light fixture? Or seal the light fixture? After all, that humidity won't be doing any favors for the LEDs either. The sealed fixture would still need to provide heatsinking, so an aluminum housing might be the best bet.

anyway.... just my thoughts on the matter.

Steve K.

p.s. something else to consider... if you build a circuit and it malfunctions and sets your house on fire, what will your insurance company say? OTOH, if you buy a AC/DC converter/LED driver with the proper UL ratings, etc., and it sets your house on fire, then there's less chance that the insurance company will deny your claim on the grounds that you're an idiot or incompetent.

 

[andersonEE]

A few things to consider:

1. Many people prefer a high CRI lightsource in the bathroom because it renders skin tones much better. (If you have a wife or girlfriend who does makeup, hair, etc. in front of the mirror, this is especially important). The warm white XML's have a CRI of 80 I believe, which is good but not great (incandescent have 100CRI). The new Bridgelux LED's have a 90 CRI option which would be much better. I think 90 CRI XPG's are also available from Cutter.

2. You should use a constant Current Supply. RapidLED.com sells very affordable Meanwell drivers. You can also check Newark.com, Futureelectronics.com, and Digikey.com for good deals on constant current drivers. I have used Roal drivers with good success.

3. Why not just put some optics on the led's to isolate them from the humidity. Something as simple as a plexiglass cover would fine... you should probably do this anyways so it will be easier to clean the dust off.

 

[purduephotog]

Check out the CXA2011. It now comes in multiple color temperatures and can go from 1000 to 4000 lumens, depending on drive current.

 

[arcaneinsomniac]

Thanks for the replies, you all bring up a lot of interesting points. Steve, I actually do buy things from digikey when I can find something that I want in their insanely huge catalog. The problem is with the selection. It's too BIG! If I were to mount it on the beams going across the ceiling i think it would be less susceptible to splash damage but I agree that it would probably be good to have some sort of cover, I hadn't even considered the dust collection aspect which is certainly important.

I am aware that CRI is important in a bathroom and I actually was playing with the idea of adding several other LED colors and balancing them to give a fuller spectrum of white. Plus if I have enough I can program an MCU in my device to give it a "party mode"! I suppose there isn't going to be a straightforward way to calculate the CRI from a combination of LED's. I know that incandescent lighting is basically a black body, so it won't have any dips in the spectrum. Looking at the datasheet for the Cree XP-E (which has six colored LED types and several white ones) there is a range from about 460nm to about 510nm which doesn't get much coverage from any of the colors. Seems like the only way to find out if a combination will work well is to try it out. I am thinking instead of trying to actually use it as a permanent fixture my first LED light ought to be a "testing platform" of sorts, best would be to have some holes for screws that fit the star heatsinks and I can swap around some combinations.

 

[andersonEE]

You can improve the CRI of your light by mixing LED's (usually cyan and red Leds mixed in with neutral or warm whites). However, high CRI with LEDs is a fairly complex topic...do a search in this forum for "high CRI mixing LEDs" ...you'll see what I mean. It might be easier to just purchase High CRI LEDs from the start.

 

[trevordurden]

p.s. something else to consider... if you build a circuit and it malfunctions and sets your house on fire, what will your insurance company say? OTOH, if you buy a AC/DC converter/LED driver with the proper UL ratings, etc., and it sets your house on fire, then there's less chance that the insurance company will deny your claim on the grounds that you're an idiot or incompetent.

That's what circuit breakers/fuses are for. Remember though, they're not for protecting the component, they're for protecting the wire.

 

[SemiMan]

That's what circuit breakers/fuses are for. Remember though, they're not for protecting the component, they're for protecting the wire.

Unfortunately electrical fires tend to start in situations where fuses would not have provided protection .... i.e. partial shorts and/or partial open .... enough resistance to start a fire but not blow a fuse or fail open.

 

[Steve K]

Unfortunately electrical fires tend to start in situations where fuses would not have provided protection .... i.e. partial shorts and/or partial open .... enough resistance to start a fire but not blow a fuse or fail open.

yep.. I agree completely. I think that's why there is stuff like circuit board material that's not flammable and metal housings for electronics. Well, and there is a considerable bit of product testing, especially with folks like Underwriters Lab. It's important to be able to show the lawyers that you followed all of the standards and recommended practices.

Does anyone here have a good summary of the regulations and standards that apply to consumer lighting products?? (I'm assuming USA, but would be interested in other standards. My guess is that the EU has the most strict standards)

 

[blasterman]

Are we talking about the driver, or LED assembly?

For the driver side, I have no idea why somebody would want to build a mains powered driver DIY when you can buy certified drivers for cheap that have every possible imagineable cert stamped on the side. If you're buying unbranded Chinese drivers from Ebay with no electrical cert then it's your funeral if they catch fire.

In terms of the LED assembly, as I understand US code if it's below 24volts it's pretty much a wild west. Somebody feel free to correct me if I'm wrong on this.

 

[Steve K]

Are we talking about the driver, or LED assembly?

I was talking about the driver. In most instances, it's a no-brainer to buy a commercially built and approved driver. In other cases... well, there may be unique requirements for the application that call for a custom build.