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|DC load on PSU
||DBS-2100 PSU load tester
|Any number of thermometers
Fan voltages / Voltage regulation
|Heath / Zenith SM-2320 multimeter
|Kill-A-Watt Power Meter
|Heath AD-1308 Real Time Spectrum Analyzer
The core PSU test tool on SilentPCReview's test bench is the DBS-2100 load tester, made (in Taiwan by D-RAM Computer Company) specifically for testing computer power supplies. The machine consists of a large bank of high power precision resistors along with an extensive selection of switches on the front panel calibrated in Amps (current) and grouped into the 5 voltage lines: +5, +12, -12V, +3.3, -5, +5SR. Leads from the PSU connect into the front panel. It is shown above with leads from a PSU plugged in.
To ensure safe current delivery, the DC output connector closest to the PSU on each set of leads is hooked up to the load tester. This ensures that the current delivered is distributed to as many short leads as possible. When pushing a PSU to its rated output, the heat generated in the wires can be an issue.
The PSU is tested at 4 DC output power levels:
- 22.5W: The total of the minimum load that can be applied on each voltage line.
- 90W: Established previously as a typical max power draw of a mid-range desktop PC.
- 150W: For higher power machines.
- Maximum: The rated maximum power of the PSU.
Care is taken to ensure that the load on each of the voltage lines exceed the ratings for the tested unit. The PSU is left running at least 10 minutes at each power level before measurements are made.
The DBS-2100 is equipped with 2 individually fused AC outlets and 4 exhaust fans on the back panel. A bypass switch toggles the fans on / off so that noise measurements can be made. The resistors get very hot under high loads.
Kill-A-Watt AC Power Meter is plugged into the AC outlet on the side of the DBS-2100 in the above picture. The AC power draw of the PSU is measured at each of the 4 power loads. The efficiency of the PSU at each power level is calculated thus: divide DC power output by AC power consumption. It always varies with load, and also temperature. PSUs seem to run more efficiently when warmer, up to a point.
The Heath / Zenith SM-2320 multimeter, a fairly standard unit, is used to measure fan output voltages and the line voltages of the PSU output. The latter is done via the terminal pin on the front panel, above the connections for the DC outputs from the PSU.
The Test Lab is a spare kitchen measuring 12 by 10 feet, with an 8 foot ceiling and vinyl tile floors. The acoustics are quite lively. The PSU under test is placed on a piece of soft thick foam to prevent transfer of vibrations to the table top. Temperature in the lab is usually 20C or lower. This is something of a problem as PSUs usually operate in environments that easily reach 45C.
In-case Thermal Simulation
Sited next to or above the CPU, the PSU is always subject to external heat. The low ambient temperature of the test lab explains why the fan in the last PSU reviewed, the Nexus NX3000, never reached 12V even at full power output for over 20 minutes. I have applied a solution to this problem as promised. The idea was first suggested some months ago by contributor John Coyle in followup communications after the publication of his article, Fanless (or Not) with TKPower 300 & VIA C3. Thanks, John!
The solution shown above is a 100W AC bulb in an empty case with the PSU mounted normally. The distance between the bottom of the PSU and the top of the bulb is about 7-8 inches. The heat generated is roughly equivalent to that produced by a typical desktop. All the back panel holes in the case are blocked with duct tape. The only significant exit for the hot air in the case (once it is closed) is the PSU exhaust, which is then subject to a fair amount of heat, probably a bit more than would be seen by a PSU in a real case because there are usually other exits. The bottom front panel case intake hole is very large. In testing, the front edge of the case is moved so it hangs over the edge of desk, over free air, to ensure good fresh convection airflow. There are no case fans.
A thermistor taped to the bottom of the PSU close to its right front is used to monitor temperature. It's the little blue nub hanging down off the black wire in the photo above. Its temperature is somewhat affected by the airflow of the PSU fans but not directly in the airflow path.
The simulation means the PSU must cope with the 100W of heat generated by the light bulb plus whatever heat it generates within itself. It is a good simulation when the PSU is actually putting out ~100W of DC voltage, although in real-life systems, there would be other air exhausts paths, resulting in a bit lower case temperature. The 100W bulb is a close approximation of the heat that would be generated by components within the case. To simulate actual PC conditions closely, the light bulb power should match the DC power load applied to the PSU. I did not do this. It was too much trouble, and I did not have 150W or other high power bulbs handy. Maybe next time if there is a major clamor about the inadequacy of this simulation.
No PSU Temperature Measurements will be done any more. Originally, it was meant to provide a more complete picture of PSU performance, but discussions in the SPCR forums have convinced me that there are far too many variables at play to make internal PSU temperature a reliable gauge of... anything. There are way too many ways to interpret the numbers. Check this thread for the full discussion. The most critical parameter for thermal performance is efficiency. If efficiency is high, the size of the heatsinks and vent openings large, and the fan can blow a lot of air at full power, then excellent cooling is ensured.
The Heath AD-1308 is a portable half-octave Real Time Spectrum Analyzer with sound level meter (SLM) functions. Below 40 dBA, its accuracy is poor, limited to 3 dB increments, down to 23 dBA. Some 15 years old, this LED-based unit has long since been displaced by digital devices with better interfaces to PCs. The "A" weighting was used; it most closely approximates the frequency response characteristics of human hearing.
The microphone on the sound meter is positioned about a centimeter to the side of the PSU fan exhaust to avoid fan turbulence in the microphone itself. The dBA obtained here cannot be compared to any other measurements due to the lack of adherence to a repeatable standard and the uncontrolled reflective environment.
The noise measurements are always accompanied by descriptions of subjective perceptions. Without these, the measurements, which are not that reliable, provide only part of the picture.
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