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Flashing LED circuit

Discussion in 'Electronic Basics' started by nick, Sep 13, 2005.

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  1. nick

    nick Guest

    Dear All,

    Please can you advise on the wiring of a flashing LED circuit.

    I want to connect 5 off 10mm red flashing HE LEDs (RS no.262-2977
    Kingbright no.L-816BID) to a 9v DC supply.

    It says on their spec sheet:

    VF 9-12v
    IF 22mA
    IF max 38-56mA
    Power dissipation 310mW
    VR max 0.5v

    Should I wire them in parallel?

    I believe I cannot wire them in series as the voltage drop across each
    LED will be too much for the supply won't it?

    It says in the catalogue that series resistors are not required and
    cannot be used to drop a higher voltage.

    Any advice will be gratefully received!
     
  2. John Fields

    John Fields Guest

     
  3. Mark VB

    Mark VB Guest

    It seems that your LEDs have built-in current-control. So wiring them
    in parallel should do.


    HTH,
    Mark Van Borm
     
  4. nick

    nick Guest


    Thanks for your advice guys!

    I was asking these questions because having constructed a few of these
    circuits in parallel arrangements for use at work, some of them have
    packed up completely. Presumably one LED blew and caused the others to
    blow.

    Is there any way I can prevent this from happening again? Adding a
    resistor to each LED? If so what size would you recommend?

    Thanks in advance for your help!
     
  5. Generally, each LED should have a resistor in series to limit the
    current through the LED. You will have to look at the spec. sheet for
    your particular LED, but most are looking for 2-3V and 20 mA operating
    current. The size of the resistor depends on supply voltage, and will
    have to be calculated accordingly. Paralleling LED's is not a good idea
    without a series restor on each LED.

    Nels
     
  6. Mark VB

    Mark VB Guest

    Probably, the polarity was reversed when they blew, so adding a resistor
    won't help much.


    Mark VB
     
  7. Rich Grise

    Rich Grise Guest

    This is probably not the mechanism. The LEDs in your original post
    DO NOT need a series dropping resistor. I'm guessing that you used
    ordinary LEDs in parallel, with only one dropping resistor. This
    will NOT work, because they have different forward voltage drops
    and one will hog all of the current until it blows, and then you
    get a cascade effect as the rest of the LEDs are overdriven by
    the lower-value resistor.
    THESE LEDS:
    do NOT need any resistor - in fact, you said yourself that the spec
    sheet specifically says not to use one.

    Any other "standard" LED DOES need a series resistor, because when
    the forward voltage reaches 1.2V (or whatever the Vf), there is
    no mechanism to limit the LED's current other than the series resistor.
    This might be helpful:
    http://focus.ti.com/lit/an/slyt084/slyt084.pdf

    Have Fun!
    Rich
     
  8. Jasen Betts

    Jasen Betts Guest

    those flashing leds detailed above are designed to operate off 9v DC. as
    long as they are connected correclty. Hook them up backwards and they'll
    die pretty quickly. if you're operating them off batteires I'd put a diode
    (eg 1N4001) in series with the group of flashing leds to protect against
    that.
    not for those LEDS.

    Bye.
    Jasen
     
  9. nick

    nick Guest

    Thanks for your helpful advice everyone!

    Nick
     
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