eTeknix Power Supply Testing Methodology

Voltage stability is determined by carrying out hot and cold load tests. The cold load test is there to see what the PSU is potentially capable of without the limiting factor of heat, whereas the hot load test tells us how well the unit operates when exposed to high temperatures.

These tests are reasonably simple but rather time-consuming. First of all, the amp values for 100% load must be determined which must be done manually. The emphasis is on stressing the 12V rail(s) rather than the less important 5V and 3.3V rails so generally I put the 12V rail up to capacity and then make up the remainder of the total wattage with the 5V and 3.3V rails. These amps for 100% load are then fed into my very own spreadsheet which, through magic and other means, calculates the amp values for the other loads I will test at which are 80%, 60%, 40% and 20%. These values are in turn entered into the load tester. VA represents 12V1, VB is 5V, VC is 12V and VE is 3.3V. Here is an example of a 20% load entered:

Once these are entered, I then press a button which switches the unit on and applies the load I have entered to the unit. If I switch it to wattage mode, it will tell me the total wattage being applied (bottom left) and the load being applied to each rail:

I then leave the unit running at that load for at least 10 minutes before measuring the voltage, to do this I simply switch the tester to voltage mode and it displays the voltages for each rail. I was originally sceptical at the accuracy of the built-in voltage meter so I got my own RS-calibrated Fluke 177 multimeter and sure enough, the load tester’s voltage was spot on. Although for the price the tester costs, I would expect it to be! Here are the voltages being displayed on the screen:

And it’s as simple as that! It takes a lot of time, especially to manually work out the correct 100% load and to key in all the numbers. However, I have now requisitioned an old laptop with a COM port which can interface directly with the tester to dial in the amps rather than using the up and down buttons which takes forever! The laptop replaces the aforementioned desktop PC after suffering an electric shock from touching the chassis due to a poor quality PSU- a double whammy of pain consisting of electricity and irony!