Date   : Sat, 17 Oct 2009 00:46:43 +0200
From   : rick@... (Rick Murray)
Subject: Dual drive power supply

Mark Usher wrote:

> I've noticed that the power supply on my dual drives is making a bit of a
> "noise" - audible, not electrical. Best described as an oscillating hum.

There are two "hums" I can think of.

A low pitched hum (50Hz), similar to what you'd hear if you put a 
sensitive microphone near the back of a TV, or a not unlike the Zzzzz 
noise you'd make if pretending to be a big bluebottle fly, is caused by 
a standard transformer. Sometimes, when operating under load, if not 
secured 100% (and this means internally too), it can rattle a little. 
Zzzzzz.

A really high pitched whine (not so much a hum), fairly similar to the 
sound camera flash units make when charging, is something a lot of 
switch-mode power supplies do. I'm not sure why they whine really high 
pitch, but I've heard it several times.


 > I wondered if this was a sign the capacitors on this psu were also
> starting to show their age and should be replaced.

No, failing components rarely make any noise. They save it all up for 
the BANG! that'll scare you half to death...


 > Or was it just a bad design in the first place.

There's a school of thought that power supplies should be designed with 
adequate capacity for the equipment being powered. This is why my 
router's power supply runs ... <touches it> it's not even warm, and it's 
been on ages.

There's another school of thought that says that power supplies are 
their most efficient when running to their capacity. I, personally, 
think this is boll*cks used as an excuse to pass off underrated (and 
perhaps cheaper) power supplies. How can you tell? The supply is Too 
Damn Hot. A great example of a TDH supply is the iomega Zip brick, you 
could fry eggs on that. A number of mobile phone chargers also get TDH, 
which is an exercise in crap design and/or irony considering we're 
supposed to be feeding maybe something in the order of 300mA?

Computer (as in generic "PC") power supplies vary wildly. I have one 
computer (Aiko) with a supply that runs "warm", and one (Ayleigh) with a 
supply that runs Blo*dy Hot (that's more than TDH! <g>). There is 
something of a difference in the insides of the supply. Aiko's is a 
component-laden switchmode.
Ayleigh? It's a switchmode of some variety, but the most noticable thing 
is the circuit is fairly sparse, save for some BIG transistor-like 
things hooked to some REAL-BIG-ASS (that's a technical term, that!) 
heatsinks. Between you and me, I rather suspect this supply spits about 
14V and the rest is done using VREGs. Cheap, but effective. That is, if 
you don't mind the PSU chunking out more heat than the Intel hunk it is 
powering!


If you've read this far, pat yourself on the back. Time to reach a point.

Step one:
Take your PSU and shake it gently.

Nothing rattling around?

Step two:
Got a multimeter? Check your outputs are correct. +5V should be close, 
old digital logic doesn't have much tolerance here. The 12V is less 
important, I've seen a lot of +12V is actually around +11.7V. No big.

All okay?


Step three:
Multimeter in place, fire up the equipment. When the drives are being 
accessed, you might see small dips on the 12V line, but the 5V should be 
steady.

All okay?


Step four: OPTIONAL
If you're brave enough to open your PSU, very gently press the circuit 
board right next to the screws to make sure it is seated correctly. Use 
EXTREME care - for switchmode capacitors carry insane voltages and can 
pack a punch for quite a while. I would recommend using a thin wooden 
stick. You can get these in the supermarket. I'm not sure what they are 
supposed to be for (some sort of skewer?), I just tend to set fire to 
one end and use it to light the cooker... Anyway, something long, pokey, 
and non-conductive. Press around the screws, make sure it's all secure.

[I trust you DID remember to unplug!]


If you get this far without faults, then proceed to step five:

Step five:
Ignore the noise. Some supplies are noisy, some are quiet. A bit like 
children. On the plus side, for children, I've yet to see one blow a 
capacitor and sit there with smoke pouring out of his/her ears. It would 
be kinda funny though. Instead, they can throw tantrums and smoke 
cigarettes. Not funny at all.



Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...