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Thread: Drawing enough current?

  1. #1

    Default Drawing enough current?

    After doing some conversion on my own, I'm wondering why anyone would bother using a Star III with the MR Force FX. The Star III is designed to run on 1000 to 1400mA. The MR battery pack supplies a forward voltage of 3.9v average, but after the sound module, it's only supplying around 500mA. That's enough for a Star/O, but in my opinion, a waste of a Star III.

    I notice, if you disable the sounds, the Star III will brighten considerably. Anyone else notice simialar results?

  2. #2

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    I have noticed the same power problem. When I hook up a Lux III to the MR board I barely get 250 mA (this was a Blue LED)!! Other people have resorted to wiring the LED directly to the battery through a relay, that is presumably turned on when the saber is activated. I have thought about replacing the 3 AA's with two 3.6V rechargeable Lithium Ion batteries, however I don't know if the MR board can handle the higher voltage (7.2 V instead of the 4.5 V provided by 3 AA's).



    MythranDir

  3. #3
    Owner of the Custom Saber shop Strydur's Avatar
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    Default

    well you guys got me curious and since I dont know how to use my multimeter worth a crap I did a little experiment.
    [img]../scripts/4aa.jpg[/img]
    This is a amber led running off of 4aa with the proper resistor.
    [img]../scripts/3aa.jpg[/img]
    This is the same setup running off 3aa in a MR with sound going.

    Like I said I am not sure how to measure amperage but as far as I can see its running just fine off of 3aa with the sound playing.

    Tim

  4. #4

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    Tim,

    I have a few more thoughts about the issue of current and brightness.

    First, the amber and red Luxeons operate at a forward voltage of ~2.95 V compared to the green and blue LED's that operate at a forward voltage of ~ 3.9 V. Empirically, I have found that I can get higher currents from 3 AA's for the red and amber LED's than I can for the blue, cyan, and green LED's. I was in the middle of documenting this more carefully when the fuse in my multi-meter blew out. Once I get it replaced I will try to complete these measurements.

    Second, the eye responds to brightness 'logarithmically'. What this means is that if you increase the luminous output of the LED from say 10 lumens to 100 lumens, the eye will easily see this 10-fold increase in brightness; however, the eye will be much less sensitive to an increase in brightness from 100 - 200 lumens. This characteristic is what enables the eye to form an image of a scene even though the brightness of the various scene objects can vary by many orders of magnitude. This leads me to believe that doubling the LED current from say 250 mA to 500 mA might not be worth the effort, since to the eye the LED brightness may not be perceived to increase by a factor of 2. I have not checked this out myself, and the extent to which an increase in current increases its perceived brightness may depend on the LED color.

    In any event, I plan on doing some experiments to check these things out more carefully.

    MythranDir

  5. #5
    Owner of the Custom Saber shop Strydur's Avatar
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    Default

    I will have to try it with one of the 1000mA led's. So far I have only tried the red and amber. That would make sense since the 1400mA ones require less voltage. You are correct that the eye can be decieved but what I was showing is that with 4aa and the proper resistor I am getting the proper mA out of the batteries and it looked exactly the same running with the 3aa and the sound going. But, like you said it might be totally different with the 1000mA led's.

    Tim

  6. #6
    Owner of the Custom Saber shop Strydur's Avatar
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    Well I finished my sons anakin just in time for halloween. I am still not having the same problem you guys had.

    [img]../scripts/blueled4aa.jpg[/img]
    Blue led running at 1000mA off of 4aa.
    [img]../scripts/blueled3aa.jpg[/img]
    Same led but running off of 3aa in the MR with sound going.

    Tim

  7. #7

    Default

    Tim,

    It looks beautiful! I think I will do the same comparison you did (4 AA's with a resistor vs. 3 AA's using the MR board) and repeat my current measurements.



    MythranDir

  8. #8

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    Well, actually, my point was that the Star/O and the Star III looked equally bright running off the MR board, as they're both drawing about 500mA or less. The only difference is that the Star III is twice as expensive, after buying the optics. So, why bother with the Star III?

  9. #9
    Owner of the Custom Saber shop Strydur's Avatar
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    If I am running a red LED that has a forward voltage of 2.95 and I am using a 2.2ohm 5w resistor and 4AA then the LED is running at 1400mA not 500mA. If I then run the same led off of the 3aa inside of a MR hilt and it is just as bright then how can you say I am only getting 500mA out of an MR hilt? That just doesnt make any sense. Can you explain where my thinking is wrong since you keep stating that I am supposedly only getting 500mA out of the MR hilt?

    Tim
    The Custom Saber Shop

  10. #10

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    You really can't know for certain how much current your Luxeon is really drawing without testing it (under load) on a good multimeter. This is exactly what I've done, and I'm testing less than 500mA off the MR board when it's under load, using alkaline batteries.

    Judging by our eyes is not terribly precise, as discussed earlier. You can't see the difference between the 4 cell and 3 cell configuration. I can't see the difference between a Star/o and a Star III when run off the MR board.

    The low amps and lack of visible difference leads me to believe that there may be no point to using a Star III over a Star/o.

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