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How to make an LED light regardless of polarity?

Discussion in 'Misc Electronics' started by Joe Strout, Sep 10, 2004.

  1. Joe Strout

    Joe Strout Guest

    Please excuse the newbie question:

    How can I make an ordinary LED light up regardless of the VDC polarity?

    I can imagine a way to do it involving four additional diodes, but
    that's more than I have room for in this application. Is there some
    clever way to do it with only two (or less)?

    Thanks,
    - Joe

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  2. Clarence

    Clarence Guest

    Just put two leds back to back so they share the same current limiting.

    Works!
     
  3. Clarence

    Clarence Guest

    No way I know of. You do know you can get dual Leds in one package don't you?
     
  4. Joe Strout

    Joe Strout Guest

    Sorry, I wasn't clear: I need to light *one* LED regardless of polarity.
    Two won't do; the spec allows for just one.

    Thanks,
    - Joe

    ,------------------------------------------------------------------.
    | Joseph J. Strout Check out the Mac Web Directory: |
    | http://www.macwebdir.com |
    `------------------------------------------------------------------'
     
  5. R.Lewis

    R.Lewis Guest

    I think you mean in parallel (so they share the same current limiting
     
  6. Ryan Wheeler

    Ryan Wheeler Guest

    what is your source voltage?
    use a bridge rectifier and connect the LED to
    the + and - terminal.
     
  7. John Fields

    John Fields Guest

     
  8. Phil Hobbs

    Phil Hobbs Guest

    You could use a two-colour LED--but then the colours would be different in
    different polarities.

    Cheers,

    Phil Hobbs
     
  9. Joe Strout

    Joe Strout Guest

    9 VDC, though of course I use a current-limiting resistor.
    Thanks, that's exactly what I was looking for!

    Best,
    - Joe

    ,------------------------------------------------------------------.
    | Joseph J. Strout Check out the Mac Web Directory: |
    | http://www.macwebdir.com |
    `------------------------------------------------------------------'
     
  10. John Fields

    John Fields Guest

     
  11. Zak

    Zak Guest

    And four diodes are usually smaller than a bridge rectifier. Though I
    haven't looked at SMD models that may be out there.


    Thomas
     
  12. Try to find those two-color LEDs that have two different color
    LEDs in one LED package, those two are installed on different
    directions. Installing such LED would make the LED to light
    on different colors on different polarity power, and third
    (combination of those first two) when AC is feed to it.
    This could be one option to consider if you specification
    allows it.
     
  13. Joe Strout

    Joe Strout Guest

    But it's one component instead of four. Is it so amazing that I have
    room for one more component (possibly even two), but not four? Or have
    I missed something?

    ,------------------------------------------------------------------.
    | Joseph J. Strout Check out the Mac Web Directory: |
    | http://www.macwebdir.com |
    `------------------------------------------------------------------'
     
  14. Zak

    Zak Guest

    An 1n914/1n4148 with leads is very small and 4 of them mounted behind
    the LED will make it a bit longer but not wider.


    Thomas
     
  15. John Fields

    John Fields Guest

    ---
    I think you've missed something.

    You said, in essence, that you didn't have room for four motorcyles,
    but that a Honda Civic would be fit in the same volume.

    That is, not that there aren't any, but I haven't seen any full-wave
    bridges in packages smaller that four tiny diodes, and they're both
    four diodes. There is another possibility, though, and that's that
    there are lots of tiny dual diodes out there, so you could make up
    your bridge that way if you could fit two of them in there and the
    full-wave bridge wouldn't.
     
  16. Have a look at the size of the components mentioned. A diode bridge
    requires (much) more space then four small diodes (1N4148 for instance.)

    petrus bitbyter
     
  17. nospam

    nospam Guest

    The Siemens BAV99S has 4 doides which can be bridge connected in a 6 pin
    SOT-363 package which is about 2mm square.
     
  18. Joe Strout

    Joe Strout Guest

    Thank you all for your input. I've already got some bridge rectifiers
    on order from Mouser, but if none of them fit the task, I'll try four
    separate diodes (or two dual diodes) instead.

    Thanks,
    - Joe

    ,------------------------------------------------------------------.
    | Joseph J. Strout Check out the Mac Web Directory: |
    | http://www.macwebdir.com |
    `------------------------------------------------------------------'
     
  19. John Fields

    John Fields Guest

     
  20. R.Lewis

    R.Lewis Guest

    Sent some gear for EMC testing.
    The leds illuminated in one of the RF immunity tests.

    Although I never tried it I guess they would illuminate whichever way round
    they were (not) connected.
     
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