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High power LED power supply?

  1. Dec 17, 2016 #1
    Wanting to know if it is possible to power a LED with a forward voltage of 29.3V and forward current of 440ma from a 1.5V source with a 2400mAh Capacity?

    No particular need for this, more for challenge/knowledge. Have tried methods with a transformer, DC-DC converter, and lm350 constant current. So far im not having any luck
     
  2. jcsd
  3. Dec 17, 2016 #2

    rbelli1

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    Are you talking ideal components or can you supply part numbers for real devices you want to use?

    BoB
     
  4. Dec 17, 2016 #3

    Merlin3189

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    It would depend on the current capacity (or internal resistance) of the source.
    29.3V x 0.44A = 12.9W soyour power supply must deliver at least this plus energy wasted in converter inefficiency.
    So your supply must deliver 12.9W / 1.5V = 8.6A plus current to cover inefficiency.
    If you had a converter with say 75% efficiency, the current drain would need to be about 11.5A while maintaining 1.5V.
    If the voltage drops with load current, as it usually does, then the needed current increases further.
    This is the killer for you, as it means (I think*) you need a 1.5V cell with an internal resistance of less than 33mΩ. Maybe there is one, but I don't know about anything close.

    * I reasoned that you need 12.9W at 75% efficiency = 17.2W.
    The cell supplies maximum power when the load resistance equals the internal resistance and half the emf is accross the internal R and half across the load.
    So you get 0.75V output to give 17.2W = 23A. This is equivalent to a resistance of 0.75V/23A = 0.033Ω.
     
  5. Dec 17, 2016 #4

    Merlin3189

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    On second thoughts, you could achieve a very low internal resistance by paralleling many cells, so it's not a problem!
    100 AA alkaline cells would probably get well below the requirement and give you more than 1200mAH.
     
  6. Dec 17, 2016 #5
  7. Dec 17, 2016 #6

    rbelli1

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    In that case just put them in series until you get to the required voltage. Plan it right and you can use the series resistance of the cells for your limiting resistor.

    Sounds like a single lithium primary AA battery to me. The short answer on that is that no you can't light the specified LED with that battery. A strobe effect may be possible with the proper circuit.

    BoB
     
  8. Dec 17, 2016 #7

    rbelli1

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    Then yes you certainly could run that led with an ideal 1.5V 2400mAh battery. Using and ideal step up supply you will get 1000 seconds of run time. This set up is often used for finding your spherical cows at night.

    BoB
     
  9. Dec 17, 2016 #8
    I forgot to consider internal resistance of battery, been using bench power supply at 1.5 V with 3 amp limiter.
     
  10. Dec 19, 2016 #9

    CWatters

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    ..and a 2400mAH could supply that for about 16mins.

    If you need longer it would be better to use more cells in series than in parallel.
     
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