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Switching wall-wart noise

Discussion in 'Electronic Basics' started by Bill Bowden, Oct 29, 2004.

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  1. Bill Bowden

    Bill Bowden Guest

    How much noise should I expect from a 5 volt 800mA, switching,
    regulated wall-wart?

    The wall-wart is powering a PIC processor and some other
    logic and works well when plugged into a surge protected
    power strip, but causes erratic operation when plugged
    into a regular unprotected AC socket.

    The processor inputs are driven with schmitt trigger
    inverters (74HC14) so I would expect the noise rejection
    to be about 2.5 volts. I can see about 150mV of noise
    on a 15mHZ scope, but apparently the noise level exceeds
    2.5 volts that I can't see.

    I added a 1 ohm resistor in series with the +5 and a 0.1uF
    cap from other side of resistor to ground which seems to
    cure the problem but I don't see much change in noise level
    on the scope.

    The load is 300mA, so what would be an optimum RC filter
    to use? Or should I replace the wall-wart?

  2. I would replace it. I use the Elpac WM series of linear regulated
    wallmounts. I use them to power custom test equipment used to measure
    noise, so you know they have to be quiet. Digi-Key & Mouser carry

    Steve J. Noll | Ventura California |
    | The Used High-Tech Equipment Dealer Directory
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