Is this power supply good?

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For the gurus who know hardware down to the voltage, please help.

I have a BTX power supply.  Please tell me if the following values are
"normal" for a PC power supply:

I test the voltage using a digital volt meter.

Expected voltage                     Measured voltage
3.3v                                        3.38v
5v                                          5.11v
5v                                          5.04v
12v                                        11.9v

For the pin number 8 (PG Gray), I got 5.11v and I am not sure what the
expected voltage for that to be.

Assume the meter is accurate and precise to 1%, would this numbers
mean the power supply bad?  It can't be concluded good because it's
not in operational mode (real load), but could it be concluded bad, or
just normal in this test mode (short pin 15 and pin 16).

Thank you very much.

Re: Is this power supply good?

vd wrote:
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Pin 8 is the power good pin and should be high. All other volts are fine
within spec. is all
you would ever want to know about ATX supplies.

<----- ATX 2.0+ main connector ----->

Pin Signal Color     Pin Signal Color
  1 +3.3VDC Orange    13 +3.3VDC Orange
  2 +3.3VDC Orange    14 -12VDC  Blue
  3 COM Black         15 COM Black
  4 +5VDC Red         16 PS_ON# Green
  5 COM Black         17 COM Black
  6 +5VDC Red         18 COM Black
  7 COM Black         19 COM Black
  8 PWR_OK Gray       20 Reserved N/C
  9 +5VSB Purple      21 +5VDC Red
10 +12V1DC Yellow    22 +5VDC Red
11 +12V1DC Yellow    23 +5VDC Red
12 +3.3 VDC Orange   24 COM Black

Re: Is this power supply good?

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Thank you for the information.  That definitely helps.  Mine is BTX,
the pin out can be found here:

Re: Is this power supply good?

vd wrote:
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They're the same, except the BTX ones are all 24 pin,while early ATX can
be 20 pin. BTX is mainly a mobo spec.

Re: Is this power supply good?

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  Those multimeter numbers say almost nothing useful.  Voltages are on
the high side which even some defective supplies will do without a
full load.  Measuring must be performed with a maximum load; not even
a partial load is sufficient.  Even a power supply tester (and its
small load) will not detect some defective supplies.  Best power
supply test is with power supply in a computer and with computer
multitasking to all peripherals.  Best test means making no
disconnections.  Even green and gray wire voltages are not useful
because a load (the power supply controller) is not connected.

  Another said a power supply must measure within 10%.  Then we add
other facts such as how and what that meter measures.  To be within
tolerances, minimum voltage numbers under maximum load should be 3.23,
4.87, and 11.7 volts.

Re: Is this power supply good?

vd wrote:
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Formfactors has specs for stuff. There is no BTX specific supply listed
that I can see, so I guess the supply is an ATX supply. This is a
recent spec revision. Section 3.2.1 lists 5% tolerance, on everything
except -12V. PG is a logic signal, and you'd expect the swing on that
signal to be consistent with other 5V rails.

Output Range       Min.   Nom.   Max. Unit
+12V1DC (1) 5%  +11.40 +12.00 +12.60 Volts
+12V2DC (3) 5%  +11.40 +12.00 +12.60 Volts
+5VDC 5%         +4.75  +5.00  +5.25 Volts
+3.3VDC (2) 5%   +3.14  +3.30  +3.47 Volts
-12VDC 10%      -10.80 -12.00 -13.20 Volts
+5VSB 5%         +4.75  +5.00  +5.25 Volts

I use a load box, for testing a PSU. That puts a light load,
like 100W, and allows testing the PSU without endangering
the motherboard. I made the load box myself, using power
resistors from a local electronics store.


Re: Is this power supply good?

On Thu, 6 Mar 2008 07:27:09 -0800 (PST), vd

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Without a suitable load on the PSU (one that comes close not
only to the wattage the system would use but also the
current per rail on (mostly 12V versus 5V rails)), the only
thing the test you have done thus far could indicate is
whether the PSU had some problem.

No problem is indicated by the above numbers.  A PSU
manufacturer generally indicates at least a bare minimum
load must be applied for proper regulation, with that bare
minimum load on the 5V rail (typically about 2A) it would
bring the 5.11V value down closer to 5.0V, and it would make
the 12.V value slightly higher than 11.9V.

In other words, the results you see are typical of a PSU
that is working properly, but they don't indicate the PSU
will still be working properly when powering the same load
as (or the actual) system it'll be powering.

The other factors to consider are whether the PSU is
accurately rated by it's manufacturer, if it's current per
rail (5V or 12V) ratio is roughly correspondant to the
current per rail the system is anticipated to use, and if
the PSU construction and design is of suitable quality to be
safe and reliable over the life of the system.  You can
either place trust/faith in the manufacturer, consult some
online reviews (realizing they can only test for short-term
fitness), or analyze it's design yourself to the best of
your ability... ideally a combination of all of these.

Re: Is this power supply good?

The various voltage output levels for a given PSU should be within 10% of
their expected value.  So, that PSU is fine.

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