SPCR's Revised PSU Testing System

Power
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REALISTIC OPERATING ENVIRONMENT TEMPERATURES

Anyone who plays around with computer hardware for any length of time quickly learns that the temperature of the air inside a typical PC easily reaches over 40°C especially near or above the CPU when the system is under load. This is precise the area in which a PSU is normally situated. For testing purposes, it would be ideal to use a real computer but the problem is that then the PSU load cannot be easily controlled; it must be done with CPU stress and system stress software, which is usually an all-or-nothing approach.

Two different strategies were used for previous PSU reviews to simulate a realistic thermal environment:

1) A hot light bulb placed at the intake vents of the PSU while it was being load tested on the open test bench, as shown below. This approach suffered from lack of thermal control, or any simulation of airflow in a case. Furthermore, the bulb needed to match the AC power drawn by the PSU. Above 150W or so, this was impractical to do, due to the absence of appropriate wattage bulbs and the tedium of changing hot bulbs.

2) A light bulb placed inside a mid-tower case in the same approximate position as the CPU. This approach simulated the rising heat aspect of the real PC environment but suffered the same problem as 1): No suitable high power bulbs.

THE NEW RIG

The total amount of heat inside any PC is equal to the total AC power it draws. That is, if 100W is the total power of a PC as measured by an AC power meter like the Kill-a-Watt, then 100W is the amount of heat that will be dissipated in the PC. Some of that heat will be generated by the power supply itself. During AC/DC conversion, there is always some loss, ranging from 40~20% of the total. The rest is all delivered as DC voltage to the components in the system and those components generate heat.

The objective was to allow the heat in the test environment of the PSU to be directly proportionate to the amount of power drawn by the PSU. In a real system, this is what happens.

With the DBS-2100 PSU load tester, the DC power from the PSU is dissipated by the large resistors inside. They get very hot under load, which is why the four 80mm fans are necessary.

It occurred to me that if the heat from the PSU load tester could be transferred into the operating environment of the PSU being tested, this would be the closest thermal simulation to actual PSU operating conditions that could be devised.

Eureka!

There seemed only two practical ways to achieve this:

1) Remove the components from inside the DBS-2100 PSU load tester and reassemble them inside a typical PC case. One look at the complex internal wiring convinced me this was not a good idea. Too much could go wrong. There were too many places where I could goof.

2) Build a PC case or other similar enclosure with ducts that the four exhaust fans on the DBS-2100 PSU load tester blow into, hence not only transferring the heat into the PSU's working environment, but also simulating the case cooling airflow that almost all PC systems depend on. This was the option chosen.

Here is the first result of a couple afternoons in the garage, working with scrap plywood.


A light in the bottom of the box: So you can see it better.

The internal dimensions of the box measure approximately 40 x 40 x 20 cm (16" x 16" x 8"). This is about 32 liters, slightly smaller than a typical mid-tower case, which is closer to 40 liters. However, this box is completely empty, while a typical mid-tower PC will be filled with a motherboard, CPU/HSF, optical and hard drives, VGA card... with all the components in place, the typical mid-tower is likely to have about the same volume of air.

There is a cutout on one side of the box that the protruding fans on the DBS-2100 PSU load tester fit into perfectly. The box has no bottom; none was required, and leaving the bottom open made it easier to get to the inside during assembly.

You can see that there is an opening on one top corner: This is where any ATX form factor PSU fits in perfectly. The bottom and back are open to accommodate multiple or 120mm fan power supplies. See the photo below.

Preliminary trials showed that the rig works pretty much as intended. There was only one problem: The tight fit of the PSU caused it to become mechanically coupled to the box, and the box resonated enough so that it added 3~10 dBA of additional noise to the PSU fan noise. This was unacceptable, so a rethink was necessary.

Back to the drawing board.



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