Advanced Transient Response Tests

In these tests, we monitor the response of the PSU in two different scenarios. First, a transient load (10 A at +12V, 5 A at 5V, 5 A at 3.3V, and 0.5 A at 5VSB) is applied to the PSU for 200 ms while the latter is working at 20% load. In the second scenario, the PSU, while working at 50% load, is hit by the same transient load. In both tests, we measure the voltage drops the transient load causes with our oscilloscope. Voltages should remain within the regulation limits defined by the ATX specification.

Real-world usage always has a PSU working with loads that change, depending on whether the CPU or graphics card(s) are busy. As such, it is of immense importance for the PSU to be able to keep its rails within the limits as defined by the ATX specification. The smaller the deviations on a rail, the steadier the system will be, which will reduce the stress applied to its components.

We should note that the ATX specification asks for capacitive loading during transient tests. However, in our methodology, our worst-case scenario is done with no extra capacitance on the rails. Although the ATX specification asks for capacitance, your system (the mainboard and other parts) may not provide it, so we have to keep that scenario in mind as well.






Transient response is mediocre at +12V, satisfactory at 5V and 5VSB, and bad at 3.3V, with the power supply failing during the second test (and being on the verge of failing during the first test).

Below are the oscilloscope screenshots we took during Advanced Transient Response testing.

Transient Response at 20% Load

Transient Response at 50% Load

Turn-On Transient Tests

We measure the response of the PSU in simpler scenarios of transient load during the power-on phase of the PSU in the next set of tests. In the first test, we turn the PSU off, dial the maximum current 5VSB can output, and then switch on the PSU. In the second test, we dial the maximum load +12V can handle and start the PSU while the PSU is in standby mode. In the last test, while the PSU is completely switched off (we cut off power or switch the PSU off by flipping its on/off switch), we dial the maximum load the +12V rail can handle before switching the PSU on through the loader and restoring power. The ATX specification states that recorded spikes on all rails should not exceed 10% of their nominal values (e.g., +10% for 12V is 13.2V and 5.5V for 5V).



The first test at 5VSB went fine; however, the slopes at +12V didn't rise smoothly. There are no high spikes and voltage overshoots, though.

Ripple Measurements

Ripple represents the AC fluctuations (periodic) and noise (random) in the DC rails of a PSU. Ripple significantly decreases the life span of capacitors since it increases their temperature; a 10 °C increase can cut a capacitor's lifespan in half. Ripple also plays an important role with overall system stability, especially when it is overclocked. The ripple limits are 120mV (+12V) and 50mV (5V, 3.3V, and 5VSB) according to the ATX specification.


Ripple suppression is good enough at +12V and pretty good at 5V and 5VSB. We would like to see less than 30 mV at full load on the 3.3V rail, though.

Ripple at Full Load

Ripple at 110% Load

Ripple at Crossload 1

Ripple at Crossload 2