Curious about 100Hz hum from wall power supply.

Discussion in 'General Electronics Chat' started by BrainFog, Sep 30, 2011.

  1. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    I have a small wall power supply (in some countries they are called wall warts) that seems to be giving off a audible hum. I think it is 100Hz hum and is easily audible upto a meter away with my very good hearing.

    I attempted to google this but all I got was results for amplifiers. The hum I hear is a noise I can hear is from the power supply itself. I intend to use it to power a pc fan. The power supply gives off the same hum regardless of whether current in being drawn or not. It is one of those voltage adjustable ones that has the option of 12v, 9v, 7.5v, 6v, 4.5v, 3v or 1.5v output. Judging by its size and weight it is a linear power supply, I can't say more without smashing it apart.

    I am just wondering what the cause of the hum is, this is more about my curiosity as I am going through a power supply phase in electronics. I have checked and none of my other power supplies produce this noise.

    Thank you
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Not to worry. It's just the transformer humming because it doesn't know the words. (That's supposed to be a joke.)

    Really, it's not a problem. Just some magnetism wobbling the bits inside.
     
    VoodooMojo likes this.
  3. BrainFog

    Thread Starter Member

    Jan 24, 2011
    122
    4
    I am not worried, just curious.

    *fails to laugh and very bad joke*

    So it would just be a poorly wound transformer and the enamel wire is a bit loose?

    I will raise your bad joke with a worse joke. *Tries not to lose his cool over loose wire.*
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
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    Usually, the loose lamination due to poor potting is the answer, but I recently learned that magnetism actually changes the size of the steel! Really huge transformers are required to hum because of this property. Tiny wall warts, not so much that you can hear them. I'd say your transformer didn't spend enough time in the enamel bath.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Line frequency in UK is 50Hz. Hence vibration at 100Hz (twice line frequency) is common.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    I thought that was clever and funny.
     
  7. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Thank you, but it's a very old joke. I probably heard it 40 years ago. Still, a new generation of people and the old jokes work again...sometimes.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Recently I passed on a very old radio (Murphy A26 ca. 1935) to a vintage radio enthusiast.

    Its mains transformer always did buzz rather annoyingly. No evidence of any arcing or breakdown, just a buzz somewhat louder than the other sources of hum and noise. On inspecting the transformer, there were signs of old damage to the screws which clamp it together, as if some technician had tried hard to quiet the thing down, a long time ago. I used it in that condition for many years, before an unrelated valve (tube) failure stopped it working.

    A replacement valve was obtained, and someone with more energy than I have these days got it working, complete with chronic buzz.
    While there are some possibly sinister reasons like overloading which can cause a transformer to be noisy, it clearly isn't always fatal.
     
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