# Question about Cree LED emitters



## kenlai32 (Jun 8, 2010)

Hi, I tried search for answers but still confuse when building a lamp using Cree LEDs. I am planning to build a lamp using 4-6 Cree P4 or Q5 XRE and will be ordering from extremedeal. 
I am not sure what I can use to run the led using the wall outlet. 

If I use a computer power supply (300w) or laptop power supply to run 6 leds:
+12V






























How would I know how much current I am running the LED? Do I need resistors since 12v / 3 = 4v running to each LED.

Thanks


----------



## Paul Baldwin (Jun 8, 2010)

Hi,
you can get help with calculations here http://led.linear1.org/led.wiz 
To get the power supply to work without being plugged in to the motherboard ground the green wire to the case (assuming you don't know that already) 
What do you plan on using for the heatsink/heatsinks?
Expect a good wait if ordering from DX but all the Crees I've had off them have been good so far.


----------



## TorchBoy (Jun 8, 2010)

:welcome:

Yes, you'll need a resistor or driver in there for each string of three LEDs. I use http://ledcalc.com/ for resistor calculations, probably because I find the URL easier to remember than Paul's link, or you can find lots of 12 V drivers in the list in my tagline.

Do you have a multimeter?


----------



## Paul Baldwin (Jun 8, 2010)

TorchBoy said:


> :welcome:
> 
> Yes, you'll need a resistor or driver in there for each string of three LEDs. I use http://ledcalc.com/ for resistor calculations,



That's top trumped my link, bookmarked, ta. :thumbsup:


----------



## mds82 (Jun 8, 2010)

you are better off using a laptop power supply, then using a constant current driver. my recomendation would be to go for the Buckpuck 3023. they have a 350/700/1000 mA outputs.


----------



## bshanahan14rulz (Jun 8, 2010)

Better to regulate current with something other than resistors at high powers. Unless efficiency doesn't matter and you can find high power resistors. Try to find an LED driver that works at 12V.


----------



## kenlai32 (Jun 8, 2010)

Thanks for all the quick response.
Have anyone experience this DC-DC driver?

http://www.sure-electronics.net/download/down.php?name=LE-LL311_Ver1.0_EN.pdf

It seems a good cheaper alternative to the buckpuck. 
According to the data sheet, I can run 6 leds at 25v input @ 758mA.



P4 and Q2 are almost the same price, but some review says Q2 are brighter than P4. Which one would be best for my application?


----------



## TorchBoy (Jun 8, 2010)

mds82 said:


> you are better off using a laptop power supply, then using a constant current driver.


Heh - when I first read that I read "than" instead of "then". Makes sense if I read correctly. 

If working off the mains efficiency might not be the main concern.


----------



## blasterman (Jun 8, 2010)

I use spare 18.5 volt laptop bricks for either 5 or 6 led runs with typical 3.5 volt emitters. You *couldn't force me *to use a buckpuck in the circuit because of their Q/C problems noted many times here. 

18.5 volts / 5 Cree P4s = approx 3watts each. If I don't need the brightness and want to have easier heat sinking I just use 6 LEDs. Works great.

12volt computer power supplies work, but with typical 3.5 volt LEDs such as P4s you need some resistance.

Ideally, if you really want to *do this right*, then get either a 700mA dedicated driver like a Xitanium, or if you want to save some money use a Meanwell. A 700mA Xitanium costs $28.00, and will drive 6 3watt Crees efficiently and practically forever. No screwing with buck-sucks or doing math.


----------



## Paul Baldwin (Jun 9, 2010)

If looking for drivers that accept 12v from your pc power supply I can recommend these https://www.candlepowerforums.com/threads/248411&highlight=ax2002

I've had 4 running at 18 hours a day for weeks with 0 failures so far. The same lamp has been run off an automotive source with no ill effects so far either.


