How do reversible polarity LED automotive lights work?

Discussion in 'General Electronics Chat' started by Razor Concepts, Jul 21, 2015.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
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    Recently I was looking at some LED replacement bulbs for automotive lights, they look like this:
    [​IMG]

    These bulbs are reversible, so you can plug them in either way and they still light up. Here's a quick schematic I drew after looking at one of them:
    [​IMG]

    There appears two be two small components in transistor packages (SOT-23-3) that ensures the LED gets +12v no matter which way 12v is applied. How does this work? What are the two components that can do this? Thanks
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    There is a full-wave bridge rectifier inside...
     
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  3. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I would've thought that the fact that the LED's contacts are offset to one side would have something to do with it... no matter how you rotate it, the contacts would also land in the same polarity.
     
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  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    A SOT-23-3 package is just big enough to fit two diodes, get two packages and you can put together a full wave bridge rectfier.

    These dual diodes come in either common anode or common cathode (or even anode to cathode) varieties. If they have part markings I would be gobstoppered to learn they share one common number.

    By using two devices you can split the power into two packages as only 1 diode in each will conduct while the other is off. A single bridge rectfier device would have all the power in one package.
     
  5. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
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    Ah sorry, that picture is not good, it's not the exact LED I have with me. The LED I have has symmetric contacts, and I can directly apply 12v in either polarity and it still lights up.

    So here is the circuit with the rectifier which looks like it will work:

    [​IMG]
     
  6. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    :confused: well that explains a lot...
     
  7. #12

    Expert

    Nov 30, 2010
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    I was going with Martinez on the displaced connectors until you said that wasn't the true picture. If the inputs really are interchangeable, the FWB trick is about as old as transistors.
     
  8. tom_s

    Member

    Jun 27, 2014
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    like MikeML typed, they do have full bridge rectification (or 4 diodes) current draw is negligible thats why they aren't bright enough.

    just pulled apart something similiar to this (12v 3 x 1w led - is bright enough for a tail light during the day) -

    [​IMG]

    got 4 diodes and little micro buck converter :)
     
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