On Thu, 22 Feb 2007 19:33:46 -0500, CBFalconer
<cbfalconer@yahoo.com> wrote:


>> Avoid the lowest cost PSUs which tend to be quite overrated,
>> generally most people can use a median priced unit unless
>> their system was particularly well endowed, but some
>> prettied-up generics also sell for more than they're worth,
>> name brands can help but first you'll need to consider the
>> specific parts in the system.
>
>Or you can measure it by using the wattmeter method I outlined in
>another thread.
>
>I suggest people build suitable test loads. You can use 12V
>automotive bulbs for the 12V supply, but you may have problems
>finding suitable 3.3v and 5V bulbs. Now make a breadboard with a
>suitable number of bulb sockets wired in parallel.
>
>When someone offers an XXX watt PSU, load up sufficient bulbs to
>dissipate XXX watts, distributed between the various lines
>according to the mfg spec for max load. Connect, turn the PS on,
>and wait. I predict 90% of PSUs will self destruct in 30 min or
>less. So do this test at the sellers location, before buying. If
>it passes, buy it. :-)

There would definitely be quite a few PSU that fail such a
test, particularly those which came free-with-case, but
there are a couple of other problems in particular.

Crossloading - Cheap designs tend to be updated less often,
it is unlikely to see a very poor PSU optimized for the high
12V current a modern system needs. A high 12V load and
lower 5V and the 5V feedback/tracking for regulation results
in the PSU shutting off, if the owner is lucky enough that
the PSU at least has reasonable overvoltage or current
shutdown feature.

Longer term stress - A unit might run for months, even a
couple of years before blowing (usually it's capacitors or
switching transistors), at which point even if the system
wasn't damaged, repair is both unfeasible and beyond the
scope of most owners, and either the better PSU that
should've been used all along is then bought, or another
poor one and the process repeats again but possibly putting
excessive ripple/wear on parts.

There's also a few now made with notibly worse quality
control than seen in most generics over the past several
years, making *some* kind of non-system load test quite
prudent, to see if it self destructs immediately upon
turn-on.