SPCR's Revised PSU Testing System

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March 22, 2004 by Mike Chin

PC power supplies are poorly understood by typical computer users. Most PC hardware web site reviewers also seem to be "typical computer users" when it comes to power supplies. Web reviewers of PSUs generally show minimal understanding of fundamental electricity / electronics concepts, standards and practice, and use inadequate evaluation equipment for reviews. Even when data is collected, little attempt is made to understand what it really means and how relevant or accurate it is. Perhaps it comes down to the simple fact that PC hardware review sites are created and maintained by computer enthusiasts, who have little or no grounding in electronics.

SPCR's PSU reviews have always tried to do more, right from the beginning when we, too, had little in the way of appropriate testing equipment. I have the small advantage, perhaps, of having studied electronics briefly, and of having run an audio hifi store for some years; my partner, a talented electronics technician, taught me a lot.


After an angel presented SPCR with the gift of a DBS-2100 PSU load tester, we basked in the luxury of having a piece of test equipment rarely seen outside a power supply design engineering lab. Still, the PSU load tester is only one device, and it does not tell us everything we want to know.

One of the more challenging issues has been to observe or measure a PSU's performance in a realistic thermal environment. Testing by safety certification agencies such as UL, CSA and others is conducted in a controlled temperature chamber. As far as I can determine, the ambient temperature is controlled to remain constant throughout testing. This procedure is probably perfectly fine if all the validation testing by agencies are done at the same temperature, preferably a high enough temperature to reflect real conditions in a PC. But it appears that agencies test to validate the manufacturer's claims only, not in accordance with a universal standard.

In other words, if a manufacturer submits a power supply specified to produce 400W total power across its various voltage lines at 25°C with 115VAC input, then that claim is verified, and safety checks are made under precisely those conditions. But not at 45°C or with 90VAC input. This is because agency regulations are concerned chiefly with safety or EMI (electromagnetic interference), not with the performance aspects of power supplies.

The test ambient temperature often specified for many retail-level PSUs is 25°C . In some technical specification documents, usually not intended for consumer viewing, a typical qualifying phrase is "0ºC~25ºC for full rating of load, decrease to zero Watts O/P at 70ºC." The meaning of this phrase has been discussed here before in the PSU forum and in the Power Shmower text box in Recommended PSUs.

It means that power output capacity declines with temperature rise from maximum at 25°C to zero watts at 70°C. The gist of the issue is that a PSU in a typical computer would never see 25°C unless it was turned off. 35°C is much more realistic, and 40°C is not uncommon in a high performance low noise PC under full load. So a PSU rated for 400W output at 0-25°C and 0 watts at 70°C cannot produce its rated power in the thermal environment of a typical PC. If you assume that this power drop is linear, then max power capacity will drop by ~9W for every degree over 25ºC. At the relatively cool internal PC case temperature of 35ºC, this "400W" PSU could only produce 310W max. At 40ºC, max output would drop to 265W. At 45ºC, you're down to 220W.

Makes you want to run and check the fine print on your PSU, doesn't it?

In most specification documents released to consumers, the test ambient temperature is not even specified. So PSU power ratings on retail packages tell little about performance in real working conditions.

The fan in most higher quality PSUs is thermally controlled to speed up with increased temperature. Naturally, ambient temperature has a big influence on the noise of the PSU, not just its load. While most makers of quiet PSUs only claim low noise at minimal loads, as end users, it is useful for us to know at what load and temperature the fan begins to spin up and make more noise: All the more reason to test the PSU under realistic ambient temperatures.

OEM and system integrator markets are much tougher to satisfy than retail markets, because their buyers are often part of the engineering team. Obsfucation and snake oil marketing techniques don't work on them. These buyers usually seek PSUs rated for full power at 50°C. In mission critical corporate systems, in servers and in other tough applications, high temperature performance is a must. Companies and brands that sell successfully to this marketplace usually make much better PSUs than those who focus solely on the retail market.


This article was originally begun to describe the latest addition to SPCR's PSU test platform, but it's necessary to understand what came before. The table below is a summary.

Measure temperature of ambient air, case, and PSU exhaust
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. (1~2W power draw.)
Measure voltages across fans and DC output line
Heath / Zenith SM-2320 multimeter. This is an ordinary multimeter. It has been compared against a much more expensive lab instrument and comes very close (within 2%) on readings of 0~20VDC.
Load PSU to specific DC output power loads for each voltage line

DBS-2100 PSU load tester. Made specifically for testing computer power supplies, it 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 6 voltage lines: +5, +12, -12V, +3.3, -5, +5SR. Leads from the PSU plug into the front panel, and there are taps for taking voltage readings for the 3.3V, 5V and 12V lines.

Measure AC power, power factor (PF), VA, AC line voltage
Kill-A-Watt Power Meter. An inexpensive consumer power meter with very good accuracy and a host of useful functions.
Measure noise in dBA from 1 meter distance B&K model 1613 sound level meter. This professional caliber SLM dates back to 1978, weighs over 10 pounds, and is completely analog in design. It has a dynamic range that spans over 140 dB. The unit's absolute sensitivity reaches below 0 dBA. A quiet environment is a prerequisite to low noise testing; the lab has been measured down to ~17 dBA at night, and a 12 dBA adjacent room is also available for any PSUs that are quieter.
Thermal environment directly related to the power delivered. Custom Test Jig. The NEW ADDITION to the lab that promoted the writing of this article. It is fully explained below.
Vary AC Voltage to consider effects of brownouts and other real-world conditions on PSU performance California Instruments 801RP Variable AC Power Supply: Coming Very Soon! 800VA capacity, 0 - 270VAC range, 16 - 500 Hz frequency range. More on this item and how it will be used when it arrives in the lab!

More About Individual Test Gear

The DBS-2100 load tester enables simplified, controlled PSU load testing from as little as a few watts to a rated maximum of 614W (in DC voltage output). It is equipped with 2 AC outlets (individually fused with 7A 250V fuses) and 4 exhaust fans on the back panel. A bypass switch toggles the fans on or off so that noise measurements on the PSU can be made. The resistors get very hot under high loads, so it is important not to leave the fans off for long.

The California Instruments 801RP Variable AC Power Supply is a professional lab-grade instrument with very tight (0.5~1%) output voltage tolerances. Full details about this product is available on their website, http://www.calinst.com/rpseries.html. This instrument will help us clearly differentiate PSUs that are honestly rated an from those that are high powered in claims only. More later after this expensive new acquisition arrives in the lab.

THANK YOU to all the sponsors, friends and patrons of SPCR whose contributions helped to make this equipment purchase possible!

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