From: Mike Poulton
Subject: Re: Using a computer power supply without a computer?
Date: 9 Dec 2002 07:30:39 GMT
Organization: MTP Technologies
On 09 Dec 2002, said:
> NleahciM wrote:
>> Hi - I recently found a 250W ATX computer power supply that didn't
>> work. I opened it up - and instantly spotted a dead 2200pf capacitor.
>> I replaced it - and the PSU worked just fine. What I was wondering is
>> whether or not it is OK to use this PSU without a computer attatched.
>> Like - I don't have a dedicated power supply for all of my little
>> electronics projects - so I have always just used batteries - but
>> that would be much better. Is ther any problem with me using it?
> If the voltages on the output seem right (12V,3.3V,5V,-12V,-5V) then
> it should be fine.
> The problem you may run into is noise on the output, and poor
> regulation on some outputs.
> If you need a low-noise power source, for any audio stuff, you may
> have to put a regulator on the input, run from the 12V line, and use
> that. Respect the current ratings, a fuse on each line you use at it's
> nominal current -30% might be a good idea.
I use a computer supply on my bench for all sorts of projects. There
are a few points to consider. In many designs, the 5V line is the only
one with active voltage regulation. The other output voltages are not
monitored. They all use the same transformer secondary, so they are
fixed multiples of the 5V output under no-load conditions. When a load
is added, this changes. Losses within the transformer are accounted for
by the regulator, but resistance losses in the output semiconductors and
wire are not. So, voltage regulation is generally rather poor when the
load is not distributed evenly between 5 and 12V. A big load on 5V
pushes the 12V line way above spec. A significant load on the 5V line
is often required to achieve solid regulation, too. These supplies have
some very good points, though. They have nearly instantaneous shutdown
in the event of a short circuit, and it is often fast enough to protect
power semiconductors (like mosfets) that do undesireable things. This
fast current limiting makes it hard to charge filter caps and start
motors, though. Also, you can usually draw much more than the rated
current from the 12V line as long as there is no other load on the
supply. My bench supply is rated 8A at 12V, but I can get 16A before
the current limit shuts it down instantly, and 14A for a couple minutes
before the thermal protector kicks in. Of course, there are many
designs out there and not all of them are even remotely similar. Yours
may behave quite differently than mine.
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