Re: faint hissing from power supply
From: SteveB (sbrads_at_nildramDOTcoDOTuk)
Date: 01/05/05
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Date: Wed, 5 Jan 2005 19:07:04 -0000
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.
"hupjack" <hupjack_nospamtoday_yahoo.com> wrote in message
news:h7adnaPQc_5EaUTcRVn-3Q@comcast.com...
>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....
> ssssssssssssssssssssssss
>
> -Ethan
>
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