Got a question on current draw.

[scubado]

I'm having a problem figuring this out in my head. I'm looking at getting some new leds to play with and trying to figure out how many can be hooked up on a channel. The led is a CREE XM-L T6 LED it uses 3000mA at 3V. If hooked up to 5V, according to the resistor calculator I need a .666ohm 6W resistor and could only hook up one per channel on a LOR DC QC controller. Is there a better way to use this led and have more than one per channel?

I currently use 10W 12V leds that require 1000mA. I can put 4 of these on one channel and not have problems, but they require signicant heat sinks. The Cree seems to not need much of a heat sink.

[plasmadrive]

I haven't looked any of these up, but is appears the XM-L T6 is a single chip.  Your 12 volt LEDS are most likely several chips in series.  You can also put the Cree units in series if you want.. If you put 4 in series they will drop approx 12v. That is a bit close on the voltage but you could possibly bump your supply to 13 volts (if adjustable). 

3 in series would be 9 volts and you would have to drop the remaining 3 volts via a resistor or controller.  If you did use three in series with a resistor you would still draw 3 amps but then you can use the 12v supply and you will have your three on the same channel.

[scubado]

Thanks.  Last night I didn't think about the idea of putting them in series, it had been a few years since I did that.  I usually run them parrallel each with it's own resistor, so if one goes bad, it doesn't effect the others.  These things need a honkin' a$$ resistor to wire up to 12V.  4 in series would be perfect for a couple of my lighting needs.  One being over 4400 lumens blasting at ya from a center burst!

[plasmadrive]

if you do put 4 in series and run them from 12 volts, make sure you put some current limiting resistor in series or you may wind up with blown chips..  Perhaps a .2 ohm resistor or something close perhaps.  at 3 amps that would drop .6 volts and you should be golden for the LEDs sake..  at least from what info you posted..

 

Craig

[FarrOut]

From things I've read, you may not need to run them at 100% of their rated voltage.

Do a test and find out if there is any observable dimming at various voltages.

 

Let us how it works out.

[scubado]

I have 4 on order.

[plasmadrive]

From things I've read, you may not need to run them at 100% of their rated voltage.

Do a test and find out if there is any observable dimming at various voltages.

 

Let us how it works out.

You don't have to run them at rated voltage.. That is the typical voltage they will drop at max current. 

[Max-Paul]

Hope your handing out sun glasses at your display.

[scubado]

For Halloween, I should be outputting 20,000 lumens for lightning!  A couple Christmas songs will even be brighter than that! 

[Bob VandenBoom]

Looking at the data sheet it looks like the typical voltage drop at maximum current of 3000mA is 3.35V.  Running at 3V would drop the current to about 1500mA or less.  If you still want to consider running one LED off 5V you would only have to drop 1.65V across a resistor.  You're still looking at a a .55 ohm 5 watt resistor - pretty hefty.  You can make the resistor easier to deal with by running multiple resistors in parallel.  For example, you could run two 1.1 ohm 3 watt resistors in parallel or three 1.65 ohm 2 watt resistors in parallel.  You get the idea.  Keep in mind that the 3000mA is the maximum current.

 

Also, you stated that the Cree did not need heat sinking.  To get the most out of the LED it is important to minimize the temperature difference between the ambient (surrounding) temperature and the junction temperature of the LED.  Simply put, the closer the led is to the surrounding air temperature, the more efficient it will be.  Matter of fact, if the thermal resistance is too high, the LED will never reach it's maximum output.  The LED chip has a thermal pad on the back for heat sinking.

[scubado]

Looking at the data sheet it looks like the typical voltage drop at maximum current of 3000mA is 3.35V.  Running at 3V would drop the current to about 1500mA or less.  If you still want to consider running one LED off 5V you would only have to drop 1.65V across a resistor.  You're still looking at a a .55 ohm 5 watt resistor - pretty hefty.  You can make the resistor easier to deal with by running multiple resistors in parallel.  For example, you could run two 1.1 ohm 3 watt resistors in parallel or three 1.65 ohm 2 watt resistors in parallel.  You get the idea.  Keep in mind that the 3000mA is the maximum current.

 

Also, you stated that the Cree did not need heat sinking.  To get the most out of the LED it is important to minimize the temperature difference between the ambient (surrounding) temperature and the junction temperature of the LED.  Simply put, the closer the led is to the surrounding air temperature, the more efficient it will be.  Matter of fact, if the thermal resistance is too high, the LED will never reach it's maximum output.  The LED chip has a thermal pad on the back for heat sinking.

Thanks guys for all the info. I'm assuming these don't need much for additional heat sinking because they are using them in small applications. The 10W leds I'm used to will burn your fingers in about two seconds w/o a heatsink. I'm curious to see what I can do with them. I also ordered a flashlight with one of these leds in it. Boasts 1800 lumens! My work light is 150 lumens and is plenty bright for the equipment I work on.

[scubado]

Update:  I got the new flashlight today and not overly impressed with it.  It's a little too blue for me.  It is brighter than my work light and seems to be as bright or slightly brighter than my 10W warm white led.  I was hoping for a better white and a little more kickass.  I asked my daughter if her photography class has a light meter I could borrow to get some measureable results. 

 

Here is the one I got: http://www.ebay.com/itm/1800-Lumen-Zoomable-CREE-XM-L-T6-LED-18650-Flashlight-Torch-Zoom-Lamp-Light-GD-/290861627774?ssPageName=ADME:L:OC:US:3160

 

Not a bad light for the price.  A friend of mine has a 450 lumen flashlight from HD that really rocks for about $20.  I think maybe the bigger reflector has something to do with it.  We had hoped this light would kick butt over the HD one.

[scubado]

I got the 4 bare leds today.  I wired them in series and ran them on 12V with no problems so far.  I was going to try to get a current draw but it seems my multimeter has a problem with the ampmeter.  Anyhow I have spots in my eyes from playing with these things.  They are a bright cool white, I found a cool white 10W led and I think it's brighter.  It's a little hard to say because the 10W has a bigger point of light being that it consists of 9 leds.  If the 10w led had 9 T6 leds in it, then it would really be kicka$$!  As far as heat output on the T6, no addition heat sink is needed, it only raised a couple degrees in my fingers.  The T6's won't work with the Holidaycoro DMX controllers for lack of current handling.