run a low voltage led(3volt) at higher voltage(12-18 volt)

[pufftmd2389]

Hello,my names Henry. new here and figured might as well say hello and ask the question at the same time.

OK so i am trying to run an LED that is rated at 3.4v and 3amps Max and i was wondering if it was possible to run it at 12-18v at 450-500 milliamps.
I got those numbers from volts × amps= watts then made sure with 18 volts the watts didn't go over what it did at 3.5v.
Let me know if that's the correct way to do that math, trying to learn as i go

 

[alfa88]

Poof!

If you were to apply 12 v directly to the LED it would flash and go poof! The LED is rated at 3.4V so the math goes 12-3.4=8.5, 8.5Volts/500mA=17ohms, 8.5V*500mA=4.2 Watts. If you don't want your LED to go poof put a 17 Ohm/ 5 Watt resistor on one leg. That is a heck of a LED they usually only draw 35mA.

 

[pufftmd2389]

I was looking for the best led i could get for 12volt but couldn't get more than 75lumens so i went lower voltage and got better.
Figured i was off by a bit with the math lol.
Ill show how it worked out in the end..time to do the physical work.

 

[Laplace]

Look at the datasheet for your LED; it should have a V-I curve. The only possible operating points for the LED are on that curve. I can guarantee that a voltage of 12-18 volts at 450-500 mA is not on that curve.

Of course, you can power the LED with 12-18 volts if you put in a series resistor that will limit the current to the maximum rating of 3 Amps, but in that case most of the power will be dissipated in the resistor.

 

[pufftmd2389]

actualy here is the LED im working with.
http://www.ledsupply.com/creexml-w280.php
and the datasheet.
http://www.ledsupply.com/docs/XLampXM-L.pdf

actually quite a robust lamp, the solder wasnt all that great on the one i got so they are replacing it but even with a weak solder i burned my eyes with it, couldnt see for a few hours. ( pushing the led against the board with some force made it light full bright)

 

[Resqueline]

What you need is called a (switchmode) LED driver. They come in all "flavors". It goes between the power supply and the LED. It won't waste power like a resistor.

 

[BobK]

I hope you plan on having a massive heat sink if you run this LED at 3A.

Bob

 

[(*steve*)]

To put that in technical terms, I would not feel safe with a heatsink less than 7.5 degC/W, and I would prefer one closer to 4.5 degC/watt.

Those figures presume that you get almost perfect mating of the LED to the heatsink, and that the heatsink is oriented so that it achieves its rated performance.

If this is going anywhere that is dusty then you would require better initial heatsink performance as it will degrade over time.

Good design would suggest that you should monitor the heatsink temperature and reduce the current if it gets too hot (better less light than *NO* light)

 

[(*steve*)]

What you need is called a (switchmode) LED driver.

And I just happen to have seen one here.