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How We Test Power Supply Units

Transient Tests

In the real world, a PSU is always working with loads that change, depending on whether the CPU or graphics cards are busy. So, a decent PSU review should contain some tests with dynamic or transient loads. We conduct a variety of transient tests:

  • While the PSU is working at a 20 percent load state, a transient load is applied to the PSU for 200ms (10A at +12V, 5A at 5V, 5A at 3.3V and 0.5 A at 5VSB).
  • While working at 50 percent load, the PSU is hit by the same transient load.
  • In the next tests we use again the same starting points, 20 and 50 percent load states, however we increase the load-changing repetition rate from 5 Hz (200ms) to 50 Hz (20ms) and 1 KHz (1ms) and we also apply higher loads at +12V (15A) and the 5V and 3.3V rails (6A). This way we push even harder the PSU.

In all tests, we measure the voltage drops that the sudden load change causes. The voltages should remain within the ATX specification's regulation limits. Finally, we should also note that the latest ATX spec requires a load-changing repetition state of 50 Hz to 10 KHz for transient response testing. The only reason that we decided to keep the "slow" 5 Hz tests is to retain compatibility with our database, however once we have enough data with the higher speed transient tests, we will most likely stop conducting the less stressful 5 Hz tests.

We also conduct three transient tests where we measure the response of the PSU in its simpler turn-on phase. In the first test, we turn off the PSU, dial a full load at 5VSB, and then switch on the PSU. In the second test, while the PSU is in standby, we dial the maximum load that +12V can handle and start the PSU. In the last test, while the PSU is completely switched off (we cut off power or switch off the PSU's on/off switch), we dial the maximum load that +12V can handle, then switch on the PSU and restore power.

The ATX specification states that recorded spikes on all rails should not exceed 10 percent of their nominal values (+10 percent for 12V is 13.2V and for 5V is 5.5V).

Aris Mpitziopoulos
Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.