SPCR Power Supply Test Rig, v.4 (and v.4.1)

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We examine power supplies in a number of different ways, starting first with their electrical characteristics. The various electrical tests are detailed in the next section.

There are two aspects of our test platform that make it unique in the world of PSU testing. They include:

  • Thorough Acoustic Testing & Analysis
  • Thermally Realistic Conditions

These particular aspects are important enough for us to present an overview about them.

1) Thorough Acoustic Testing & Analysis
We use three methods of assessing noise:

  • Sensitive lab-grade sound level meter (SLM) to record sound pressure level (SPL) from one meter distance in a test room with very low ambient noise (<20 dBA ambient) at many power levels from 40W on up to full rated output.
  • Audio recordings made with a sensitive pro quality microphone and prosumer digital sound system to capture the noise made by the power supply at selected points, from 1m and 30cm (1ft) distances. These recordings are converted to high quality MP3s for readers to download for listening comparisons. It's the next best thing to actually hearing the PSUs yourself in our lab.
  • Careful listening and detailed descriptions of the sound level and quality. We consider this the most important part of the acoustic analysis. 90% of what we know about a products acoustics can be learned with careful listening under varied conditions. In combination with our measurements, listening helps us to identify and confirm the effects of measured parameters on noise, and any other effects not documented or detected in other ways. These include instances of tonal noise, periodic or cycling noise,
    (Note: When measuring or recording the noise, the power supply loader's internal fans and all other noise sources are turned off.)

2) Thermally Realistic Conditions
A primary feature of the test system is a simulated mid-tower case with modest airflow. The heat generated in the load tester by the output of the tested PSU is forced directly into the simulated mid-tower case. The higher the output power, the greater the heat in the test box. It replicates the thermal conditions faced by a PSU in a real PC: The total power a computer draws at the AC outlet is precisely the amount of heat that it generates. Our test box is a close simulation of actual-use conditions for a PSU in a typical quiet mid-tower case. A very quiet 120mm fan fan performs the same role as a back panel exhaust in a mid-tower case. This fan is decouple-mounted in foam to minimize noise and is voltage limited to provide under 20 CFM of measured airflow. Noise from the PSU is measured from a meter away at every test load point, from 40W up to maximum rated power, with all other noise sources in the room turned off.

There are several advantages to this setup:

  • Because almost all PSU fans today are thermally controlled, the noise produced by a power supply in our test rig is very close to the noise it would produce at the same ambient temperature and the same loads in a real computer. In contrast, most other PSU test schemes we've seen measure the noise only at idle and in typical <25°C room temperature . The result is unrealistically low noise readings. Others use SLMs that cannot read below 30 or 40 dBA, and so end up placing the SLM microphone just a few inches away from the noise source, which is far too close for any chance at accuracy, even for comparison's sake. In truth, few PSU reviews actually consider noise in any but the most casual way.
  • Temperature affects electronic performance. At high load, high temperature can limit maximum power and/or reduce efficiency. Our tests show the PSU's power conversion efficiency and stability in thermally realistic conditions (read: hot) rather than the typical undemanding static 20~25°C of most other test setups.

Photo shows wooden case used for thermal simulation of quiet, low-airflow mid tower case. Note exhaust fan decouple-mounted in foam. A thermal sensor is placed at the exhaust of the PSU, and its fan lead tapped to monitor voltage. The
DBS-2100 PSU load tester pressed up against the wooden box actually feeds its internal heat into the box via four slow 80mm fans.

The four cooling fans of the DBS-2100 PSU load tester feeds the heat generated by the loading resistors into the thermal simulation case. NOTE: The wooden box as shown above was in an earlier incarnation.

It is important to note that the room in which the power supplies are tested is almost always very quiet, typically <20 dBA, more often <18 dBA. If it is a noisy day (due to rain, wind, or lawn mowers) we wait until it's quieter to conduct any recordings. listening tests or sound measurements.

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