Corsair RM850x (2021) Power Supply Review

The Corsair RM850x is the new leader in the 850W Gold category.

Corsair RM850x (2021)
(Image: © Tom's Hardware)

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Advanced Transient Response Tests

For details about our transient response testing, please click here.

In the real world, power supplies are always working with loads that change. It's of immense importance, then, for the PSU to keep its rails within the ATX specification's defined ranges. The smaller the deviations, the more stable your PC will be with less stress applied to its components. 

We should note that the ATX spec requires capacitive loading during the transient rests, but in our methodology, we also choose to apply a worst case scenario with no additional capacitance on the rails. 

Advanced Transient Response at 20% – 20ms

Swipe to scroll horizontally
VoltageBeforeAfterChangePass/Fail
12V12.001V11.813V1.57%Pass
5V4.969V4.909V1.21%Pass
3.3V3.282V3.141V4.30%Pass
5VSB4.945V4.889V1.13%Pass

Advanced Transient Response at 20% – 10ms

Swipe to scroll horizontally
VoltageBeforeAfterChangePass/Fail
12V12.001V11.820V1.51%Pass
5V4.968V4.903V1.31%Pass
3.3V3.281V3.136V4.42%Fail
5VSB4.945V4.884V1.23%Pass

Advanced Transient Response at 20% – 1ms

Swipe to scroll horizontally
VoltageBeforeAfterChangePass/Fail
12V12.001V11.812V1.57%Pass
5V4.968V4.888V1.61%Pass
3.3V3.282V3.149V4.05%Pass
5VSB4.944V4.891V1.07%Pass

Advanced Transient Response at 50% – 20ms

Swipe to scroll horizontally
VoltageBeforeAfterChangePass/Fail
12V11.944V11.851V0.78%Pass
5V4.961V4.905V1.13%Pass
3.3V3.278V3.127V4.61%Fail
5VSB4.930V4.882V0.97%Pass

Advanced Transient Response at 50% – 10ms

Swipe to scroll horizontally
VoltageBeforeAfterChangePass/Fail
12V11.944V11.870V0.62%Pass
5V4.961V4.896V1.31%Pass
3.3V3.278V3.126V4.64%Fail
5VSB4.931V4.870V1.24%Pass

Advanced Transient Response at 50% – 1ms

Swipe to scroll horizontally
VoltageBeforeAfterChangePass/Fail
12V11.943V11.888V0.46%Pass
5V4.961V4.893V1.37%Pass
3.3V3.278V3.117V4.91%Fail
5VSB4.935V4.882V1.07%Pass

Transient response is about the same as the previous model on all rails but 3.3V, where the new model performs notably worse. 

Turn-On Transient Tests

In the next set of tests, we measure the PSU's response in simpler transient load scenarios—during its power-on phase. Ideally, we don't want to see any voltage overshoots or spikes since those put a lot of stress on the DC-DC converters of installed components.

There is only a tiny voltage overshoot at 5VSB, which is nothing to worry about. 

Power Supply Timing Tests

There are several signals generated by the power supply, which need to be within specified, by the ATX spec, ranges. If they are not, there can be compatibility issues with other system parts, especially mainboards. From year 2020, the PSU's Power-on time (T1) has to be lower than 150ms and the PWR_OK delay (T3) from 100 to 150ms, to be compatible with the Alternative Sleep Mode.

Swipe to scroll horizontally
PSU Timings Table
T1 (Power-on time) & T3 (PWR_OK delay)
LoadT1T3
20%49ms135ms
100%50ms137ms

The PWR_OK delay is within the 100-150ms region, so the PSU supports the alternative sleep mode recommended by the ATX spec.

Ripple Measurements

Ripple represents the AC fluctuations (periodic) and noise (random) found in the PSU's DC rails. This phenomenon significantly decreases the capacitors' lifespan because it causes them to run hotter. A 10-degree Celsius increase can cut into a cap's useful life by 50%. Ripple also plays an important role in overall system stability, especially when overclocking is involved.

The ripple limits, according to the ATX specification, are 120mV (+12V) and 50mV (5V, 3.3V, and 5VSB).

