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Power from one or both of 2 sources

  1. Mar 28, 2012 #1
    Hi all,

    Apologies if this is the wrong place to post this but I can see lots of similar questions on this forum which aren't a 100% match for my question.

    I want to add a new interior light to my car.

    I want it to be on either when the headlights are on (power from the illumination circuit of the radio) or when the existing interior lights are on. It is possible that both could be on at the same time.

    Both sources are (I assume) +12v

    The new light will be grounded to the car chassis via the cig lighter -ve.

    Is this possible? How do I ensure that the 2 power sources do not affect eachother? (i.e. I don't want the existing interior light coming on when I switch the headlights on!)

  2. jcsd
  3. Mar 28, 2012 #2


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    Staff: Mentor

    Hi doowruc! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    One power diode in series with each lead from each source should work.
    Last edited by a moderator: May 5, 2017
  4. Mar 28, 2012 #3
  5. Mar 28, 2012 #4
    And is this the basic circuit?

  6. Mar 28, 2012 #5
    What would happen if both were live? would the new light then get too much current or voltage?
  7. Mar 28, 2012 #6


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    Staff: Mentor

  8. Mar 28, 2012 #7
    No, it will draw just as much current it requires. Nothing wrong will happen.

    (As a side note: Calculating the current shares of the lines, when both line is live, is a difficult and quite unpredictable thing. Depends upon Diode Imperfection and Line resistances.)
  9. Mar 28, 2012 #8

    jim hardy

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    Gold Member

    You might check something on the interior light before getting too far along.
    In old days they worked by completing a circuit to chassis, on side of bulb opposite the battery. The little buttom in doorjamb made that connection.
    On those automobiles your interior light would have 12V on both sides when OFF and on one side only when ON.

    Nowadays they are controlled by the computer and i dont know which wire the computer switches, could be either the "hot" or the "return".
    The button in doorjamb apprises computer there's a door open and computer decides whether to light light and for how long.

    So get your meter and check that interior light socket to see which side changes state when it comes "on".

    old jim
  10. Mar 28, 2012 #9
    Thanks Jim. The kit I am buying comes with specific instructions for my car and wiring into the interior light circuit so I hope that this will be covered!
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