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Low-alloy
05-13-2012, 02:04 AM
Hello Folks,

As I mentioned in a previous thread, I am transitioning from fluorescent lighting to LED. The last time I had to pay to dispose of the burn-out tubes I swore off dealing with fluorescents.

Anyway, I thought I'd try the CREE led emitters that Deal Extreme has for sale. Model XMLAWT 1000-Lumen LED Emitter White Light Bulb (3.0~3.5V) to be exact <dealextreme.com/p/xmlawt-1000-lumen-led-emitter-white-light-bulb-3-0-3-5v-51989>. The problem I am having is locating an appropiate driver! Each emitter pulls 3000mA, which appears to me make none of the drivers Deal Extreme sales suitable. As I am totally new to LED's and probably overlooking an apparent driver on the Deal Extreme website could someone point a good one out to me?



Thanks in advance

danlb
05-13-2012, 11:13 AM
This is not actually a reply to your query, so forgive me if it comes off as pedantic.

There are many power supplies available that will provide 3 amps at 3 to 3.5 volts. The problem is that the LEDs will not be consistent in the exact voltage that will make them draw that 3 amps. If you feed 3.5 volts to one that only needs 3 volts you may find it drawing 4 or even 5 amps and it will burn out early ( minutes to hundreds of hours ). To complicate things, the voltage need also changes a little with age and / or heat.

The two points above limit how you can provide power to the LEDs. If you get 3 matched ones that all draw the same amps ( and less than 3 amps at 3.5 volts), then you can wire three in series and use a voltage regulated 10.5 volt supply that is rated for 3 amps or more. If the LEDs are not closely matched they will use wildly different amounts of current.

The Cree data sheet (http://www.cree.com/products/pdf/xlampxm-l.pdf) gives a lot of information if you know what to look for. It says that TYPICALLY it will use .7 amps at 2.9 volts and will use 1.5 amps at 3.1 volts. Typically it will use 3 amps at 3.35 volts. The problem is that those values can vary by about 1/2 volt within the batch of LEDs.

The safest way to do it (if you want maximum light output) is to use a separate 3 amp constant current driver for each LED, and feed those converters whatever voltage they need. You can get a commercial supply that will provide a regulated 3 amps from a 120V AC source, but they are pricey.

I'm lazy. I'd pick up a supply for each lamp, then use a resistor to adjust the voltage as needed to keep the amps below that magic number. example:
3.6 V, 2.2amp constant voltage power supply (http://www.mpja.com/36V-22A-Desktop-Supply-KETI/productinfo/18357+PS/) for about $5 and a low resistance resistor in series to limit the current to less than 2.2 amps. The 2.2 amp is the max that this particular power supply will safely provide. Draw 3 amps and it will melt eventually. :)

MPJA has a lot of power supplies to draw from. A list is at http://www.mpja.com/Single-Output-Supplies/products/3/ and I've had good luck with them in the past. :)


BTW, the LED you chose is rated between 164 and 300 lumens at .7 amps, and 406 to 742 lumens at 2 amps. The output at 3 amps should be about 3 times the .7 amp rating. By using a bit lower than the max amps the heat problems are easier to deal with. Running lower than max power also makes it easier to deal with the voltage fluctuations as the LED ages.

Hope that helps some.

Dan

PS; from that data sheet, the table:
"Flux Characteristics" gives the lumens at each current level for each model of LED.
"Characteristics" gives the voltage at each current level.
"Relative Flux vs. Current" gives the lumens (flux) at each current level.

PSS; Note that all output ratings are given when the LED mount is at 25C. Without adequate heat sinks and airflow it will rapidly climb above 100C and will self destruct around 150C

Low-alloy
05-13-2012, 03:44 PM
This is not actually a reply to your query, so forgive me if it comes off as pedantic.

I'm lazy. I'd pick up a supply for each lamp, then use a resistor to adjust the voltage as needed to keep the amps below that magic number. example:
3.6 V, 2.2amp constant voltage power supply (http://www.mpja.com/36V-22A-Desktop-Supply-KETI/productinfo/18357+PS/) for about $5 and a low resistance resistor in series to limit the current to less than 2.2 amps. The 2.2 amp is the max that this particular power supply will safely provide. Draw 3 amps and it will melt eventually. :)


BTW, the LED you chose is rated between 164 and 300 lumens at .7 amps, and 406 to 742 lumens at 2 amps. The output at 3 amps should be about 3 times the .7 amp rating. By using a bit lower than the max amps the heat problems are easier to deal with. Running lower than max power also makes it easier to deal with the voltage fluctuations as the LED ages.

Hope that helps some.

Dan


PSS; Note that all output ratings are given when the LED mount is at 25C. Without adequate heat sinks and airflow it will rapidly climb above 100C and will self destruct around 150C

This my first real forray into LED electronics, so I appreciate the education and I didn't find you pedantic at all and was very helpful.

I have several old CPU heatsinks laying around, so I should be okay from a cooling stand point.

Originally, I was going to use an old desktop power supply as that I have several of them as well. Is this a reasonable idea? The label on one of the PSU's says 3.3 volts / 20 amp max. Those numbers are kinda why settled on the Cree XML emitter.

Thanks for your help!

Low-alloy

danlb
05-13-2012, 05:20 PM
Sure, you can use the PSU. The 3.3 volt SHOULD be OK, but it's a good idea to check the voltages each time you add or change any thing.

That power supply would be CV ( constant voltage ) so the amps that are consumed will depend on what is attached to it.

Over simplifying for ease of understanding:

A resistor in series with an LED is known as a 'current limiting resistor'.

You can attach up to 6 LEDs to that supply in parallel. You can accurately adjust the amps used by each LED by putting a 1 watt resistor in series with each LED The ohms value of the resistor will have to be determined by either experimenting unless you have a variable PS that you can use to determine the exact volts needed for each LED. The idea is that the resistor will consume a small portion of the voltage and the balance of the voltage will be used by the LED. Test the assembly by putting an ammeter in series with each led and checking the amps.

IN THEORY: the 3.3 volt supply should be safe for the LEDs without a resistor in almost all cases. Unfortunately there CAN be some LEDs of that type that will draw much more current at 3.3 than it should. There might also be some that under perform at that voltage.

caution; do not run the LED at full power without mounting it to a heatsink. They do get damaged from the heat fairly quick. They also burn your fingers, Ask me how I know. :)


Dan