Swipe to scroll horizontally
Test12V5V3.3V5VSBPass/Fail
10% Load4.7 mV2.6 mV3.2 mV5.9 mVPass
20% Load4.5 mV2.7 mV3.0 mV5.7 mVPass
30% Load6.5 mV2.7 mV3.3 mV5.9 mVPass
40% Load5.6 mV2.6 mV3.3 mV5.6 mVPass
50% Load5.4 mV2.8 mV4.3 mV6.4 mVPass
60% Load5.2 mV2.9 mV3.6 mV6.1 mVPass
70% Load5.4 mV3.1 mV4.8 mV6.5 mVPass
80% Load5.6 mV3.1 mV9.5 mV6.3 mVPass
90% Load5.8 mV3.3 mV11.4 mV7.3 mVPass
100% Load8.2 mV5.4 mV11.2 mV7.3 mVPass
110% Load8.1 mV5.6 mV11.3 mV7.2 mVPass
Crossload 15.6 mV4.1 mV12.4 mV7.2 mVPass
Crossload 28.4 mV4.5 mV4.9 mV6.8 mVPass

Ripple suppression is excellent.

Ripple At Full Load

Ripple At 110% Load

Ripple At Cross-Load 1

Ripple At Cross-Load 2

EMC Pre-Compliance Testing – Average & Quasi-Peak EMI Detector Results

Electromagnetic Compatibility (EMC) is a device's ability to operate properly in its environment without disrupting the proper operation of other nearby devices.

Electromagnetic Interference (EMI) stands for the electromagnetic energy a device emits, and it can cause problems in other nearby devices if too high. For example, it can be the cause of increased static noise in your headphones or/and speakers.

(Image credit: Tom's Hardware)

EMI emissions are low, with both detectors we used. This means that the PSU's corresponding filter does a great job. 

MORE: Best Power Supplies

MORE: How We Test Power Supplies

MORE: All Power Supply Content

Aris Mpitziopoulos
Contributing Editor

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

  • Isokolon
    shame they changed the fan. the 2015 & 2018 RMx were so great for low-noise enthusiasts.
    how the review classifies an almost 30db unit as "quiet" is beyond me.
    Reply
  • Udyr
    What makes this unit better than the Toughpower GF1?

    https://www.tomshardware.com/reviews/thermaltake-toughpower-gf1-850w-power-supply
    Reply
  • g-unit1111
    I have an RM750x and it is a really great PSU. You can't go wrong with this unit.
    Reply
  • jsz031
    Hi Aris.. like your reviews..

    small question. I noticed the Bulk/hold up caps get higher with wattage. can you explain how this impacts a general PSU?

    for example/off topic, I have a 5 year old 550w G2 and it has a 400v 470uf cap. Just purchased a 550w g3 and the cap is lower capacity @ 400v 390uf.

    Does this mean anything or am I overthinking it? is the 550w g2 better than "base" spec? I understand leakage can occur over time, I'm just not sure if its a downgrade or upgrade in specific areas.

    thank you.
    Reply
  • jayjr1105
    jsz031 said:
    Hi Aris.. like your reviews..

    small question. I noticed the Bulk/hold up caps get higher with wattage. can you explain how this impacts a general PSU?

    for example/off topic, I have a 5 year old 550w G2 and it has a 400v 470uf cap. Just purchased a 550w g3 and the cap is lower capacity @ 400v 390uf.

    Does this mean anything or am I overthinking it? is the 550w g2 better than "base" spec? I understand leakage can occur over time, I'm just not sure if its a downgrade or upgrade in specific areas.

    thank you.
    As long as they are meeting the ATX spec of 17ms hold up time, it doesn't really matter. The G2 was probably overkill so they cut costs by going with a lower capacity one.
    Reply
  • tomkawal
    Admin said:
    The new Corsair RM850x is among the best PSUs in its category.

    Corsair RM850x (2021) Power Supply Review : Read more
    ..and worst quality coming from production line! Mine was covered with thick layer of metal salts from soldering. It stopped turning on. Works again after thorough cleanup.
    Reply
  • tomkawal
    Beautifully presented. I was not so lucky with the unit I got, which stopped working after a month. Sent back to eBuyer guys and received it again not fixed (generally they requested ALL cables where I wanted only brick swap - this is totally unacceptable for me when the wires are in place tied to enclosure!) Raised ticket with Corsair with no answer in 2 days. Fixed on my own with one hour of cleaning the PCB from thick layer of soldering flux on the bottom side of PCB which caused leakage currents all over the board. Now it works. Horrendous experience and I don't recommend none of these businesses.
    Reply