- Date:
- Tuesday , May 13, 2008
- Author:
- Paul Johnson
- Editor:
- Kyle Bennett
- Share:

Getting the Best Buy on a Power Supply
Best Buy has certainly stepped up its game in the last few years when it comes to better servicing the DIY computer builder or upgrader. But is Best Buy doing you a service or disservice by selling you its own computer power supply?
Load Testing
For those of you that are curious as to some of the reasoning and equipment behind our PSU testing program here at HardOCP, we have put together a living document that shares a lot of the behind the scenes of the program. The testing we are conducting today is exactly as described in that document and will begin with a range of loads tested at 120v input including our torture test and then move on to the same set of tests at 100v input but without the torture test.
120v and 100v Load Testing Results

Test #1 is equal to approximately 25% of the rated capacity of the Dynex 400w at 45c. This makes Test #1 equal to 102w by loading the 12v rails to a combined 4a, the 5v rail to 4a, the 3.3v rail to 2a, the +5vsb to 2a, and the -12v to 0.5a. The results of Test #1 at both 120v and 100v are mixed. The units starting DC output voltages are all very good, however the units efficiency is not doing nearly as well at ~76% at 120v and ~74% at 100v.
Test #2 is equal to approximately 50% of the rated capacity of the Dynex 400w at 45c. This makes Test #2 equal to 201w by loading the 12v rails to a combined 11a, the 5v rail to 8a, the 3.3v rail to 4a, the +5vsb to 2a, and the -12v to 0.5a. Test #2 shows mixed changes on the DC output voltages from Test #1 as the 12v rails have risen while the 5v and 3.3v rails have dropped. The units efficiency has moved up to ~79% at 120v and ~78% at 100v while the exhaust temperatures have jumped to 53c/54c.
Test #3 is equal to approximately 75% of the rated capacity of the Dynex 400w at 45c. This makes Test #3 equal to 300w by loading the 12v rail to 16a, the 5v rail to 13a, the 3.3v rail to 8a, the +5vsb to 2a, and the -12v to 0.5a. Test #3 saw the first Dynex 400w blowout on the primary side destroying our Variac. The second sample we obtained would not transition past Test #2 at all and on the third attempt to do so shutdown quietly and would not restart for several hours. Once it would restart the unit would intermittently shutoff in Test #2.
Load Testing Summary
During the first half of the Dynex 400w’s advertised DC output capacity the unit was marginal in regards to its load testing results. The unit’s efficiency was certainly not great peaking at just ~79% in Test #2 at 120v but it could have been worse. However, whenever the unit was pushed beyond 200w DC the unit was a complete and utter failure, which is in stark contrast to what the 400w label on the unit promised. Indeed, the first unit blew out on the primary side and in doing so it damaged our Variac requiring replacement. To date primary side failures have typically taken out the power meter being used not the Variac. So in the end, the Dynex unit did manage to excel in one category, sadly that category was the test equipment destroying category where it somehow one upped the PowertekPro 500w which is certainly not a good thing to excel at in our estimation.
Transient Testing
The Transient Testing portion of this review marks the first revision to our living testing methodology. Readers can read more about this inclusion in our testing here in our methodology section, but briefly we will be examining the response of the power supply to a short duration load such as a RAID array spinning up or load change due to power draw from video cards etc. Ideally we would not see a deflection from the baseline voltage output when this occurs but that is simply not the case for the majority. We will be using the ATX12v specification for transient response as a guide.
The Transient Load Tester adds an additional 9.25A to the 12v rail for 10ms and an additional 3.75A to the 5v rail for 10ms at 25% total load and 50% total load.
*Due to the magnitude of the change exhibited by the 12v rails the voltage divider has been changed for that rail to 100mV from 50mV.
Test #1 is equal to approximately 25% of the rated capacity of the Dynex 400w at 45c. This makes Test #1 equal to 102w by loading the 12v rails to a combined 4a, the 5v rail to 4a, the 3.3v rail to 2a, the +5vsb to 2a, and the -12v to 0.5a before the addition of the transient load. The results of Test #1 show a peak change of ~400mV when the 12v rail is loaded and ~100mV change when the 5v rail is loaded. During the 12v load an unloaded 12v rail shows a peak change of ~200mV while the 5v rail shows a peak change of ~50mV.
*Due to the magnitude of the change exhibited by the 12v rails the voltage divider has been changed for that rail to 100mV from 50mV.
Test #2 is equal to approximately 50% of the rated capacity of the Dynex 400w at 45c. This makes Test #2 equal to 201w by loading the 12v rails to a combined 11a, the 5v rail to 8a, the 3.3v rail to 4a, the +5vsb to 2a, and the -12v to 0.5a before the addition of the transient load. The results of Test #2 show a peak change of ~325mV when the 12v rail is loaded and ~100mV change when the 5v rail is loaded. During the 12v load an unloaded 12v rail shows a peak change of ~200mV while the 5v rail shows a peak change of ~50mV.
Transient Load Testing Summary
The results of the Transient Load Tests are really not all that bad considering how great a percentage of the units total DC output the transient loads represent, the performance of other 400w power supplies, and how just absolutely horrible the unit was otherwise during the load testing. Overall we saw peak changes on the loaded 12v rails of ~400mV in Test #1 and ~325mV in Test #2 while the 5v rails when loaded hit ~200mV. While in specification these values did result in the units dropping very low in the acceptable range for the ATX12v specifications.