----------



## doctaq (Jun 9, 2010)

you could look up a transistor based driver if you dont need dimming, ive had pretty good luck so far and its much cheaper to build it than buy a buckpuck

look up instructables i think a guy named dan has a few different models, otherwise even simpler would be a linear regulator like an lm317


----------



## arek98 (Jun 9, 2010)

Look at one of these http://wattsupply.com/s.nl/sc.30/category.2239/.f
Choose constant current supply, for 6 LED, 20W or 35W depending on current you want. E.g., for 6 LEDS ~1A you may choose LPC-35-1050.
If you want dimming ELN-30-27 may be good, but you will need external PWM or 1-10VDC source to dim it.
I ordered few others from them and they are fast and power supplies are nice, fairly small and dust/water resistant (most is IP67 or IP64).

For the LED, why you planning to buy XR-E? XP-G is brighter and probably less expensive?


----------



## TorchBoy (Jun 9, 2010)

doctaq said:


> you could look up a transistor based driver if you dont need dimming, ive had pretty good luck so far and its much cheaper to build it than buy a buckpuck


I gave up making my own when inexpensive drivers (much less than the buckpuck) began appearing. I can't make them for the same price.


----------



## doctaq (Jun 10, 2010)

http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/

i built this without too much hassle. 
i guess the only one that ive really seen that is also very cheap is the amc based one that i had a thread on, but that one needs low voltage, ive put 15v through this for testing a single led and it stayed on for 10 seconds before i ended the test but the mosfet will shut itself down when it gets too hot


----------



## TorchBoy (Jun 10, 2010)

Good on you for giving it a go. Am I right in thinking the circuit is a form of linear regulator? Burning off all the extra power in the MOSFET, it would get very hot. What current were you running the LED at? How much did your MOSFET cost?

Sadly, good MOSFETs are expensive here. FWIW here are some inexpensive 15 volt buck drivers, some of which are very efficient so heat isn't a problem running on 15 V.


----------



## doctaq (Jun 10, 2010)

based on a few accounts its supposedly more effecient than a linear regulator but ive read it going both ways

ive used it in another project of mine that has 100 hours of use so far, 6 leds 24v, so its matched fairly closely, i might be burning off 3v or so, i do heatsink the fet and i used a fairchild mosfet with a tiny ram sized heatsink on the back.

the voltage drop on it is supposed to be only .5v whereas like a lm317 is 3v, giving it a higher potential to be more effecient. 
ill find the part number when i get home but i did make one with the one he specified


----------



## TorchBoy (Jun 10, 2010)

doctaq said:


> based on a few accounts its supposedly more effecient than a linear regulator but ive read it going both ways


Both this and other linear regulators such as an AMC7135 will have to burn off the same amount of excess volts, and the AMC7135 uses only about 200 microamps to run - no room for significant savings there. That just leaves their overhead. AMC7135 is 0.12 V, LM317 is quite a bit more. If you're running just one LED from 15 V the efficiency is going to be horrible on any linear regulator.


----------



## doctaq (Jun 13, 2010)

the amc7135 is an awesome linear regulator, i wish it could handle more voltage though, even just a couple volts more so it could run two leds in series would have made it that much more useful.


----------



## TorchBoy (Jun 13, 2010)

doctaq said:


> the amc7135 is an awesome linear regulator, i wish it could handle more voltage though, even just a couple volts more so it could run two leds in series would have made it that much more useful.


You can just put the extra LED in series with the AMC7135, on either the + or - side. (Common positive for the AMC7135 and the LED.) The extra current used to drive the AMC7135 is only about 200 µA, so both LEDs will see almost identical current.


----------



## LukeA (Jun 15, 2010)

doctaq said:


> the amc7135 is an awesome linear regulator, i wish it could handle more voltage though, even just a couple volts more so it could run two leds in series would have made it that much more useful.



A visual depiction of an implementation of the circuit proposed above.


----------



## doctaq (Jun 18, 2010)

that is very useful, i will check it out later tonight


----------

