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faint hissing from power supply

Discussion in 'General Electronics' started by hupjack, Jan 3, 2005.

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  1. hupjack

    hupjack Guest

    I've got your joe-average ATX case and I can hear a faint hissing coming
    from the power supply even after the computer is shut down. I can kill the
    sound by flipping the switch on the back of the power supply (which kills
    power into the psu). I know "faint hissing" isn't super descriptive, but
    can anybody tell me whether I should be afraid of this psu? Is in the
    process of frying itself? its capacitor? or some other part that I'd rather
    not fry? Or is it no big deal and some psus make a bit of noise? I don't
    have any past experience with this hardware, so I'm afraid I can't say if it
    used to be quieter or not.

  2. eddumweer

    eddumweer Guest

    Looks like the electrolytic capacitors lost capacity (have bad esr), due to
    cost we normally replace them when not ok.

    If you have an esr-meter available you can measure then the caps in-circuit
    and replace the bad ones. But a new power supply is probably cheaper then
    the repair cost.

    Greetings Peter
  3. mc

    mc Guest

    This may be perfectly normal. Recall that the PSU outputs 5 volts even when
    "off" so that the computer's wake-up functions can work. That means it's
    still rectifying the incoming AC, chopping it at an ultrasonic frequency,
    and running it through a transformer. I've always believed -- without a lot
    of proof -- that ultrasonic transformer whine can sound like a hiss, because
    the vibration has irregular components that are audible.
  4. Guest

    The PSU is on whenever the line power is on, it's waiting for
    the PC's "on" button to be pressed. For this you need power.
    Whether it should hiss or not is another matter, PSU's
    have a high frequency switch mode inverter, and like TV's
    with similar power supplies, they can make a high frequency
    sound. Whether it's safe cannot be determined unless you can
    determine which part is making the hiss, and it needs somebody
    technically competent to run the PSU uncased and connected
    to a PC.
  5. DMF

    DMF Guest

    I think your power supply is dying. The high-pitched whine indicates
    bad capacitors -- power supplies should be silent when the computer
    is off but still plugged in. I would replace it before it fails, especially
    it is more than 3 yrs old. Power supplies are cheap and easy to replace
    so why take a chance. Bad voltages can create havoc on a PC resulting
    in spurious reboots, corrupted data or damaged hardware (e.g. hard

  6. CJT

    CJT Guest

    To me, a whine and a hiss are entirely different things.

    A whine I might be concerned about. I doubt a hiss portends more than
    the usual evil associated with PCs.
  7. hupjack

    hupjack Guest

    I absolutely agree with the "safety first" standpoint of fix it before it
    causes me havoc, and I didn't have much hope that my description of a "faint
    hiss" would give ya'all THAT much to work with.

    Definitely what I'm experiencing IS a hiss rather than a whine. Nothing
    high pitched about it.. No squeak.. More like a faint rustling....

  8. DMF

    DMF Guest

    You've got me stumped. I've never heard a peep out of a power
    supply when the PC was off except when they are failing due to
    bad caps. Perhaps you can email the manufacturer and see if the
    noise is normal. Also, how old are your ears? I know that we
    lose the ability to hear high pitch as we age, so maybe it is a high
    pitch but all you hear is rustling... just a thought. I've serviced PCs
    that had power supplies that were squealling so loud you could
    hear it across the room -- but the client (around 70 y.o.) didn't
    hear a thing even when he put his ear right next to the computer.

    BTW, how old is this power supply?

    Good luck!

  9. SteveB

    SteveB Guest

    As an electronics engineer who works on many types of rather expensive
    switch mode power supply (but not PC ones) I believe I can answer your
    question. Yes, it could be dried out caps gone low in capacitance or high
    in ESR causing an instability, but it can also be a function of the supply
    being too efficient for it's own good. When a switched mode supply is asked
    to deliver very low output power (i.e. standby) then it has to reduce the
    switching waveform width to a very narrow pulse so as to achieve a low
    average power output. The pulse width modulator (PWM) IC will only work
    reliably down to a certain width pulse which may still be too wide to
    achieve the low power required. The result is that the feedback that is
    there to maintain a constant output voltage controls the PWM IC by
    'chopping' the switching pulses, so you may get a string of pulses followed
    by a period of no pulses, then another string of pulses, another gap etc
    etc. This isn't too precise, so the number of pulses and gap times varies a
    bit; the overall choppiness usually occurs at audio frequencies and can
    sound very much like a hissing noise being radiated by the output inductors
    or transformers.

    A resistor across the standby 5v to waste a bit of power would cure this if
    it's the cause, possibly something like 47 - 100 ohms 1 watt but I'm
    guessing. A lot of switch mode supplies have a minimum load rating so doing
    this is normal practice with a lot of designs anyway. If it doesn't fix it
    then I would say change the supply as it's probably a cap failing.
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