Powering My First LED.

UrbanExplorer

Newly Enlightened
Joined
Feb 16, 2017
Messages
39
Location
California.
Hi! Hope all are doing well.

I am currently in the process of making a solar powered DIY light and for this application I have chosen a CREE XP-L HI LED. I have my LED's, my power sources, etc. However, I am struggling to determine how to power this light to my desired output. All I know is that it's max output is roughly 1045 ish lumens and has a drive current of 2.95v @ 1.05A (whatever exactly that means besides what it explicitly states)

For my application I need 254 lumens so about a 1/4 of the max output. How do I figure out how much power is needed to get this output? Is there a chart somewhere I need to pull out? Also is it voltage I should be concerned about or Amperage? I was told higher voltage makes the LED put out more light but from my understanding it's Amperage that determines that.

Either way this should help me determine which driver it is I need to get.

Thanks to all the folks on CPF. You guys have always been a life savor.
 

UrbanExplorer

Newly Enlightened
Joined
Feb 16, 2017
Messages
39
Location
California.
I've looked at that data sheet but it doesn't seem to show much else besides a rough chart. Is there no way to get any more specific of an answer? Also would voltage still be 2.95?
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,725
Location
Connecticut, USA
The datasheet does have the information you need, if you know how to look for it. You'll want to find the rated output (lumens) at rated current (1050 mA) from the charts starting on page 3. Say you were using the first part in the chart. Minimum flux at 1050 mA is 480 lm.

You want 254 lm, so this is 254/480 x100 = 53% of rated flux. You could say that means you want 53% of rated current (0.53 x 1050 = 556 mA). You'd be close, but you can get closer. Go to the graph at the bottom of page 13, and you see that you will get just about 53% of rated output at 500 mA.

Keep in mind these are not exact values. There are variations from one LED to another, even if they have the same part number. It will also change with die temperature and age.

As for the voltage, check the upper graph on page 13. It tells the typical voltage to expect at various currents. At 500 mA, it shows just over 2.8V. Note that at 1050 mA, it shows right about 2.95V as expected. Also, you should be aware that the voltage has even more variation than the luminous flux. At 500 mA you might get 2.8V, or you might get 2.7 or 2.9V. This is why we don't try to drive LEDs with voltage. We drive them with current, because they are much more predictable that way.

You don't get to pick both the voltage and the current. You set the current, the LED determines the voltage. This is opposite of the way many things work. Plug a lamp into the wall, and the power company (and some other factors) determine the voltage, and the lamp determines the current at that voltage.
 
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