Powering the LUXEON K2?

[Jbcourt]

Powering the LUXEON K2
I have 28 LUXEON K2's (8 blues and 20 reds) and have been struggling with getting power to them. I have a heat sink to attach them too, and want to use my homes a/c outlet to power it. Someone told me that Field-Effect Transistor or something called a Driver will work? I thought that just hooking up transistors to give me the 350ma per LED would be fine? I also want the LEDs and their power supply to work for many hours with no parts wearing out or burning up.
Does anyone know the real answer to this question?

 

[HarryN]

So, if you think about it, you have roughly 3-4 watts x about 30 LEDs, so something over 100 watts of LED power there. I guess you can run them at 350ma, but that is a lot less than they are rated for. More typical would be 1000ma each.

The correct way to drive that many LEDs from AC is to use a product from Advanced Transformer called a Xitanium driver. It does all of the work for you, is UL rated, etc. so you don't have to worry about buring down your house.

http://advancedtransformer.com/products/led.jsp

You might be interested to know that Philips (the same company that makes the K2) also owns Advanced Transformer.

They come in various power outputs and the price reflects that. I have seen them for sale at leddynamics and The Sandwich Shoppe as well as other places.

I just noticed that you have 20 K2 reds, which you correctly point out can only take about 350ma. The Lux III version can take 1 amp, so that is actually a closer match to the blue K2s. The other way to do this is to run the K2 reds in 3 parallel strings - a very crude drawing here

Power ---------------- 10 x red K2 (about 25 volts Vf) + small resistor
---------------- 10 x red K2 (about 25 volts Vf) + small resistor
---------------- 8 x r blue K2 (about 25 volts Vf)

There is some trial and error to get this fully balanced on one power supply. Ideally, you would put each string on its own Xitanium driver.

Feel free to ask more if that is not clear enough and welcome to CPF.

-

 

[Jbcourt]

Sorry, 700ma

 

[HarryN]

Hi, Regardless of the ma, basically you need to choose a driver to control the current flow.

In a perfect world, you would buy a driver that puts out your desired current (700ma in this case) and just run them all in a string like Christmas lights.

In practice, each LED has a "forward voltage (Vf)" that will be around 2.5 volts for red and about 3- 3.5 volts for the blue. If you put 10 blues in a row, that is 10 x (about) 3.5 = (about) 35 volts. IIRC, the Xitanium driver maximum is around 25 volts, so you can't put 10 of them in one string.

If you break it up into 2 strings, 5 each, then 5 x 3.5 volts = (about) 17.5 volts. If the driver can put out 1400 ma, then it will feed approx 1/2 to each string of 5, as long as the sum of the Vfs is the same.

The trick gets to be "how close do you need to be" with this whole Vf thing, because every LED is slightly different, the Vf is not a "number", it is a curve (rises with current flow) and also changes a little with temperature and time. (exciting huh). The general approach is to try it and see how it turns out, or you can sometimes add just a few ohms of resistance to each string and that helps balance them out if they are relatively close.

It helps a lot of you have a meter that can take some measurements, and especially if you have an adjustable power supply to play with.

We are all looking forward to watching your project come together.

 

[Changchung]

Check this out...

http://www.candlepowerforums.com/vb/showthread.php?t=195715