Corsair RM850x Shift 850W ATX 3.1 Power Supply Review

Corsair is a leading American company in the PC hardware industry, founded in 1994, known for producing high-quality components and peripherals. Initially focusing on memory products, Corsair has since expanded its portfolio to include power supply units (PSUs), cooling solutions, cases, and gaming peripherals. Corsair is particularly well-regarded for its dedication to performance and reliability, making it a popular choice among both enthusiasts and professional users.

In this review, we will be examining the Corsair RM850x Shift, an 850W power supply unit that greatly stands out with its unique side-mounted modular connector design. This 80Plus Gold certified PSU is engineered to deliver reliable power with high efficiency but also requires a very careful case selection to ensure compatibility. Although our unit came with an ATX 3.0 badge on the box, Corsair is already marketing it as ATX 3.1 compliant and will be changing its boxes soon. Fundamentally, all ATX 3.0 units are already compliant with the less-severe ATX 3.1 specification requirements, with the 12VHPWR connector on the PSU being the sole physical difference - the 12V-2x6 connector has shorter sense pins and that is all about it.

Specifications and Design

In the Box

The Corsair RM850x Shift comes in a sturdy cardboard box featuring Corsair's signature black and yellow color scheme. The packaging is designed to protect the unit effectively during transit, with a picture of the PSU prominently displayed on the front. Inside, the power supply is further safeguarded by a nylon pouch and protective foam, ensuring it arrives in pristine condition.

The bundle included with the RM850x Shift is minimal but sufficient for most installations. It includes the necessary mounting screws, an AC power cable, and a few cable ties to assist with cable management. There are no cable straps or other accessories, which is understandable in this case, as the very concept of this design is to hide the cables almost completely.

The RM850x Shift features all-black, ribbon-style cables with matching black connectors. The cables are flat and free from nylon sleeving, making them both flexible and aesthetically pleasing. The PSU includes a 12VHPWR connector and four 6+2 pin PCI Express connectors, each on individual cables without piggybacking, ensuring proper power delivery to high-performance components.

External Appearance

The Corsair RM850x Shift is housed in a chassis measuring 160 mm in length, 20 mm longer than the standard ATX dimensions specified by the ATX design guide. This additional length allows for the integration of a 140 mm cooling fan, which enhances thermal management and acoustics. Compatibility with ATX-compliant cases (see our list of the best PC cases) is not of much concern here as this unit was meant to be compatible with a limited number of cases to begin with.

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The unit features a unique design with a matte black, slightly grainy finish that gives it a refined and durable appearance. The RM850x Shift stands out due to its innovative modular connector layout, which is shifted to the left side of the chassis. This design choice allows for better cable management by enabling cables to be routed directly behind the motherboard tray, though it requires a compatible case to match the PSU. Decorative stickers cover the right and rear sides of the unit, adding to its sleek aesthetic without being overly flashy.

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On the top side, the standard comprehensive sticker provides details on the electrical specifications and certifications. The fan guard is integrated into the chassis and features a distinctive triangular geometric design. The front side of the PSU includes the standard on/off switch and AC cable receptacle, while the left side houses the modular cable connectors, accompanied by a subtle legend for easy identification. The modular cable connectors are unique and smaller than the ones we are typically used to.

Internal Design

The Corsair RM850x Shift 850W PSU is equipped with a Corsair NR140P 140 mm fan, featuring a fluid dynamic bearing (FDB) engine. This fan type is highly regarded for its excellent balance between acoustics and reliability, making it an ideal choice for high-performance power supplies. FDB fans are known for their low noise levels, even at higher speeds, and their relatively long lifespan, making them suitable for units with such long warranties. Although the specific datasheet for this fan is not available, its maximum speed has been measured at 2000-2100 RPM, more than sufficient for the PSU’s efficiency and power output needs.

The Corsair RM850x Shift is produced by Channel-Well Technology (CWT), a respected Taiwanese OEM with extensive experience in the power supply industry. CWT has a long history of designing and manufacturing mid-to-high-end PC power supplies, contributing to numerous platforms that serve as the foundation for a variety of premium PSUs currently available on the market. The RM850x Shift utilizes a platform similar to other high-quality units, with the core difference being a PCB that has been essentially rotated 90 degrees to accommodate its unique design.

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The RM850x Shift relies on robust and proven topologies to deliver reliable performance. The PSU features a strong input filtering stage, consisting of six Y capacitors, two X capacitors, and two filtering inductors. After the filtering stage, two rectifying bridges mounted on their own heatsink can be found near the edge of the PCB. The Active Power Factor Correction (APFC) circuit is composed of three MOSFETs and one diode as active components. CWT employs a configuration that includes two powerful MOSFETs (60R125P6) and an enhancement MOSFET (SPN5003) to optimize low-load performance. The APFC also includes an encased inductor and two Nippon Chemi-Con 470 μF capacitors as passive components.

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The primary inversion stage of the RM850x Shift utilizes two ST Microelectronics 33N60M2 MOSFETs arranged in a half-bridge LLC configuration. These components are positioned on a large heatsink adjacent to the main transformer. The secondary stage features six Onsemi NTMFS5C430N MOSFETs located on the underside of the PCB, generating the primary 12V line using a synchronous rectification topology. Small nickel heatsinks can be seen at the upper side of the PCB, assisting in their cooling. The 3.3V and 5V rails are produced through DC-to-DC conversion on a vertical daughterboard. The capacitors on the secondary side are a mix of high-quality electrolytic and polymer types from Nippon Chemi-Con and Nichicon, reputable Japanese manufacturers known for their reliability and performance in premium PC PSUs.

