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LED 'ON' light across power switch

  1. Jul 11, 2009 #1

    DaveC426913

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    I wish to attach an LED indicator light across the main power to the 12V battery on my boat. This light will let me know from a convenient location whether I've remembered to switch off the main power.

    How do I wire it up?

    The LED is obviously 3V.
    The light would only be ON if the power switch is on.
    If the LED fails, I don't want it to kill the power.
    Do I simply splice it in parallel with the hot side of the battery wire?
     
  2. jcsd
  3. Jul 11, 2009 #2
    The LED is not "obviously" 3V at all! And this is not how you calculate LED circuits. If it's a vanilla LED without internal resistor, then it has a voltage drop of 1.7 V and the remaining Voltage over the pre-resistor will be used to adjust the current.
    I would try a 800Ohms resistor in series with the diode which would give around 15mA with 12 V or a 700Ohms one if it is indeed specified to run directly on 3V. But you would do best to check the specs of the diode to see what currents you can use. If your diode burns then it shouldn't shortcut, but even if it did it will not be a problem.

    http://www.theledlight.com/LED101.html [Broken]
     
    Last edited by a moderator: May 4, 2017
  4. Jul 11, 2009 #3

    Danger

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    You could also (as I would) just use a small 12V incandescent bulb such as from a car dashboard wired in parallel.
     
  5. Jul 11, 2009 #4

    negitron

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    (12 - 1.7) / .015 = 686.7 Ohms.

    A more conservative drive level for typical GaAs LEDs (which normally have a 20 mA max forward current) is 10 mA, so use a ~1k resistor.
     
  6. Jul 11, 2009 #5

    dlgoff

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    You could just go to Radio Shack and purchase a http://www.radioshack.com/family/index.jsp?categoryId=2032293" which has a current limiting resistor built in.
     
    Last edited by a moderator: Apr 24, 2017
  7. Jul 11, 2009 #6

    vk6kro

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    How do I wire it up?


    You would wire the resistor in series with the LED (as above) between the cold side of the switch and ground.
    When you apply power to the load, you would also apply power to the LED / resistor combination.

    LEDs have to be the right way around and usually have one lead longer than the other and this is the positive lead.
     
    Last edited: Jul 11, 2009
  8. Jul 11, 2009 #7

    DaveC426913

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    Last edited by a moderator: Apr 24, 2017
  9. Jul 11, 2009 #8

    DaveC426913

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    Got it.

    I feel embarrassed, having taken basic electronics in night school. Seem to have forgotten much of it.

    Yes. This much I knew.


    Thank y'all.
     
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