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

Discussion in 'LEDs and Optoelectronics' started by pufftmd2389, Apr 1, 2011.

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  1. pufftmd2389


    Apr 1, 2011
    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
  2. alfa88


    Dec 1, 2010

    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.
  3. pufftmd2389


    Apr 1, 2011
    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.
  4. Laplace


    Apr 4, 2010
    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.
  5. pufftmd2389


    Apr 1, 2011
  6. Resqueline


    Jul 31, 2009
    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.
  7. BobK


    Jan 5, 2010
    I hope you plan on having a massive heat sink if you run this LED at 3A.

  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    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)
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    And I just happen to have seen one here.
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