Hi All,

I have a project for which I chose the xm-l2 led which according to the specs have a maximum current of 3A at 3.18V.

To my understanding, if a component can draw xA it will do it as long as the power supply will be able to deliver it.

In this case, the maximum drawn current at the specified voltage is 300mA.

can someone explain why is that?

Thank you in advance for your replies

 

hi,
How are you driving the LED.?
E

 

right now for the test, throw a power supply directly

 

Hello,

Does the power supply have a current limiting function?
If so set a current and raise the voltage.
Leds are current driven elements.

Bertus

 

Hi,
What voltage/current is the power supply to output.?
E

 

The power supply has a maximum supply current of 10A. I tried setting a higher voltage and that indeed increased the current consumption and therefore the brightness of the led. The question is why if the power supply can supply higher currents the led is not drawing them, at the end of the day I planned to power the led from a li-ion cell.

 

If you vary the Voltage output of the power supply does the current follow the plot of published chart.???

 

This is the C/V chart for this specific led

 

Hi marc,
Please answer this query.
If you vary the Voltage output of the power supply does the current follow the plot of published chart.???

I have already posted the plot in post #2

E

 

Sorry I missed you posted the chart, in answer to your question, yes, you can say it follows some what the C/V relation shown in the chart, but of course not at the same values, otherwise I wouldn;t be asking

 

hi,
You will find that most published charts are only 'typical' , so an exact match would be unusual.

Many LED's are powered by a constant current supply, that is set to operate within the specified current range, in order to control the brightness.

E

 

Thank you for your reply, the only thing that I find odd or don't understand it I guess is, current is something the circuit draws right, so if the led consumes xA at xV and the power supply can deliver it should do it no?

I understand that the typical chart values might be off, bu I can tell you that I set the power supply to 5.6V and it draw only 1A which is way less than what the chart show.

Thanks again

 

hi,
Reading thru the Cree datasheet, if you are sure that your power supply is not set to a 1Amp current limit and you are applying 5.6Vdc to the LED, I would recommend that you contact Cree for an explanation. [ oy vey.. ]

I assume you have the recommended heatsink fitted.

E

 

The current vs voltage graph shows that one LED typically draws 1A when its forward voltage is a little higher than 2.8V, but your voltage is double 2.8V at 5.6V so you probably have two LEDs in series.

 

I see what you're saying but I definitely don't have 2 in series

I have contacted Cree, maybe they'll be able to explain.

Thanks for all your replies. Will update if I get response from Cree

 

The trouble with these types of LED package is that they need soldering on a good thermal board like IMS. Not many have the facilities to surface mount devices. It can be done but needs a hotplate or controlled heat source that can reach solder temperature in the recommended cycle - which is relatively quick.