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Noise filter for ATX power supply

Discussion in 'Electronic Design' started by daveatfernie, Apr 26, 2007.

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  1. daveatfernie

    daveatfernie Guest

    Hi All,

    I use a number of ATX power supplies as bench supplies. However the
    voltage changes when the load changes and there is quite a bit of
    noise accross all frequencies on it. I plug in and disconnect devices
    from the PSU randomly.

    I'd like a way to clean up the noise from the supply. Can anyone
    suggest the circuit to make a nice clean powersupply from it. I would
    like to get rid of as much noise form as many frequencies as

    Many thanks in advance

  2. Mike Monett

    Mike Monett Guest

    Switching power supplies are great for high power and digital work, but if
    you are having problems with noise, you are probably working with low-level
    analog signals.

    In this case, once the switching noise is generated, there no good way to
    get rid of it.

    Filtering, grounding and shielding have limited effect. Without completely
    shielding everything, the noise simply goes around them. So you need a
    linear supply with special attention to eliminating the switching noise
    from diode turnoff in the bridge rectifier.

    Even if you use a linear supply for dc power, simply connecting the circuit
    to a pc to transfer the data can often introduce large amount of noise into
    the circuit. Optoisolators can be used to reduce the noise, but the
    inexpensive ones are slow.

    When working with low-level circuits with reasonable bandwidth, the amount
    of effort you have to spend reducing noise increases as the signals get
    weaker. Often, you can amplify the signal at the point where it is
    generated, and only have to shield and filter a small area. The resulting
    high amplitude signals won't give as much problem with noise.

    Everyone has their special tricks and approaches. Listen to them carefully,
    since they are telling you things that cost them dearly to learn:)


    Mike Monett
  3. Tim Williams

    Tim Williams Guest

    I don't see why it can't be fixed with filtering. I mean, you do worse with
    an analog supply with crud down to 120 or 60Hz. You just need smaller
    filters to get rid of the 1MHz-ish harmonics without coupling it back in by
    parasitic capacitance.

    I would use a common-mode choke, then a pi filter or two to kick HF noise.
    Then another pi filter if I need low hum too.

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