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Hold Up Time
Although modern power grids are stable and of fairly high quality, there are always some power fluctuations. Modern power supplies have capacitor banks to handle such power fluctuations for a certain period, called the hold up time. The ATX standard for hold up time is a minimum of 16 milliseconds.
Tom's Hardware measures this by permanently monitoring the PSU's output voltages. After cutting off the power supply, the oscilloscope records the voltage curves in order to accurately measure how much time passes before the voltages drop. These measurements are carried out both at 115V and 230V.
A PSU's power rating normally describes the maximum amount of power it can supply over an extended period of time without being damaged. Here, the total is the sum of the individual 3.3V, 5V, 12V, -12V, and 5V standby rails combined. To check whether a PSU operating at its maximum specified load is truly at its limit or whether the manufacturer has left the PSU with some headroom, we perform an overload test. The load on the 12V rails are set to 110% of its specified maximum. If the PSU can run the test for five minutes without dropping voltage below the ATX specification's 11.4V (a maximum 5% deviation), the test passes.
Short Circuit Test
A critical short circuit doesn't happen very often, but it's not inconceivable, and the result is often a dead PSU. Today, however, many PSUs have circuits to protect them from short circuit damage. To test the protection circuit, Tom's Hardware manually triggers a short circuit on a 12V rail. The power supply should turn off immediately so it doesn't overheat and becomes permanently damaged. If the PSU can be used normally again after deactivating the short circuit, the test passes.
Like any electrical appliance, a PSU operates at a certain efficiency. The energy not used for powering the computer and its peripherals is released as heat into the environment. Tom's Hardware continuously monitors the temperatures of the inflowing and exiting air to track the warming effect, and the biggest difference measured between these two values is reported as the test result.
Accessories and Ergonomics
In addition to the results of the numerous performance tests mentioned above, the PSU's accessories and ergonomics are also taken into account in our final results. The testers turn their attention to details, such as included mounting solutions, housing construction quality, cable modularity, and cable lengths. Anything else making the user's life harder or easier gets mentioned as well.