MicroPuck and 5mm Leds

Discussion in 'The Projects Forum' started by Bombadil, Apr 1, 2011.

  1. Bombadil

    Thread Starter New Member

    Apr 1, 2011
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    In the past I've always used individual resistors with 5mm leds but the Micro Puck has me intrigued for a project involving leds in a guitar fretboard.

    If I have 12 leds (5mm) that can handle 30ma forward current and 3.5 forward voltage, could I simply wire them up in parallel and wire a 350ma micropuck between them and a 2AA battery pack, would it be that simple? Any help is much appreciated :D
     
  2. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    I'd put a small value resistor in series with each LED. Variations in LEDs mean that in parallel some of them will draw more than their fair share of current.
    I just did a simulation using a constant current source and the forward voltages of a recent batch of LEDs I used. With no resistors the LED currents were from 19.5 to 39 ma. With 27 ohm resistors in series with each the current range was 28.1 to 30.9 ma.
    With 18 ohm resistors it was 27.5 to 31.5 ma.
    The higher value the resistor the more power is wasted but the better the regulation.

    With resistors:
    [​IMG]

    No resistors:
    [​IMG]
     
    Last edited: Apr 1, 2011
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  3. Bombadil

    Thread Starter New Member

    Apr 1, 2011
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    Thanks, it looks like resistors are still in the equation then :rolleyes: I was hoping to do less soldering hehe.

    I was wondering what value of resistor though, the leds are rated for 3.5 voltage, the 2 aa batteries will supply 3 volts to the micropuck. Something with a very small value?
     
  4. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    The output of the micropuck can be up to 8V according to the documentation. It will only rise as high as it needs to so that it is supplying 350mA.
    I think that 27 ohms should be good.
    If 30mA is the maximum for the LEDs then their lifespan may be reduced slightly if any are a little over or the ambient temperature is high. It's usually best to design for a little lower to increase reliability. Adding a few more LEDs should do it.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    EEK! Your schematics are a negative with yellow dots all over the wires.
     
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