SPCR's Revised PSU Testing System

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POSTSCRIPT — April 13, 2004

Shortly after this article was originally published, a member of the SPCR forum posted a strong criticism about the new PSU Testing System, especially the Thermal Simulation Box. Rory B. wrote:

"I am going to raise a stink about SPCR's new thermal box. In every picture of that PSUtester that I have ever seen, there are four fans on the rear. Mike described the box as being not air-tight with air being able to escape around the cables. He went on to say that he could feel the air coming out the holes. The logic is that the power resistors in the PSUtester will supply the heat for the box.

"But as long as the heat for the box is being blown in by the four 80mm fans which also provide back-pressure, the testing methodology is flawed.

"In many systems, the power supply's fan will be operating against negative pressure in the box, and the fans will need to spin faster due to the axial fan's relative inefficiency at low speeds. The thermal box, however, is a POSITIVE PRESSURE testing situation, and the four fans will be pressurizing the box. Because the pressure inside the box is already forcing air out through the power supply, the fans don't NEED to spin up! There are already four 80mm fans working to cool the PSU!"

Rory B.'s comment forced me to reexamine all the various aspects of this thermal simulation box. It helped to set down all the salient points:

1) The PSU Load Tester absorbs all the power output of the PSU, and thus gets very hot. Generally, at least twice as hot as the PSU. (Because at least ~65% of the AC power drawn by a PSU is sent to the Load Tester, while only ~35% or less of the power remains as heat in the PSU.)

2) The design and shape of the PSU Load Tester does not allow convection alone to keep the big resistor banks cool when more than ~150W is being dissipated: They must be fan cooled or eventually suffer heat damage.

3) Each fan gets 12VDC across its terminals, and there is definitely positive pressure inside the box. It means that unless the PSU cooling fans are blowing huge amounts of air, turning the PSU Load Tester fans on increases the flow of air through the PSU. This effect can be felt as increased airflow at the PSU exhaust outlet.

4) The fans themselves are Max Flow brand, model 8025D1-HSPL, 12VDC, 0.18A. The brand does not have its own website, but I found a site that lists some of its products. However, this particular model was not listed. Their AC version of the same fan (also rated for 0.18A) is said to push 22/25 cfm.

Checking my fan database for 80mm 12VDC fans rated close to 0.18A, there is a fairly broad range of airflow specs, from under 20 cfm to over 30 cfm. On the basis of simple feel and ballpack averaging, I will chose 25 cfm as a guesstimate for this model's rate airflow (free and unimpeded).

There are four fans in parallel, so one could say they represent 100 cfm, but we know this is not true, because the airflow is always measured in free air without resistance. The air intake for the load tester is a row of slots on the front edge that represents a much smaller area than that of the fan blades. Then the air has to pass through a huge jumble of cables as well as the large cylindrical resistors themselves.

My guess is that the total real airflow into the Thermal Box is somewhere in the vicinity of ~60 cfm.

What does this ~60 cfm airflow do?

1. It helps to cool the big hot resistors. The cooling is not 100% effective. In other words, the cooling airflow does not drop the temperature of the resistors down to ambient; some of the heat is retained by the resistors.

2. It forces a lot of the heat from the hot resistors into the Thermal Box and increases the air temperature inside the box.

3. From a case simulation point of view, it's kind of like running 2~3 Panaflo 80L fans at 12V for front intake. This is definitely more airflow than any serious PC silencer would use (because of the noise). However, the absence of any large exhaust vent or vent makes it a strange case.

What happens if the airflow is reduced?

Without getting into thermodynamics or fluid dynamics, it is safe to say that just how much heat the PSU gets exposed to in the Thermal Box depends on not only the power that's fed into the resistors, but also the amount of airflow across those resistors. The following was verified experimentally:

  • Without any airflow, the PSU actually "sees" less of the heat because it tends to stay in and around the resistors.
  • With a lot of airflow, the amount of heat is spread across a bigger volume of air, and so the average temperature of the air in the box is also not that high.
  • The trick is to get a level of airflow that allows the Load Tester resistors to be cooled well enough, and transfer enough of the heat into the Thermal Box to get it hot, yet not allow too much in the way of positive pressure so that the PSU is unduly helped in keeping itself cool.


1. PSU Load Tester Fans set to 5V —

Some exploration in the maze of wiring inside the PSU Load Tester revealed a 5V terminal that is easily accessed for the fans. Making the switch means all the fans in the Load Tester are driven off the 5V line of the PSU being tested. (This represents a load of ~2W, which is compensated for in the PSU review data.)

The result is much reduced but still steady airflow. The pressure is low enough that it is difficult to discern any increase in the PSU exhaust with a PSU that uses a low airflow Panaflo 80L fan. If we guesstimate by assuming a linear relationship between airflow and voltage, we're looking at 25~30 CFM for the Load Tester fans at 5V. (It is also far quieter, registering just ~26 dBA/1m.)

2. 80mm Exhaust Fan in the Thermal Simulation Box

Virtually every PC has one, even quiet PCs: Why not this simulation box? A Panaflo 80mm Medium speed fan was chosen. This fan is rated for 32 CFM at 12V, and produces about 13~14 CFM at 5V, which is the intended setting. Why? Because 5V is about the level that SPCR readers would find this fan acceptably quiet, and it still provides a useful amount of airflow. This exhaust fan also ensures that the PSU does not benefit unfairly from positive airflow.

As you can see in the photo above, a rough 4" square hole was made on the "back" panel of the Thermal Box, about where an exhaust fan would go in a real case. A hole was cut out for the fan in a 4" square piece of 1" thick foam. Then the fan was inserted into the foam, and the foam+fan then wedged into the hole. The foam insulates the fan's vibrations from getting into the structure of the box and maintains a good seal.

The end result of these changes is a PSU Thermal Simulation box that closely resembles a typical SPCR PC case for airflow, and still uses the DC output of the PSU to heat the case as in a real PC.

For a practical illustration of what this change means for testing results, please check the Postscript to the Enermax NoiseTaker 475 PSU review.

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