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LED alternatively powered by different voltages

  1. Dec 3, 2016 #1
    Hello this is my first post here and I am not very good at electric circuits, so I'm not familiar with the jargon either, I'll describe the practical use of the circuit I'm trying to make, basically I have a LED underneath the pedals in my car which will be permanently connected to something around 6 volts which will make a faint light but when I turn the overhead dome light I want the LED under the pedals to receive the full 12 volts to have maximum intensity. I have made a small scheme which may explain the problem better, thank you and I hope you understood my problem despite my difficult description. My question is: Will this work? plaf.png
     
  2. jcsd
  3. Dec 3, 2016 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF. :smile:

    What you need is to "Diode-OR" your power sources into your LED circuit (which will contain a current-limiting resistor). Something like the figure below. In your specific case, you will feed your 6V supply to the resistor+LED through one of the diodes. Your 12V source you could either feed through a diode, or just connect it directly to the resistor+LED, since it is switched, and always a higher voltage than your 6V source.

    So you would only have one diode in your circuit, with the 6V supply connected to the anode of the diode, and the switched 12V source connected to the cathode of the diode. You would size the resistor to allow about half current to the LED with the 6V supply, and full current to the LED with the 12V supply. Makes sense? Can you post a link to your LED datasheet?

    http://www.learningaboutelectronics.com/images/Diode-OR-gate-circuit.png
    Diode-OR-gate-circuit.png
     
  4. Dec 3, 2016 #3
    Thank you for your reply, I don't have a datasheet for my led, however could you please explain what would happen if I apply the diagram I made?
     
  5. Dec 3, 2016 #4

    berkeman

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    If the little arrow thingies in your diagram are diodes, then all you are missing is the series current-limiting resistor. :smile:
     
  6. Dec 3, 2016 #5

    rbelli1

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    If the LED in question is a 12V LED then it already has the resistor included. Consult the datasheet from the LED. If you put an LED across 12V and there is not resistor you will get a brief very bright/wrong color flash. This is often accompanied by one of the most awful smells in existence. LED's contain brown smoke unlike the more common blue smoke in other electronics.

    You can make the 6V operation dimmer by adding a resistor in series with the 6V source.

    BoB
     
  7. Dec 4, 2016 #6
    The LED in question is one made specially for cars so it can be conetected to 12v without any issues, another question I have would be if both sources are on (6v and 12v) would their output add and result 18 volts? Even if so the LED resistor should be able to handle this? For now I will just tie the cathodes of the diodes with the wire to the LED and the anodes to the separate power sources (6v & 12v) I believe that would work. Thank you all very much, this issue was 24/7 on my mind. :)
     
  8. Dec 4, 2016 #7

    rbelli1

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    If both are on then the 6V leg diode will be reverse biased and that source will do nothing.

    BoB
     
  9. Dec 4, 2016 #8

    berkeman

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    Staff: Mentor

    :smile:
     
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