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USB device interfering AM radio

Discussion in 'Electronic Basics' started by Man-wai Chang, Mar 20, 2011.

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  1. I just discovered this. And I also found that the AC to USB power
    adaptor does not have a metal ground pin.

    So would this AM interference be gone after getting a better AC to USB
    power adaptor?

    --
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  2. Jamie

    Jamie Guest

    If that is where it's really coming from? You could have a bad insulator
    out side your place, on the lines or, some equipment else where causing
    it. You can tell better if you operate it via battery. If the noise
    goes away, then you need a line filter for your plug.. Other than that,
    if that does not take care of it, you have a cheap set up.

    If it really is coming from your outlet power, you could use a 1:1
    transformer to isolate it. I've found that to work very good. But I am
    thinking that maybe you are operating a cheap switching supply and the
    radio is picking that up..

    Jamie
     
  3. it. You can tell better if you operate it via battery. If the noise
    Could the outside mesh shield of USB cable contain the noise?

    Line filter? Is it a chip or a circuit? I am not a EE guy. :)
    It's just a cheap AC to USB power adaptor.

    --
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  4. This is the simple circuit.

    AC ---> AC-to-USB power adaptor
    ---> 3-feet USB cable (5V)
    ---> 2x100ohm resistors
    ---> 2 LEDs in parallel

    The AM/FM radio is about a feet away the circuit.

    --
    @[email protected] Might, Courage, Vision, SINCERITY.
    / v \ Simplicity is Beauty! May the Force and Farce be with you!
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  5. ehsjr

    ehsjr Guest

    From your description, I assume your circuit is like this:

    +5 ---+---[R]---[LED1]---+---Gnd
    | |
    +---[R]---[LED2]---+

    If your adapter is truly providing 5 volts, and those are
    standard LEDs, you may cook them. They are running near
    the max with 100 ohm resistors.

    It would be better for the LEDs if it was like this:

    +5---[R]---[LED1]---[LED2]---Gnd

    That would cut the LED current from about 32 mA to about 15 mA.
    and they would still be plenty bright.

    As to the noise, I would not be surprised to find crappy
    electronics (cheap USB adapter or whatever) causing it,
    so replacing the adapter with a better one might cure it.

    Ed
     
  6. +5 ---+---[R]---[LED1]---+---Gnd
    No. It's this:

    +5 ---[R=100ohm]--+--[LED1]--+
    | |
    GND ---[R=100ohm]--+--[LED2]--+


    --
    @[email protected] Might, Courage, Vision, SINCERITY.
    / v \ Simplicity is Beauty! May the Force and Farce be with you!
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  7. ehsjr

    ehsjr Guest

    No, it can't be like that - there would be no current
    through the LEDs. The way it's drawn, the LEDs have a
    short circuit across them (the vertical line between the
    resistors and the LEDs). If that line was not there,
    and the LEDs are in series with the correct polarity,
    it would work fine. You'd have about 14 mA through the
    LEDs, and they would shine nicely. They would not be
    at full brightness, but they would not be too dim.

    Maybe it's really wired like this:

    +5 ---+---[100R]---+---[LED1]---+
    | | |
    +---[100R]---+ |
    |
    Gnd -------------------[LED2]---+

    That would give you about 28 mA through the LEDs which
    is still higher than I like, but should be ok with
    LEDs rated for 30 or 35 mA max.

    Ed
     
  8. +5 ---+---[100R]---+---[LED1]---+
    My mistake drawing the diagram. Thanks.

    +5 ---[R=100ohm]--+----+
    | |
    LED1 LED2
    | |
    GND ---[R=100ohm]--+----+

    --
    @[email protected] Might, Courage, Vision, SINCERITY.
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  9. Guest

    Not a good idea. The LEDs are unlikely to share current evenly. Use one
    ballast resistor for each. It doesn't matter which side it's on.
     
  10. Not a good idea. The LEDs are unlikely to share current evenly. Use one
    Already soldered, don't wanna change it. Next project... :)

    http://img843.imageshack.us/i/gn002withleds.jpg/

    --
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    / v \ Simplicity is Beauty! May the Force and Farce be with you!
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  11. Guest

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