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Corsair RM750x review

Correctly testing power supplies is a complex procedure and KitGuru have configured a test bench which can deliver up to a 2,000 watt DC load. Due to public requests we have changed our temperature settings recently – previously we rated with ambient temperatures at 25C, we have increased ambient temperatures by 10c (to 35c) in our environment to greater reflect warmer internal chassis conditions.

We use combinations of the following hardware:
• SunMoon SM-268
• CSI3710A Programmable DC load (+3.3V and +5V outputs)
• CSI3711A Programmable DC load (+12V1, +12V2, +12V3, and +12V4)
• Extech Power Analyzer
• Extech MultiMaster MM570 digital multimeter
• Extech digital sound level meter
• Digital oscilloscope (20M S/s with 12 Bit ADC)
• Variable Autotransformer, 1.4 KVA

We test with the +12V in single rail mode.

DC Output Load Regulation

Combined

DC Load

+3.3V
+5V
+12V
+5VSB
-12V
A
V
A
V
A
V
A
V
A V
75W
1.24
3.35
1.19
5.05
4.95
12.08
0.50
5.03
0.20
-12.05
150W
2.16
3.35
2.83
5.05
10.07
12.06
1.00
5.02
0.30
-12.05
375W
6.17
3.35
6.06
5.03
26.17
12.04
1.50
5.02
0.50
-12.07
565W
10.31
3.34
10.85
5.01
38.34
12.01
2.00
5.01
0.60
-12.07
750W
10.71
3.34
13.85
5.01
53.60
11.97
3.00
5.00
0.80
-12.07

Load regulation is excellent within a 1.5% variable.

Corsair RMx Series 750W Maximum Load
835W

We managed to reach around 835W before the unit would shut down gracefully. Or another 85 watts over the rated maximum.

Next we want to try Cross Loading. This basically means loads which are not balanced. If a PC for instance needs 500W on the +12V outputs but something like 30W via the combined 3.3V and +5V outputs then the voltage regulation can fluctuate badly.

Cross Load Testing +3.3V +5V +12V -12V +5VSB
A V A V A V A V A V
734W 1.0 3.35 1.0 5.05 60.0 11.96 0.2 -12.05 0.50 5.03
154W 15.0 3.33 15.0 5.00 2.0 12.06 0.2 -12.06 0.50 5.02

The unit passes our Cross Load testing with both +5V and +3.3V rails holding steady. The +12V rail drops a little when hit quickly with 60A, but it is a great result.

We then used an oscilloscope to measure AC ripple and noise present on the DC outputs. We set the oscilloscope time base to check for AC ripple at both high and low ends of the spectrum.

ATX12V V2.2 specification for DC output ripple and noise is defined in the ATX 12V power supply design guide.

ATX12V Ver 2.2 Noise/Ripple Tolerance
Output
Ripple (mV p-p)
+3.3V
50
+5V
50
+12V1
120
+12V2
120
-12V
120
+5VSB
50

Obviously when measuring AC noise and ripple on the DC outputs the cleaner (less recorded) means we have a better end result. We measured this AC signal amplitude to see how closely the unit complied with the ATX standard.

AC Ripple (mV p-p)
DC Load +3.3V +5V +12V 5VSB
75W 5 5 5 5
150W 5 5 10 5
375W 10 5 10 5
565W 10 10 15 5
750W 10 10 15 5

Ripple suppression is stellar with both +3.3V and +5V rails peaking at 10mV. The +12V peaked at merely 15mV at full load, which is a superb result.

Efficiency (%)
75W
87.62
150W
91.33
375W
92.28
565W
91.71
750W
90.95

The power supply delivers great efficiency results, peaking at just over 92% at 50% load. This drops to around 91% at full load.

We take the issue of noise very seriously at KitGuru and this is why we have built a special home brew system as a reference point when we test noise levels of various components. Why do this? Well this means we can eliminate secondary noise pollution in the test room and concentrate on components we are testing. It also brings us slightly closer to industry standards, such as DIN 45635.

Today to test the power supply we have taken it into our acoustics room environment and have set our Digital Sound Level Noise Decibel Meter Style 2 one meter away from the unit. We have no other fans running so we can effectively measure just the noise from the unit itself.

As this can be a little confusing for people, here are various dBa ratings in with real world situations to help describe the various levels.

KitGuru noise guide
10dBA – Normal Breathing/Rustling Leaves
20-25dBA – Whisper
30dBA – High Quality Computer fan
40dBA – A Bubbling Brook, or a Refrigerator
50dBA – Normal Conversation
60dBA – Laughter
70dBA – Vacuum Cleaner or Hairdryer
80dBA – City Traffic or a Garbage Disposal
90dBA – Motorcycle or Lawnmower
100dBA – MP3 Player at maximum output
110dBA – Orchestra
120dBA – Front row rock concert/Jet Engine
130dBA – Threshold of Pain
140dBA – Military Jet takeoff/Gunshot (close range)
160dBA – Instant Perforation of eardrum

Noise (dBA)
75W
<28.0
150W
<28.0
375W
<28.0
565W
32.7
750W 33.3

The NR135L 135mm fan doesn’t spin until around 300-330 watts are demanded from the unit. In the last 200 watts of power, it gradually spins up until full load, when it is generating around 33dBa of noise. This fan will be likely masked by other fans in a system build – it can be considered quiet at all times.

Temperature (c)
Intake
Exhaust
75W
36
38
150W
38
46
375W
39
46
565W
45
54
750W
47
58

The power supply maintains a great thermal curve throughout the full range of load.

Maximum load
Efficiency
835W
89.80

For those interested, we measured efficiency when stressing the unit to breaking point. 89.80 percent efficiency at 835W … hardly practical, but interesting regardless.

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