Archive: SPCR's PSU Test Platform V.3

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Sept 19, 2005 by Mike Chin

PSU reviews have become bread-and-butter articles at SPCR over the years. This is partly because of our close attention to detail, fundamentally sound test procedures, and decent test gear. It is also because there are very few serious PSU reviews done by hardware web sites. In this context, SPCR PSU reviews really stand out.

The old saying about not resting on your laurels holds true for PSU testing, however. In the 18 months since the last refinement to SPCR's PSU testing system, PC power supplies have undergone tremendous changes. As a result, weaknesses in our testing setup have become evident. This article documents those weaknesses and the changes we've made to address them. Our goal is greater accuracy and precision for future PSU reviews.

There are two main issues, both related to the need to test increasing more powerful power supplies, increasingly with much of the output power concentrated in the 12V line:

  • The accuracy of our AC/DC conversion efficiency results; and
  • The adequacy of the test system to provide high enough load on the 12V line.

Before we dive into these topics, let's take a brief look at SPCR's existing PSU testing system.

PSU TEST SETUP, Q1 2004

As documented in the SPCR's Revised PSU Testing System article, the test platform has been refined in steps over the years, sometimes with useful critical input from readers. To summarize the article, our PSU test system seeks to characterize:

  • The quality and level of noise emitted at various standard output levels, all the way to maximum rated output.
  • The AC-to-DC conversion efficiency at each output level.
  • The voltage regulation at each output level.
  • The rise in temperature as air flows through the PSU.

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 into the simulated mid-tower case. This means that the higher the output power level, the greater the amount of heat there is in the test box, which replicates the thermal conditions faced by a PSU in a real PC. It is a close simulation of actual-use conditions for a PSU in a typical quiet mid-tower case. A modest airflow 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 just 10~12CFM of measured airflow.


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. A Kill-a-Watt meter used to monitor AC power input to the PSU can be seen at the bottom of the photo. The
DBS-2100 PSU load tester pressed up against the wooden box actually feeds its internal heatinto 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 before the back exhaust fan was installed and top of the PSU mounting area was cut away.

Here's a list of all the gear used currently for PSU testing:

  • Digital readout thermometer. There are several in the lab that are used. (Like DigiDoc.) They measure within ~1°C of each other, which is good enough for our purposes. Powered by the PSU being tested. (<1W power draw.)
  • Digital readout multimeters. A couple of ordinary DMMs are used to measure DC voltage.
  • DBS-2100 PSU load tester. Made specifically for load-testing computer power supplies, it consists of a large bank of high power precision resistors along with an extensive selection of electronic switching on the front panel calibrated in Amps (current) and grouped into 6 voltage lines: +5, +12, -12V, +3.3, -5, +5SB. Leads from the PSU plug into the front panel.


Front panel of DBS-2100 PSU load tester.

  • Kill-a-Watt or Seasonic Power Angel AC power meter is used to monitor the AC power input to the tested PSU. These AC meters also measure Power Factor for the PSU.
  • B&K model 1613 sound level meter. This professional caliber SLM is >20 years old, weighs over 10 pounds, and is completely analog in design. It has a dynamic range that spans 140 dB. The unit can measure accurately down to about 16~18 dBA. A quiet environment is a prerequisite to low noise testing; the lab has been measured down to ~17 dBA at night, and a <16 dBA dBA adjacent room is also available for any PSUs that are quieter.



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