Modifying a LED dynamo lantern

Discussion in 'The Projects Forum' started by freswood, Nov 20, 2008.

  1. freswood

    Thread Starter New Member

    Nov 20, 2008
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    Hey all!

    Got a little project I'm working one with a friend. We bought a dynamo camping lantern which consists of 6 white LEDs connected in parallel. The power is supplied to the LEDs via a few AAA batteries, which are charged by turning the handle. We'd like to add more LEDs to make the lantern brighter.

    My physics is a little rusty, so it'd be great if someone could explain why this happened: we removed one of the 6 LEDs, which didn't affect the brightness of the other 5 when it was switched on. We replaced it with a red LED found lying around the house, which caused the other 5 LEDs to dim. We took the red one out and replaced it with the original white LED and it was back to normal. Is the red LED introducing some kind of resistance that the white ones aren't?

    Any advice you could provide on this project would be much appreciated - otherwise we'll probably end up destroying the lantern! I suppose the most important thing to know is whether it's possible.

    Do these LEDs look any good?

    http://cgi.ebay.com.au/5mm-white-LE...286.c0.m14&_trkparms=66:2|65:15|39:1|240:1318

    Thankyou in advance!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    What I believe you are seeing is the effect of the RED led's lower forward voltage causing it to take an unequally greater share of the current than the remaining white LEDs.

    hgmjr
     
  3. freswood

    Thread Starter New Member

    Nov 20, 2008
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    Oh ok that's interesting.

    Will adding more LEDs make the lamp brighter overall? Or is that not possible.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    The thing to keep in mind is that there is only a finite amount of the power available. It has been optimally designed to power the number of LEDs that are present. Putting more LED's will increase the brightness but the downside will be that the finite power source will be drained more rapidly.

    hgmjr
     
  5. freswood

    Thread Starter New Member

    Nov 20, 2008
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    Currently 2 minutes of cranking the handle provides 50 minutes of light. IFf we add 20 LEDs that means that the power would drain ~four times as fast. Hmm that's not too good is it.

    Other than the limits imposed by the rate of power drainage, do you think there could be any other limits to the number of LEDs we could add to the lantern?
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    The only other limit would be the additional area that you would need to accomodate the additional 20 LEDs and the associated 20 current limiting resistors.

    hgmjr
     
  7. freswood

    Thread Starter New Member

    Nov 20, 2008
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    Would we need resistors if we connected the LEDs in series with the existing LEDs? Like if we piggybacked the new LEDs on the existing ones. It all gets very complicated if we need to buy other components, and it's also hard to see exactly how the components are currently wired.

    Thankyou so much for your advice! You are an absolute champ :)
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
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    A few AAA's.real explicit; just guess 3. Most[some] LED flash lights do not use a resistor with ultra bright LED's. I converted some incadescent bulb flash lights to LED ,3 N cells & 3 AA's no resistor, no problems.
     
  9. freswood

    Thread Starter New Member

    Nov 20, 2008
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    Yeah I've forgotten how many AAAs there are - I don't have the lantern on me right now. Pretty sure it was 3, going by memory. There is in fact a resistor in there, and a few other components (not sure what they are).
     
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