Re: PSU fan blows a gale - options?

The message <5rednQhwu_ZmIIjeRVnytA@xxxxxxxxxxxxxx>
from "Derek Baker" <me@xxxxxxxxxxxxxxxxxxxxxxxxxxx> contains these words:

> "Tiny Tim" <_> wrote in message
> news:4315bafa$0$97116$ed2619ec@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > Johnny B Good wrote:
> >> The message <MPG.1d7fa4e250f5a59e9896da@xxxxxxxxxxxxxxxxxxx>
> >> from Rob Morley <nospam@xxxxxxxxxxxx> contains these words:
> >>
> >>> In article <431582aa$0$1289$ed2619ec@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
> >>> "Tiny Tim" <_> says...
> >>>> I've been given an old unbranded PC of pretty low spec - Athlon XP
> >>>> 1100MHz,
> >>>> 384MB, 10GB 5,400rpm, 52X CD-ROM. I've popped on a clean install of
> >>>> Win XP
> >>>> to check it out and everything seems fine.
> >>>>
> >>>> But there is one annoying problem - the PSU fan is blowing a gale. I
> >>>> don't
> >>>> know the make but it is rated at 300W and runs completely cool to
> >>>> the touch
> >>>> and the exhausted air is barely above room temperature. I can't see
> >>>> any way
> >>>> to reduce the fan speed either on the PSU or in the BIOS. Other than
> >>>> replacing the PSU is there anything I can do to shut the thing up
> >>>> without spending money?
> >>>>
> >>> If the fan has a pressed steel grill rather than a wire one you can
> >>> probably quieten it by cutting that out (make sure that stray bits of
> >>
> >> This will certainly reduce turbulence noise but the excessive CFM
> >> performance of the fan means it's still likely to be objectionably
> >> noisy. If the psu box interior vent slots _are_ slots, rather than
> >> hundreds of small holes, he can use a suitable pair of pliars to twist
> >> the vent slot metalwork to reduce turbulance and airflow resistance
> >> which will benefit slow speed operation of the fan as per the
> >> undervolting suggestion.
> >>
> >>> metal don't get where they shouldn't). Other than that you could
> >>> fit a quieter fan in the PSU, or possibly run the existing fan at 7v
> >>> instead
> >>
> >> This is not as straight forward as the "7volt mod" normally described
> >> for case fans where the 0v wire from the fan is transfered to the +5v
> >> line to run the fan at the 7volt difference between the 12 and 5 v
> >> rails of the supply.
> >>
> >> The most certain and safest way to reduce the fan voltage is to put a
> >> string of 6 or 7 cheap silicon diodes (the 1N400x series being a
> >> typical choice) in series with the 12 volt line (red wire to the fan)
> >> insulated with heat shrink or push tight fit plastic sleeving.
> >>
> >> You'd only need to seperate the fan wires and cut and strip the red
> >> wire to insert the diode string. This avoids messing about modifying
> >> any PCB circuit traces or tapping into a 5 volt line with the black
> >> fan wire (disconnected from it's origional 0v point).
> >>
> >> Although this latter idea isn't too difficult to achieve, it does
> >> require either soldering to the PCB trace or very carefull removal of
> >> a small section of insulation from an accessable 5 volt (red) wire in
> >> order to solder the black fan wire to and the application of
> >> insulating tape to protect said joint from shorting out.
> >>
> >>> of 12v, but for anything that involves opening the PSU be sure that
> >>> you are aware of the dangers and know what you are doing, as there
> >>> can be high voltages in there even when it's switched off and
> >>> unplugged.
> >>
> >> Most PSU's have 1 to 5M ohm 'bleed' resistors wired across each of the
> >> two 200v rated smoothing caps and so would be discharged to a safe
> >> voltage after a couple of minutes (the time taken to disconnect from
> >> the mains, remove from the PC case and open up).
> >>
> >> However, not all PSUs are so blessed and there is the possibility that
> >> any such bleed resistors may have failed open circuit. It's a good
> >> idea to either pause the boot process or boot to a dos prompt and then
> >> disconnect the mains supply. This will leave a minimum charge in the
> >> smoothing caps which you can then individually bridge with an
> >> insulated handled screw driver to confirm the existence and efficacy
> >> of any such bleed resistors. If the resistors don't exist or are open
> >> circuit, you'll get a modest bang and a spark.[1]
> >>
> >> [1] Shutting down from the OS (or via the front panel button) would
> >> leave the individual caps with a residual charge of some 170 volts
> >> each. Removing the mains supply whilst paused or at a realmode dos
> >> prompt will reduce this to 130 volts which will somewhat reduce the
> >> fireworks when shorting out with a screwdriver.
> >>
> >> HTH
> >
> > EEEEEEKKKKK!!! Thanks for the replies. I was hoping the designers of
> > power
> > supplies would have the brains to build in provision for varying fan speed

> The designers of good PSUs do.

It might be more accurate (in this context at least :-) to say "The
designers of _expensive_ PSUs do." :-)

> > and that I had simply overlooked something. I may have a look at
> > butchering the thing or might just leave things as they are. I suppose a
> > new PSU is not out of the question but I was hoping to be able to simply
> > twiddle or click something and lower the fan speed the easy way.
> >
> > Cheers,
> > Tim.
> >

I assumed you were prepared to "Get down and Dirty" with a screwdriver
and soldering iron and just wanted some pointers. Unfortunately, the
cheap to mid-range PSUs _never_ have such fan speed controls.

Your cheapest option is to get a new PSU for anywhere between 10 to 20
quid from a local dealer who will be prepared to verify that it isn't as
noisy as your existing one (or at least let you audition it on a
benchtest setup).

I stock the Mercury 400W ones (model number 2603_400W_CE_PPFC) [2]
which I retail at 17 quid each. You might be able to acquire one locally
for give or take a couple of quid on that price. A batch of the earlier
350W (or was it 400W?) models had high CFM fans, but the current crop
are back down to a more reasonable noise level now.

I've actually transplanted the innards of one of those 'noisy' units
into a PSU box which has the interior vent slots where _I_ need them. I
added my own temperature dependent fan speed controller board to,
essentially, run the fan at half voltage unless the ambient goes above
35 deg C. It's nice and quiet for 99.9% of the time. :-)

[2] I've been selling these and Mercury cases/system boxes (fitted with
Mercury PSUs) for the last 3 to 5 years now, and, AFAIR, have yet to see
a faulty one. IME, they seem to be quite reliable.

I could stock an even cheaper brand at half the trade price of the
Mercury ones, but to be honest, I just don't want to be _that_ shabby.
The cheap ones _might_ be just as good, but the only practical way to
tell this would be to use my customers as guinnea pigs. :-(

--
Regards, John.

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