Ten ATX Power Supplies Compared
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We evaluated each power supply unit (PSU) on features and
performance. A full range of equipment
was used to test each PSU under controlled load conditions. style='mso-spacerun:yes'> In addition to measuring the power going in
and coming out we looked at voltage regulation, electrical noise (AC ripple),
airflow, sound level, efficiency and cost.
Here is a block diagram of the test bench setup and a list of the
equipment I used during testing.
- FLUKE 87-III
True RMS digital multi-meter ( style='font-size:10.0pt;font-family:Arial'>Accuracy +/- 0.05% of 3-digit
- WattsUp? Pro –
digital wattmeter and power analyzer (Accuracy 3% of displayed value)
V-650F 60 MHz dual trace oscilloscope (Accuracy +/- 3% Hitachi
of input range)
Variable Autotransformer, 1.4 KVA, 0-140 VAC
style='mso-spacerun:yes'> 52-II digital thermometer (Accuracy +/-
- Extech Model
407736 digital sound level meter (Accuracy +/- 1.5 dB)
V1-10kg digital balance (Accuracy +/- 1g)
- Homemade power
supply load tester – 300 watt combined load
be ATX compliant, a power supply must be designed and built according to a set
of standards created by Intel ( href="http://www.formfactors.org/FFDetail.asp?FFID=1&CatID=2">ATX Specification
– Version 2.1), which along with motherboard specifications also defines
the size, form factor, connectors, voltage outputs, etc., that a power supply
must incorporate. More detailed
information can be found in the href="http://www.formfactors.org/FFDetail.asp?FFID=1&CatID=2">ATX12V Power
Supply Design Guide. All of the
power supplies in this roundup conform to the ATX 2.03 specification.
supply manufacturers generally include the maximum total output in the name of
their products. This is frequently a
major selling point. But just because a
particular unit is advertised as being a 400 watt model does not mean it will
operate reliably and for any length of time if fully loaded 24/7 at the maximum
general rule-of-thumb is not to exceed 70% of the total rated output. style='mso-spacerun:yes'> For a 400 watt PSU that means the total
continuous load (not intermittent loads) being drawn by the PC should not exceed
280 watts. Doing so may shorten PSU
components life. The AC ripple present on all DC outputs also increases as the
load goes up.
of the biggest variables in determining a power supplies true output load
potential is the temperature that the power supply components were designed to
operate at. Manufacturers don’t always
divulge these operational temperature limitations. style='mso-spacerun:yes'> To illustrate this point, consider two very
different power supplies: each advertised as a “400 watt ATX PSU”. style='mso-spacerun:yes'> One unit is designed to produce 400 watts as
long as the power supply’s internal temperature does not exceed 25ºC (not
realistic). The second unit is designed
to produce 400 watts as long as the temperature of internal components does not
exceed 50ºC (realistic with a built-in safety margin). style='mso-spacerun:yes'> Something else you will notice about these
two power supplies is one weighs and costs considerably more than the other.
switching-mode power supplies used in modern PCs are designed to convert
alternating current (AC) into direct current (DC) used by the computer’s
internal components. A standard ATX
power supply will produce three different voltages to power the motherboard,
CPU, memory, hard drives, optical drives, etc.
These are: +3.3 volts, +5 volts, and +12 volts. style='mso-spacerun:yes'> In addition, the PSU also generates several
other voltages: -5 volts, -12 volts, and +5 volts STBY.
a controlled load is critical to testing and evaluating a PC power supply. style='mso-spacerun:yes'> I built my own power supply load tester using
10 wire-wound, ceramic resistors of various sizes. style='mso-spacerun:yes'> This unit places a ~300 watt combined load on
the power supply being tested.
style='font-size:10.0pt;font-family:Arial;color:#00000B'>Here is the Ohm’s Law
key if you are trying to remember how the different variables relate… :)
all ATX power supplies incorporate over-voltage and over-current shutdown
circuits to help protect the PSU from these conditions. style='mso-spacerun:yes'> What many people do not realize is that some
ATX, switching-mode power supplies (usually older models) will self-destruct if
turned on without any load connected! To
be safe, never turn on a switching-mode power supply without a minimal load
connected to one or more of the outputs.
In general, placing a 5 ohm, 25 watt resistor across the +5 volt output
is sufficient. Be sure to check and see
what your particular PSU manufacturer recommends. style='mso-spacerun:yes'> Inexpensive power supply testers are
available that incorporate a load resistor.
style='font-size:8.0pt;font-family:Arial;color:#00000B'>PC Power & Cooling
ATX PSU Tester ($10.00)
before we get into the nuts-n-bolts (arcs-n-sparks) of testing lets take a
quick look at each of our ten ATX power supply’s specifications and features.
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