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Corsair RM750x PSU Review

Corsair released its RMx PSU line, which the company claims will offer good performance along with silent operation. Unlike the RMi models, the RMx units lack a digital interface, a fan test button, and uses a Rifle bearing fan instead of an FDB version.

Transient Response Tests And Ripple Measurements

Advanced Transient Response Tests

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

In these tests, we monitored the response of the PSU in two different scenarios. First, a transient load (10A at +12V, 5A at 5V, 5A at 3.3V and 0.5A at 5VSB) was applied to the PSU for 200 milliseconds while the PSU was working at 20-percent load. In the second scenario, the PSU was hit by the same transient load while operating at 50-percent load. In both tests, we used our oscilloscope to measure the voltage drops caused by the transient load. The voltages should have remained 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, an instant 100-percent load of CPU/GPUs, etc.). 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

VoltageBeforeAfterChangePass/Fail
12V12.048V11.971V0.64%Pass
5V5.042V4.976V1.31%Pass
3.3V3.317V3.213V3.14%Pass
5VSB5.016V4.967V0.98%Pass

Advanced Transient Response at 50 Percent

VoltageBeforeAfterChangePass/Fail
12V11.984V11.910V0.62%Pass
5V5.037V4.968V1.37%Pass
3.3V3.309V3.199V3.32%Pass
5VSB5.000V4.946V1.08%Pass
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The +12V rail behaved very well during these tests, and the 5V and 5VSB rails kept their voltage drops at low levels. On the other hand, the 3.3V rail didn't manage to keep its voltage above 3.2V during the second test, although its deviation was at normal levels (for this rail).

Here are the oscilloscope screenshots we took during the Advanced Transient Response Tests:

Transient Response At 20 Percent Load

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Transient Response At 50 Percent Load

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Turn-On Transient Tests

In the next set of tests, we measured the response of the PSU in simpler transient load scenarios — during the PSU's power-on phase.

For the first measurement, we turned off the PSU, dialed in the maximum current the 5VSB could output and switched on the PSU. In the second test, we dialed the maximum load the +12V could handle and started the PSU while it was in standby mode. In the last test, while the PSU was completely switched off (we cut off the power or switched off the PSU by flipping its on/off switch), we dialed the maximum load the +12V rail could handle before switching on the PSU from the loader and restoring the 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).    

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A tiny voltage overshoot at the 5VSB rail and two small waves on the waveform before the +12V rails settle down during the last test are nothing to worry about. Overall, the PSU showed good performance here.

Ripple Measurements

To learn how we measure ripple, please click here.

The following table includes the ripple levels we measured on the rails of the RM750x unit. The limits, according to the ATX specification, are 120mV (+12V) and 50mV (5V, 3.3V and 5VSB).

Test12V5V3.3V5VSBPass/Fail
105.3 mV6.1 mV7.3 mV20.7 mVPass
205.7 mV6.0 mV7.5 mV4.0 mVPass
306.8 mV5.6 mV7.8 mV4.3 mVPass
407.0 mV5.7 mV9.8 mV5.2 mVPass
507.6 mV6.1 mV9.6 mV6.1 mVPass
608.5 mV6.6 mV11.2 mV6.9 mVPass
709.3 mV7.6 mV11.9 mV8.2 mVPass
8010.5 mV8.4 mV12.8 mV10.4 mVPass
9011.3 mV9.0 mV13.6 mV10.6 mVPass
1013.3 mV10.7 mV16.9 mV11.9 mVPass
1114.1 mV11.0 mV18.3 mV12.7 mVPass
CL18.0 mV9.6 mV10.8 mV6.9 mVPass
CL212.7 mV10.7 mV16.7 mV11.3 mVPass
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We saw excellent ripple suppression on all rails. This platform is a great performer, thanks to Corsair's interventions. Besides the high-quality capacitors on the main PCB, the extra capacitors on the ATX, EPS and PCIe cables likely did their part as well.

Ripple Oscilloscope Screenshots

The following oscilloscope screenshots illustrate the AC ripple and noise registered on the main rails (+12V, 5V, 3.3V and 5VSB). The bigger the fluctuations on the screen, the bigger the ripple/noise. We set 0.01V/Div (each vertical division/box equals 0.01V) as the standard for all measurements.

Ripple At Full Load

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Ripple At 110-Percent Load

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Ripple At Cross-Load 1

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Ripple At Cross-Load 2

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