Cold Test Results

Cold Test Results (25°C Ambient)

For the testing of PSUs, we are using high precision electronic loads with a maximum power draw of 2700 Watts, a Rigol DS5042M 40 MHz oscilloscope, an Extech 380803 power analyzer, two high precision UNI-T UT-325 digital thermometers, an Extech HD600 SPL meter, a self-designed hotbox and various other bits and parts.

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During cold testing, the Corsair RM850x Shift 850W PSU comfortably meets the 80Plus Gold certification requirements across different input voltages. When tested with a 115 VAC input, the PSU achieves an average nominal load efficiency of 89.9%, which increases greatly to 92.4% when using a 230 VAC input. The efficiency peaks at around 50% of its capacity, which is typical for most PC PSU topologies. Although there is a 2.5% efficiency drop when using a 115 VAC input, the RM850x Shift still exceeds the certification requirements. The unit also maintains good efficiency at very low loads.

The fan in the Corsair RM850x Shift remains off at lower loads, thanks to its zero-RPM mode, contributing to a completely silent operation until the load exceeds 400 Watts. As the load increases, the fan begins to spin, but it maintains relatively low speeds throughout the entire load range. Even at maximum load, the fan operates quietly, never getting anywhere close to its maximum speed. The internal temperatures of the unit are not very low but not high either.

Hot Test Results

Hot Test Results (~45°C Ambient)

During hot testing, the Corsair RM850x Shift 850W PSU experiences a substantial decrease in efficiency under heavy loads, with efficiency figures dropping to 87.9% at 115 VAC and 90.3% at 230 VAC, compared to 89.9% and 92.4% during cold testing. This reduction in efficiency is not typical for a high-performance unit but occurs almost consistently across the entire load range, with the PSU showing signs of thermal stress only when operating at maximum load. It would appear that high ambient temperatures reduce the efficiency of some of its active components significantly but they are designed so as to handle it.

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In these hotter ambient conditions, the fan of the RM850x Shift activates sooner than during cold testing, with its speed ramping up more quickly once the load exceeds 400 Watts. Even as the speed of the fan increased compared to room temperature testing, the fan still does not reach its maximum speed, thanks to the PSU’s efficiency, good fan, and well-engineered heatsinks. The fan does reach easily perceptible noise levels but not painfully loud. The internal temperatures remain within safe limits but there is a lot of headroom for the fan to cool the unit down further, demonstrating that the designer chose to opt for acoustics performance but without neglecting thermals altogether.

PSU Quality and Bottom Line

Power Supply Quality

The Corsair RM850x Shift 850W PSU delivers excellent electrical performance, with a few standout features. Voltage regulation is particularly impressive on this unit, with the 12V rail maintaining a tight regulation of 1.1%. The 5V and 3.3V lines, often neglected in modern designs, strangely exhibit even better regulation, at 0.5% and 0.4%, respectively.

Ripple suppression is another strong point, with the RM850x Shift achieving maximum ripple values of 32 mV for the 12V line, 14 mV for the 5V line, and 12 mV for the 3.3V line. These figures are well within the design guide recommendations and they might not be setting any records but are competitive with what most competitive products in the market offer. The PSU’s ability to minimize ripple ensures cleaner power delivery, a crucial feature for the longevity and stability of connected components.

During our thorough assessment, we evaluate the essential protection features of every power supply unit we review, including Over Current Protection (OCP), Over Voltage Protection (OVP), Over Power Protection (OPP), and Short Circuit Protection (SCP). The Corsair RM850x Shift PSU successfully passed all critical protection tests without issue. 

The OCP settings for the 3.3V and 5V rails are slow to react, activating at 134% and 136% respectively, but that was to be expected given the high power rating of the 3.3V/5V lines this PSU has. The 12V rail’s OCP activates at 130%, which is a high setting, probably chosen so as to avoid unnecessary triggering due to graphic card power excursions while ensuring safety.

Bottom Line

The Corsair RM850x Shift 850W PSU emerges as a robust and innovative offering in Corsair's lineup, featuring a unique design with its modular cable connectors shifted to the left side of the chassis. This distinct layout facilitates exceptional cable management by routing cables behind the motherboard tray providing a sleek, refined appearance to the system as a whole. However, while this design enhances aesthetics and cleanliness inside the case, it does limit compatibility to certain cases, which is a major consideration point for potential system builders. Despite this, the RM850x Shift excels in its core functions, combining high efficiency and reliability.

In terms of electrical performance, the RM850x Shift stands out with its impressive voltage regulation and ripple suppression. The PSU maintains a tight 1.1% regulation on the 12V rail and superior regulation on the 5V and 3.3V lines, at 0.5% and 0.4% respectively. Ripple suppression is also commendable, with maximum values well within design guidelines: 32 mV for the 12V line, 14 mV for the 5V line, and 12 mV for the 3.3V line. These results underscore the PSU's ability to deliver clean, stable power, crucial for protecting and extending the lifespan of high-end components.

The thermal and acoustics performance of the RM850x Shift is similarly impressive. During cold testing, the PSU operates with high efficiency and maintains a quiet operation due to its zero-RPM fan mode, which only activates above 400 Watts. Even under high load conditions, the fan remains relatively quiet and does not reach its maximum speed. This balance between cooling and noise levels ensures that the PSU performs efficiently without generating excessive noise, which is a key benefit for users seeking to build a quieter system.

In summary, the Corsair RM850x Shift is a highly capable power supply unit that offers excellent performance and efficiency at a reasonable price of $125. Its blend of high-quality components, effective thermal management, and competitive pricing paired with a 10-year warranty offers significant value. While its unique design and premium features justify the cost, potential buyers should consider the compatibility constraints imposed by its innovative connector layout. For those with compatible cases or building a system using one, the RM850x Shift provides top-notch electrical performance and cooling efficiency, as well as the potential of a very sleek aesthetic appearance, making it a compelling choice.

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