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usb powered, flashing LEDs

Discussion in 'Electronic Basics' started by Branden Faulls, Aug 26, 2005.

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  1. I am trying to build a simple circuit with 2 flashing LEDs drawing power
    from a USB power source. I am using 5mm LEDs with an integrated flashing
    circuit from Maplins.

    The spec sheet for the LEDs says they have an operation voltage of 3.5 -
    14V and a forward current of 20mA each.

    Since these LEDs can operate between 3.5 - 14 then I assume that they will
    be fine at the USB's 5V 500mA. Is that right?

    Similarly, since they can take variable voltage, will I need a resistor in
    this circuit? My assumption was no.

    Is the following a safe way to wire these?

    V+ -+-[LED]--gnd

    How safe will the USB host bus be? I don't want to fry a motherboard with
    my first project!

    I'm a total beginner and I have tried to pick up as much from the other
    LED posts in the group, but I'm not confident enough to build this without
    some advice. I am especially confused how the 20mA
    of current for each LED relates to the 500mA of the USB.

    Thanks for any help

    Branden Faulls
  2. Ban

    Ban Guest

    If you get familiar with the units and specs, you will notice that the
    current spec is a max. value, which shouldn't be exeeded. The voltage OTOH
    is always present and can be measured with a meter. Look at a battery, even
    when not connected it has the voltage on its terminals. The current is the
    number of electrons/s that is pushed by the voltage through the circuit.
    So if you connect your LEDs(in parallel) they will draw the specified
    current, since they have a built in current sink.
    If you use normal LEDs you will need a particular resistor for each applied
    voltage, but with the built-in sink you do not need to care. Be just aware
    that the USB can only deliver 100mA, or else you have to signal the
    controller that your circuit needs more current(up to 0.5A).
    So don't worry, just be sure the LEDs are connected in the right polarity,
    the reflector is usually tight to gnd.
  3. OK, so the voltage is essentially irrelevant because the LED can handle
    the range? Is the current demand cumulative? Meaning, the USB bus can
    deliver 100mA, but my LEDs (in parallel) are only drawing 40mA? Would the
    demand be different if they were wired in series?

    Thanks for the RAPID response!

    Branden Faulls
  4. PeteS

    PeteS Guest

    Whether they would operate in series depends on the internal circuitry,
    although they probably would, but then you would need 7V (min) (3.5V
    min per device) to power them, but the current would then be the same
    for both.

    You won't get 7V from USB, though - it is specified that the device at
    the end of the cable always gets at least 4.4V under worst case
    conditions. It is nominally 5V.


  5. John Fields

    John Fields Guest

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