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My solution in the end was to cut one side and the top away so that the PSU would be placed on the rig, rather than mounted in it.
One side is a block of foam, and all the surfaces the PSU touches are lined with soft damping materials or foam weather-stripping that keeps it mechanically decoupled from the box. You can see in the above photo the shadow of the thermal sensor that has been fixed about 1" below the back bottom edge of the PSU being tested.
The above photo
shows a Seasonic Super Tornado PSU in the rig. It's plugged into a Kill-a-Watt AC power meter. The output cables from the PSU are long enough to reach the input panel on the load tester; this may be an issue with some PSUs that have short leads. On the top right is a digital thermometer powered via a 4-pin Molex from the PSU. The long row of slots along the front edge of the PSU load tester are the intake vents for the four fan that draw air across the hot resistors. (They are equivalent to the front intake vent in a typical PC case.) All the parts of the box that make contact with other surfaces (the bottom rim, the contact area around the load tester fans) are lined with weather-stripping for good air sealing (though it does not have to be perfect) and minimal vibration transfer.
One item missing in the above photo is the thermal sensor for the PSU exhaust air. It is positioned near the center of the exhaust grill, about 1/3 of the way down from the top, within 1cm of the grill. This is usually the hottest spot, but the sensor is moved around a bit for each PSU to find the hottest spot before being fixed in place.
Some quick testing showed this setup to be perfectly neutral acoustically, at least when the PSU fan is spinning slowly. At very high PSU fan speeds, there is some added resonance from the air being vibrated in the case, and perhaps a bit from the PSU panels as well. This needs to be examined in more detail. But in the range that matters to PC silencers (PSU noise of under 30 dBA), there is no difference in noise level whether the PSU is in the rig or placed on a piece of foam on the bench.
The whole assembly sits on a sturdy cabinet with drawers. Equipped with castors, it can be easily wheeled into the very quiet room adjacent to the main test room for super quiet PSUs (should they appear): The minimum noise level in the lab is about 16~17 dBA; in the adjacent room, I've measured as low as 12~13 dBA.
After many trials and experiments, I found that the following procedures provide the most consistent results:
A. Record the ambient temperature and noise.
B. Warm up the PSU before testing by turning it on at 65W load, leaving it running for a couple minutes, then setting the load to 50% of the rated output power and alllowing it to run for 15 minutes. Like all electronic devices, PSUs work more efficiently when they are warm.
C. Start measuring with the lowest power load and move up to max power. The test load points will be:
- Max power
D. Allow the PSU to run at each power level for 3~5 minutes to ensure stability before taking any measurements.
E. Check and record voltages across each output line several times at several power levels, monitoring it as small variations in load are introduced on the 3.3V, 5V and 12V lines in turn.
F. Leave the PSU load tester fans on until just before temperatures are recorded. At lower power levels, there's hardly any change in the internal temperature. At higher power levels, turning the fans on actually increases the internal test box temperature (and sometimes, the PSU exhaust temperature) up by a degree or two. Convection is certainly not enough to move the hot air from the PSU loader into the test box.
G. Record the noise level of the PSU at each power level at the same time that the temperature is recorded.
H. Monitor and record the voltage across the internal PSU fan(s) after 3~5 minutes at each power level. This is not always possible, as some PSUs are wired so tightly that tapping into a fan line requires the precision and patience of a surgeon, which cannot always be mustered up.
The above refers only to documentation using instrumentation. There is always much listening and taking of notes regarding the quality of the noise as I hear it.
That's pretty much covers the current PSU testing system... until the new variable AC power supply arrives. :)
Look for reviews of new Enermax, Zalman, Fortron, and Seasonic PSUs in the near future.
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POSTSCRIPT ADDED April 13, 2004: Please check the next page.
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