Luxeon K2 DC Power Help

[The_Savior]

First post and I'll admit I'm very new to the LED scene. I need some help trying to determine what resistor I need to get for the K2 Luxeon LED I purchased.

Here's the scenario:
- I plan on running the LED off of a AC/DC charger.
- Two options for the charger - 5.2v @ 850mA or 5v @ 1000mA

The charger will be hooked up to a light switch that will turn the LED on and off. I'm trying to determine what resistor I need to purchase so I don't burn the LED out. Target mA is supposed to be 1300mA, but the LED works fine with 850mA and 1000mA so I'm told.

I'm trying to determine what resistor I need for the two supplies I have mentioned.

Any help would be appreciated.

Savior

 

[evan9162]

I wouldn't use either supply. Being chargers, you need to map out their behavior with respect to current vs. voltage.

If they are a dumb NiCD charger, then they are a transformer, probably a half-wave rectifier, and a diode. So the output waveform is going to be tricky, and output voltage will depend on the current being drawn. These really aren't the right power supplies for a beginner to attempt to use for their first LED experiments.

How do you plan on drawing 1300mA from a 1000mA power supply when you're just using a resistor? It's not a good idea to run power supplies at their rated output current (unless they're designed to do so, which these are not), and an even worse idea to try to draw more current than the supply is rated for.

You'd be far better off to get a 5V regulated (switching) DC power supply. Look for a 5V, 2A regulated supply if you're going to draw 1300mA. Expect the Vf of the K2 to be around 3.9V at 1300mA.

With the LED Vf at 3.9V, 5V in, you'd need to drop 1.1V in the resistor. The resistor would need to be 0.8 ohms, and would be dissipating 1.4W of power, so you'd need at least a 2W power resistor for this. That's not a standard value. But using a 1 ohm resistor would get you around 1.1A. You'd still want a 2W resistor for that.

 

[The_Savior]

How do you plan on drawing 1300mA from a 1000mA power supply when you're just using a resistor? It's not a good idea to run power supplies at their rated output current (unless they're designed to do so, which these are not), and an even worse idea to try to draw more current than the supply is rated for.

Thanks so much for the detailed response. I meant to say that these are DC power supplies. I apologize. I wouldn't be drawing the 1300mA from the power supply. I guess that's what the LED's targeted mA is. Again, I'm green to all this so excuse the ignorance. I guess I was just wondering if I could power the LED off a 5v power supply with the 850mA? Would that be ok?

I don't mind going out an purchasing a 5v or 9v 1300mA power supply (Radio Shack).

Thanks again!

 

[jayb79]

I am using a motorola cell phone charger to run 3 R bin luxes. They are each using 1 ohm resistors and they draw about 325-350mah each and are hocked up in parallel . The charger is rated at 1100mah 4.2v, if i remember correctly. The charger gets pretty warm but seems to handle it fine. They have been running 24-7 for about 2 weeks now.

 

[The_Savior]

That's what I have. A Motorola power supply that is rated at 5v and 850mA. I'm just trying to determine what resistor I'd need for this kind of setup.

 

[evan9162]

That's what I have. A Motorola power supply that is rated at 5v and 850mA. I'm just trying to determine what resistor I'd need for this kind of setup.

If it is a cell phone charger, than it is almost undoubtely a switching DC power supply. I wouldn't draw more than 500mA from the power supply. At 500mA, the Vf would be around 3.4-3.5V. Voltage dropped in the resistor would be 1.5V. To get 500mA, use a 3 ohm resistor. The resistor will be dissipating 0.75W, so you need at least a 1 watt power resistor.

 

[The_Savior]

Fantastic. Thanks so much for the assistance with this.

 

[Gunner12]

:welcome:

Well, you could also buy 4 relatively cheap 3.7-9v driver at Dealextreme. Or even a multi mode driver. While you are there maybe also a more efficient(double the efficiency of the K2) Cree XR-E P4 LED. Don't forget a good heat-sink.

Of course, a resister would be cheaper.

May I ask what you'll use the light for?

 

[jayb79]

This is were my 3 luxes are

I pulled the guts out of an 18" under counter fluorescent and mounted the stars on a small piece of 1/8 aluminum.

Here is the cell charger

 

[Illum]

I was just wondering if I could power the LED off a 5v power supply with the 850mA? Would that be ok?

a K2 will light up fairly well as low as 4v at 250ma from my experience
I'd use a 4.7ohm half watt resistor for that, but I'm fairly certain the K2 wont burn out with 5V

keep in mind the K2 was designed to withstand high current inputs but that doesn't mean it can't be used at low current applications like the rest of its luxeon brothers

unlike jayb79, I didn't make something very useful...I simply hacked a 5mm LED goose neck LED light brighter by using a north bridge heatsink and a K2. I had a n00bish idea to integrate my desk lamp into something LED based and plugs into my desktop....since it has 8 USB ports in the back I started designing LED "resistor driven light engines" that run on USB [5V500ma]. I thought ceiling bounce looks nice...okay enough words heres some pics


this was before I clamped the Hsink on the gooseneck [K2 is the onle on middle left]

That's what I have. A Motorola power supply that is rated at 5v and 850mA. I'm just trying to determine what resistor I'd need for this kind of setup.

theres a few websites that does the calculations for you
LED series/parallel array wizard
[luxeonstar] LED Resistor Calculator