Why you can trust Tom's Hardware Our expert reviewers spend hours testing and comparing products and services so you can choose the best for you. Find out more about how we test.
Transient Response Tests
Advanced Transient Response Tests
For details on our transient response testing, please click here.
Ιn these tests, we monitor the 850 BQ's response in two different scenarios. First, a transient load (10A at +12V, 5A at 5V, 5A at 3.3V, and 0.5A at 5VSB) is applied for 200ms while the PSU works at 20 percent load. In the second scenario, EVGA's contender is hit by the same transient load while operating at 50 percent load. In both tests, we use our oscilloscope to measure the voltage drops caused by the transient load. The voltages should remain within the ATX specification's regulation limits.
These tests are crucial because they simulate the transient loads a PSU is likely to handle (such as booting a RAID array or an instant 100 percent load of CPU/GPUs). We call these tests "Advanced Transient Response Tests," and they are designed to be very tough to master, especially for a PSU with a capacity of less than 500W.
Advanced Transient Response at 20 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.195V | 12.107V | 0.72% | Pass |
| 5V | 5.071V | 4.922V | 2.94% | Pass |
| 3.3V | 3.384V | 3.198V | 5.50% | Pass |
| 5VSB | 4.998V | 4.945V | 1.06% | Pass |
Advanced Transient Response at 50 Percent
| Voltage | Before | After | Change | Pass/Fail |
|---|---|---|---|---|
| 12V | 12.123V | 12.026V | 0.80% | Pass |
| 5V | 5.021V | 4.872V | 2.97% | Pass |
| 3.3V | 3.336V | 3.154V | 5.46% | Pass |
| 5VSB | 4.948V | 4.890V | 1.17% | Pass |
The +12V rail's performance is quite good in these tests, and the 5V rail manages to stay within 3%. Deviations on the 5VSB rail are low as well. By far, the worst-performing rail is the 3.3V one; it registers close to 5.5% deviations on both tests. Thanks to a high initial voltage, though, the 3.3V rail manages to stay above the corresponding ATX limit (3.14V) the moment our transient load is applied.
Here are the oscilloscope screenshots we took during Advanced Transient Response Testing:
Transient Response At 20 Percent Load
Transient Response At 50 Percent Load
Turn-On Transient Tests
In the next set of tests, we measured the PSU's response in simpler transient load scenarios—during its power-on phase.
For the first measurement, we turned off the 850 BQ, dialed in the maximum current its 5VSB rail could output, and switched the PSU back on. In the second test, we dialed the maximum load the +12V rail could handle and started the 850W supply while it was in standby mode. In the last test, while the PSU was completely switched off, we dialed the maximum load the +12V rail could handle before switching it back on from the loader and restoring 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 5.5V for 5V).
There are no voltage overshoots or spikes to mention. Instead, we observe good performance, especially for a budget-oriented PSU.
