# How Specific Does A Power Supply Need To Be?

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I have a machine where (I believe) the power supply died.  I'm looking
to buy another one, but before I spend the money for a good power
supply I'd like to know a little bit more about what I'm buying.  Now,
I'm not entirely familiar with the science behind the voltages/ampages
(college was a long time ago and I only took 2 courses on the subject).
But how specific do the volt/amp pairs need to be?

The allegedly dead power supply is 350W.  So I looked for a 350W power
supply online, but the volt/amp pairs in the details of the product
didn't match the ones on my power supply.  Summary:

115V - 8A/50-60Hz
230V - 4.5A/50-60Hz
+5V - 30A
+12V - 12A
+3.3V - 20A
-5V - 0.5A
-12V - 0.8A
+5VSB - 2A

Possible New PSU:
115V - 10A/60Hz
230V - 6A/50Hz
+5V - 26A
+12V - 18A
+3.3V - 20A
-5V - 0.3A
-12V - 0.8A
+5VSB - 2A

What do these differences mean, specifically?  Are they terribly
important, depending on the hardware in the machine?  Any
recommendations?

Regards,
David P. Donahue
ddonahue@ccs.neu.edu
http://www.cyber0ne.com

## Re: How Specific Does A Power Supply Need To Be?

cyber0ne wrote:

The amp ratings specify the maximum possible current draws, not what
the power supply will always put out because the computer determines
that, limited by the capacities of the power supply.  The new supply
should meet your needs despite its lower +5V and -5V amp ratings
because:

1) Any computer built in the past 5 years loads the negative voltage
outputs only negligibly, if at all;

2) The 12A rating for the +12V is low for a 350W supply, indicating the
supply is of low quality;

3) If your motherboard uses a square 4-pin connector in addition to the
20-pin connector, it draws few amps from the +5V output and more from
the +12V.

The best manufacturers include Fortron-Source Power, which puts out
many brands, including Sparkle, Hi-Q, and anything whose model number
starts with "FSP" (no all FSP model numbers do), Zippy-Emacs, Astec,
Newton, Lite-On, Delta, PC Power & Cooling, and Antec (actually not a
manufacturer, but they market the better products from Channel Well
Tech.).  Fortron-Source should be strongly considered due to their
combination of highest quality and unusually low prices.

## Re: How Specific Does A Power Supply Need To Be?

Awesome, thanks.  Actually, by the time I'd come back and read your
now I found a low-priced one (I'd love a higher quality one, but
money's too tight right now) that matches the amp ratings on all
voltages except allows higher values on the +/-12V values.  So, by what
you've said, that's likely a better power supply than what I was going
to buy and better than what's in the machine.

Now I just have to cross my fingers and hope that the power supply is,
in fact, what's wrong with the machine...

Regards,
David P. Donahue
ddonahue@ccs.neu.edu
http://www.cyber0ne.com

## Re: How Specific Does A Power Supply Need To Be?

On 26 Sep 2005 08:37:23 -0700, "cyber0ne"

Absolutely not!

You cannot use amperage ratings as a guide to purchase of a
cheap power supply.  It should be (maybe is) fraudulent how
most cheap power supplies are rated.  Sadly there is usually
a closer correspondance between price and actual, true
amperage capacity, than the  amperage rating and true
capacity.

If you choose a quality name-brand PSU you can have more
confidence in the amperage rating.  It's quite common for
so-called 500W PSU to be closer to 300W in a good name-brand
PSU, regardless of what's printed on the generic's label or
a website.

## Re: How Specific Does A Power Supply Need To Be?

cyber0ne wrote:

I hope it's true, but I'm afraid manufacturer and distributor ratings
vary so greatly in accuracy (honesty) that they can't be trusted
without independent verification.  I've casually verified this a few
times myself and have found some OEM supplies from major brand
computers being much more capable than some low-quality supplies rated
for even twice the power or 50% higher current.  Those low-quality
supplies didn't weigh any more than the OEM units.

## Re: How Specific Does A Power Supply Need To Be?

The replacement PSU that you found should work fine in your system
electrically.

--
DaveW
__________