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Can I filter noise from power supply?

Discussion in 'Electronic Basics' started by Peabody, Apr 22, 2009.

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  1. Peabody

    Peabody Guest

    I have 5VDC wall wort power supply, which is a switching supply, and
    it has a typical barrel connector. But I rigged up a small one-inch
    square pc board with a USB socket and a barrel socket mounted on it.
    So, I can use the power supply as a USB power source for charging my
    MP3 player and other things.

    But, when I used this power supply to play music, rather than just
    charge the player, it was clear there's a good bit of noise in the
    supply which doesn't show up when batteries are used.

    I can't regulate the supply because I need the full 5V, but I
    wondered if I can add things to the little circuit board that would
    reduce the noise - such as a series inductor, or a large capacitor
    to ground, or both. Will those things work, or do they screw things

    Pics of this setup are here:

    By the way, for future reference, a dumb USB power supply should
    have its data pins shorted together.
  2. Tom Biasi

    Tom Biasi Guest

    What you say may work, it would help to know more about the noise but that's
    a little more advanced.

  3. Eeyore

    Eeyore Guest

    Was the equipent the mp3 player was connected to grounded / earthed ?
    That'll be the problem. Most low power SMPS wall warts have a Y
    capacitor between primary and secondary sides that gives rises to a
    leakage current at mains frequency and harmonics thereof. That current
    will run to ground along your audio cable and add a nice buzz ( worse
    still if the audio equipment has poor / old fashioned attention to
    internal grounding ).

  4. westom

    westom Guest

    Filtering is the power supply's job. Why do you want to build what
    should already exist in the supply? Simpler and probably just as much
    are power supplies from electronic supply houses such as,,,,, etc. Either buy
    a 5 volt wall wart that says the filtering already exists, or buy the
    inductors and capacitors from the same sources to construct a filter.

    Anyone with minimal electrical knowledge can build this filter.
    Simply create a chain of capacitors between the plus and minus wires;
    with coils (inductors) between each shunt capacitor. Inductors are
    in series. Capacitors connect in parallel. Every "parallel capacitor
    and series inductor" chained together means even better noise

    Need more information? Any basic power supply primer will show this
  5. Eeyore

    Eeyore Guest

    You can add more as required for the task. Dead easy.

  6. whit3rd

    whit3rd Guest

    Try another wall-wart (even if it's an old analog type).
    They're cheap, widely available, and your immediate
    friends and family probably have dozens in the garage
    for things that broke years ago.

    Pickup or ground loop problems can account for this behavior, too; can
    you hear a difference with the ground disconnected at the wall-wart,
    or with the plug reversed in the socket? Can you find a USB cable
    with those ferrite-bead clamp filters on it, and see if that helps?
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