What is wrong? (Leds/DPDT)

Discussion in 'The Projects Forum' started by Banquo281, Nov 19, 2011.

  1. Banquo281

    Thread Starter New Member

    Nov 19, 2011
    2
    0
    Hi I'm a novice and don't know what the heck is wrong here, the batteries keep melting lol. I am trying to wire 4 leds in parallel to a on/off/on DPDT switch just to turn on when the switch is up and off when in the middle or down, here is how I have it set up the pink 3v is the hot wire from,two AA batteries in a holder, the black is the minus:

    [​IMG]

    Any help is greatly appreciated, I am anticipating a doh! moment:p...
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    You need a current limiting resistor.

    LEDs look like a short circuit once their forward voltage has been reached. The Forward voltage varies, but the current is usually 8mA for colored lens LEDs and 18mA for clear LEDs as a rule of thumb.
     
  3. Banquo281

    Thread Starter New Member

    Nov 19, 2011
    2
    0
    I got it, I just didn't understand the switch. Negative from the leds goes to the bottom middle, negative from the battery goes to the bottom left... doh!

    Melted batteries and plastic stinks!:rolleyes:
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Whoops - you have wired the switch so that when you turn it on, you create a dead short across the batteries! :eek:

    Tell us more about these LEDs. What is their typical Vf at what current? The datasheet or package they came in should tell you that.

    You need to use current limiting resistors in series with each LED.

    The negative (black) wire from the battery pack goes to all of the cathodes of the LEDs (the shorter leads, or the side of the LED that has the flat spot in the rim).

    The positive wire from the battery pack goes to either the middle switch terminal or the left side terminal. another wire goes from the other of the above two terminals to one side of the resistors for the LEDs. The other side of the resistors go to the anodes of the LEDs (the longer leads).
     
  5. BrainFog

    Member

    Jan 24, 2011
    122
    4
    I many be incorrect but at a mear 3v is a resistor needed? LEDs are current devices however at only 3v the current drawn by the LEDs would be minimal without a resistor. Of course due to the non linear relationship between current and voltage driving an LED at 3.3v would very much need a resistor to prevent the LED drawing too much current and burning itself out. Not to mention the fact the resistance within the AA batteries. I am assuming you are using LEDs rated for 3-3.3v here such as green, blue or white.
     
  6. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    You always need a current limiting resistor when working with LEDs. Suppose that those green LEDs drop 2.5V across them and the battery resistance is 2Ω. With I=.5/2=.25A running constantly through your LEDs you are sure to fry them, possibly shorted and then deplete your battery in a very short time.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Even at a mere 3V, there could be a real problem with some LEDs. The OP has drawn his LEDs as green, but not all greens are equal. Here is a chart applying to certain LEDs - but note that others may differ. http://www.oksolar.com/led/led_color_chart.htm

    In general, it is always best to arrange for some means of current limiting when using LEDs, or the current will be too unpredictable.

    The OPs real mistake though seems to be in connecting the switch in parallel with the batteries. I can remembering doing that with a "Merit" "batteries and bulbs" set that I got for my birthday when I was a kiddie. The batteries were too feeble to burn anything up, but of course they ran flat fast. That soon taught me where a switch should go!

    @Banquo281: Do you understand that the switch needs to go in series with the load?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here, this should help you visualize what I was talking about:

    [​IMG]

    As far as the resistors; I don't know what value to suggest, as you have not told us what your LED's specifications are. We need to know the typical Vf @ current.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Maybe for tiny surface-mount LEDs.
    A normal size LED is 5mm in diameter and its typical current is 20mA with 30mA as its absolute max.
    Whether the lens is colored or clear has no affect on the allowed current.
     
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