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TESTING
For a fuller understanding of ATX power supplies, please read
the reference article Power
Supply Fundamentals & Recommended Units.
Those who seek source materials can find Intel's various PSU design
guides at Form Factors. The ATX12V Power Supply Design Guide is the most referenced PSU standard in the computer industry. This guide is part of Intel's Power Supply Design Guide for Desktop Platform Form Factors, now in Revision 1.2, dated Feb 2008.
For a complete rundown of testing equipment and procedures,
please refer to SPCR's PSU Test Platform V.4.
The testing system is a close simulation of a moderate airflow
mid-tower PC optimized for low noise.
In the test rig, the ambient temperature of the PSU varies
proportionately with its output load, which is exactly the way it is in
a real PC environment. But there is the added benefit of a high power
load tester which allows incremental load testing all the way to full
power for any non-industrial PC power supply. Both fan noise and
voltage are measured at various standard loads. It is, in general, a
very demanding test, as the operating ambient temperature of the PSU
often reaches >40°C at full power. This is
impossible to achieve with an open test bench setup.
The 120mm fan responsible for "case airflow" is deliberately
run at a steady low level (~6-7V) when the system is run at "low"
loads. When the test loads become greater, the 120mm fan is turned up
to a higher speed, but one that doesn't affect the noise level of the
overall system. Anyone who is running a system that draws 400W or more
would definitely want more than 20CFM of airflow through their case,
and at this point, the noise level of the exhaust fan is typically not
the greatest concern.
Great effort has been made to devise as realistic
an operating environment for the PSU as possible, but the thermal and
noise results obtained here still cannot be considered absolute. There
are too many variables in PCs and too many possible combinations of
components for any single test environment to provide infallible
results. And there is always the bugaboo of sample variance. These
results are akin to a resume, a few detailed photographs, and some
short sound bites of someone you've never met. You'll probably get a
pretty good overall representation, but it is not quite the same as an
extended meeting in person.
SPCR's
high fidelity sound recording system was used to
create MP3 sound files of this PSU. As with the setup for recording
fans, the position of the mic was 3" from the exhaust vent at a 45°
angle, outside the airflow turbulence area. The photo below shows the
setup (a different PSU is being recorded). All other noise sources in
the room were turned off while making the sound recordings.
A TWIST IN FAN SPEED MONITORING
In our usual test methodology, the voltage to the fan is monitored with a multimeter by tapping the fan wires. This was not possible to do with the fan in the Modu82+ 625 because it is controlled with PWM, and thus the voltage would always read as 12V.
4-pin connector indicates PWM fan control.
Another method of fan speed monitoring had to be used. We opted for our optical (laser) tachometer. It requires a reflective piece of tape to be affixed to one of the blades, then counts the rotations to determine RPM.
Reflective tape affixed to one of the blades for RPM monitoring.
This meant we could not monitor the fan speed continuously, as the fan is located on the bottom of the PSU, which faces the inside of our thermal simulation box. The exhaust fan had to be removed from the PSU thermal simulation box
in order to gain access to the bottom of the PSU where the fan could be seen. We did a fan speed check just before and right after each power load change.
The only way for the optical tachometer to "see" the blades of the PSU fan.
INTERPRETING TEMPERATURE DATA
It important to keep in mind that fan speed varies with temperature,
not output load. A power supply generates more heat as output
increases, but is not the only the only factor that affects fan speed.
Ambient temperature and case airflow also influence the PSU fan speed. Our PSU
test rig is a thermal challenge for a power supply:
A portion of the heat generated inside the case must be exhausted
through the power supply, which causes a corresponding increase in fan
speed. This replicates conditions inside a typical PC.
When examining the thermal data, the most important indicator of
cooling efficiency is the difference
between intake and exhaust. Because the heat generated in the PSU
loader by the output of the PSU is always the same for a given power
level, the intake temperature should be roughly the same between
different tests. The only external variable is the ambient room
temperature. The temperature of the exhaust air from the PSU is
affected by several factors:
- Intake temperature (determined by ambient temperature and
power output level)
- Efficiency of the PSU (how much heat it generates while
producing the required output)
- The effectiveness of the PSU's cooling system, which is
comprised of:
- Overall mechanical and airflow design
- Size, shape and overall surface area of heatsinks
- Fan(s) and fan speed control circuit
The thermal rise in the power supply is really the only indicator
we have about all of the above. This is why the intake temperature is important:
It represents the ambient temperature around the power supply itself. Subtracting
the intake temperature from the exhaust temperature gives a reasonable gauge
of the effectiveness of the power supply's cooling system. This is the only
temperature number that is comparable between different reviews, as it is unaffected
by the ambient temperature.
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