Christmas Lights in a Car

Discussion in 'The Projects Forum' started by JellyWoo, Dec 10, 2013.

  1. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    I bought a set of Christmas lights. They are powered by 3 AA batteries, for a total of 3.6 volts. I would like to use these in a car by connecting them to a car charger like this one. The adapter uses a microUSB cable and outputs 12 to 14 volts (I think?). Is it possible to somehow remove the battery pack from the lights and connect it to the microUSB cable and add some resistors? I have a good soldering background, so that wouldn't be a problem, but I'm a little iffy with the electronics.
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
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    USB outputs are by definition 5V. You can usually modify the DC/DC converter to output the voltage you want but you might do well to simply put a rectifier diode in series to drop the 5V down to 4.4V.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    3 x 1.5 = 4.5..
    Its probably close enough to just use the charger directly without a diode even..
     
  4. Experimentonomen

    Member

    Feb 16, 2011
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    Atleast where i live, its illegal to put lights in a vehicle, though many truckers but these christmas light stuff on the dash anyways.
     
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  5. KJ6EAD

    Senior Member

    Apr 30, 2011
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    True, considering the fact that new alkaline cells often measure as high as 1.7V each without a load.
     
  6. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    Disregarding the legality of the lights, what I'm getting at is that i can solder the lights directly to the USB cable without any diodes or resistors?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    I'd add the diode or a low value resistor. Without it, your string will likely work perfectly for a while but it's life may be shortened. It's hard to tell without more details, but if it was designed for 4.5V, a 10% increase in voltage and current might stress it. If you don't care too much if it burns out early, just go for it.
     
  8. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    I'd prefer for it to last a while. What details would you need/what diode or resistor would I need?
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Your diode needs to be rated for the current drawn by the lights, plus a nice safety factor. I'm guessing a 1A diode would be fine, for instance 1N400x. Readily available at Radio Shack. But do check the current drawn by your lights.
     
  10. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    How do I check the current drawn by the lights?
     
  11. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    (Sorry I'm a noob at this.)
     
  12. #12

    Expert

    Nov 30, 2010
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    Get a $4 digital multimeter at Harbor Freight and measure the current.
     
  13. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    Okay, so I know I need a resistor to limit the current. Do I also need a voltage regulator to limit the voltage? What is the purpose of the diode? I'm a little confused.
     
  14. #12

    Expert

    Nov 30, 2010
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    It's either/or. A diode will use up about .6 to .7 volts and thus lower the voltage a bit just by existing in the circuit. A resistor would have to be calculated by measuring the current so it uses up the right amount of voltage.

    Opinion: Don't buy the meter, don't calculate the resistor, just stick a diode in and be done with it.
     
  15. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    Why do you suggest not using a resistor?
     
  16. #12

    Expert

    Nov 30, 2010
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    Because you have to buy a meter and measure the current to calculate the resistor size.
     
  17. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    So if I did have access to a meter, it would be ideal to use both a diode and a resistor? Would they be connected in series?
     
  18. KJ6EAD

    Senior Member

    Apr 30, 2011
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    No, not both; just use a diode.
     
  19. JellyWoo

    Thread Starter New Member

    Jun 2, 2013
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    Why not both? Don't the two limit different things?
     
  20. KJ6EAD

    Senior Member

    Apr 30, 2011
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    That's actually a good question. They limit the same thing (current) in different ways.

    http://en.m.wikipedia.org/wiki/Diode

    A diode drops a certain voltage dependent mostly on it's construction materials but a resistor in a series circuit drops a voltage that varies as a function of the current passing through it. This current is dependent on the properties of other circuit components which are in the present case only partially known.
     
    Last edited: Dec 11, 2013
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