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Switching Power Supplies A - Z, Second Edition
Switching Power Supplies A - Z, Second Edition, by Sanjaya Maniktala (Newnes), starting at $48.82
Home » Power
Hardware Secrets Power Supply Test Methodology
Author: Gabriel Torres
Type: Articles Last Updated: June 23, 2009
Page: 5 of 8
Temperature

As we have already explained, temperature is a very important aspect on true power supply reviews, since semiconductors lose their ability to deliver current (and thus power) as the temperature increases.

Several power supply manufacturers label their products at 25° C, which is an unrealistic temperature. Inside the computer case the temperature is far higher than that. Because of this we will conduct our tests with a room temperature between 45° and 50° C.

We measure temperature with a Fluke 52 II precision digital thermometer, which has accuracy of 0.05% + 0.3° C. This instrument was bought in May 2009 and on reviews posted before that we used a CompuNurse digital thermometer, which isn't so precise (that is why the Fluke thermometer costs over USD 200 and the CompuNurse costs less than USD 20). This thermometer has two probes. One is used to measure temperature inside our "hot box", while the other is used to measure the temperature on the surface of the power supply, measured on its top side.

Fluke 52 II
click to enlarge
Figure 8: Fluke 52 II precision digital thermometer.

Instead of buying a thermal chamber (a.k.a. “incubator”), which would allow us to set the exact temperature we want the power supply to run under, we decided to build our own "hot box". The goal of this "hot box" is to keep temperature around the power supply between 45° C and 50° C (between 113° F and 122° F). To increase the temperature inside the "hot box" we installed a duct connecting the exhaust system from our load tester, which blows hot air, to inside the box. The box itself is constructed in MDF. We built this box on 06/23/2009. On reviews posted before this date we used a cardboard box (we used the box from our Sony home theater receiver), which, despite its amateur appearance, worked just fine.

Hot Box
click to enlarge
Figure 9: Our “hot box.”

Hot Box
click to enlarge
Figure 10: Duct connecting the exhaust system from our load tester to the "hot box".

Hot Box
click to enlarge
Figure 11: Location of the thermometer probe.

Next to our box we have a five pound ABC fire extinguisher (not shown on the picture) for any emergency.

Before starting our load tests we will keep the power supply running until we get the temperature inside the box with at least 45° C. We can increase or decrease the temperature inside the box by opening or closing its front acrylic cover. We also added a 110 V fan connected to a dimmer on one of the sides of the box so we can better manage the temperature inside the box.

Hardware Secrets Lab
click to enlarge
Figure 12: Fan and dimmer.

Some may argue that we could install the power supply inside a case. Actually this isn’t a good choice for several reasons. First, the length of the power supply cables wouldn’t allow us to do this. Second, we would need to keep the case open to install the cables from the power supply on the load tester. Third, we would need to have a running system inside the case in order to generate an amount of heat compatible with the one produced by a real PC, and that would be impossible to accomplish as we would need another power supply feeding this system – and where should we install it? So we would need to keep the case open for using this second power supply and since the case is open, the whole idea of using a PC case goes down the drain (we would need the case closed to really simulate a typical PC).

In fact even using a commercial thermal chamber we would be faced with some of the challenges exposed above: the length of the power supply cables and the need to open a hole on the chamber to pass these cables from the power supply to the load tester.

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