Corsair may today be one of the largest companies in the PC market, offering literally hundreds of products, but that was not always the case. The company started off as a manufacturer of memory-related computer products and took them many years to even consider diversifying into other segments of the market. Their first significant diversification attempt away from RAM-related products was towards the power supply unit (PSU) market, with the company initially releasing just a couple of PSUs. That move was a great success for the company and the beginning of Corsair's expansion towards multiple segments of the PC market.

Although the company today produces hundreds of products, they still place a significant amount of attention on their power supply products, with Corsair having not only one of the largest selection of products but excellent reputation in the field as well. The company today markets dozens of PSUs through nine different product series. In today's review, we are testing a unit of their most popular series, the RMx.

Corsair initially released the RM series several years ago, with the products designed to be reliable, quiet, and to offer good overall performance, yet still affordable. More recently, the company added some new features that bifurcated the series in two, giving birth to the RMi and RMx series. With the former, Corsair added software monitoring and control to the RM series, a feature that was previously reserved only for the premium HXi and AXi units, however the added features did come at the cost of higher prices. On the other hand, the RMx series is true to the original vision of the family, consisting of quality modular PSUs that are primarily designed to offer good overall performance and quiet operation. The unit that we are reviewing today is the latest (2018) revision of the RM850x.

Packaging and Bundle

We received the RM850x in a large cardboard box with the yellow-black artwork that is becoming the company's color theme. The front of the packaging is clean, focused on a picture of the unit itself and noting its output and only the most important features. Detailed information about the unit and its performance can be found on the sides and rear of the box. Inside the box we found the PSU protected inside thick foam paddings and packed inside a synthetic cloth pouch.

The bundle of the RM850x is simple and straightforward, which is not something unexpected for a product of this class. We received a typical AC power cable, four black M3.5 mounting screws, a thorough manual that is printed in 10 languages, a case badge and a few short cable ties. Corsair does not include thumbscrews, cable straps, or other accessories with this PSU.

Like its previous version, the RM850x is fully modular, meaning that even the always-necessary 24-pin ATX cable is removable. The SATA and Molex cables are flat, ribbon-like, but the larger PCIe and ATX/EPS power cables are normal round cables with black nylon sleeving. The designer could not use ribbon-like wires for everything because small filtering capacitors are hidden inside the sleeving of the larger cables. The cables are supplied inside a nylon storage pouch. No cable straps are supplied here either, Corsair is using simple wire ties.

External Appearance

The new revision of the RM series is proportionally and aesthetically identical to the unit it replaces, with the only exception being the stickers. Corsair has been using the same chassis with the chamfered edges for nearly all of their advanced PSUs. Satin black paint covers the chassis and the designer went with neutral colors on the stickers as well, forming a classy design. Further aesthetic improvements include embossed parallel ridges aligned with the fan’s finger guard wires and a small badge with the unit’s model at the rear side of the chassis. The chassis has been shortened in comparison to the previous version and now is 160 mm long, which is just slightly more than the length of a standard ATX PSU. This is unlikely to be an issue with any modern advanced ATX case but users should be careful with low-cost and atypical case designs.

We find the sticker with the unit's specifications and certifications at the top of the chassis. It is possible for the user to choose whether to hide the sticker or not by choosing whether to install the RM850x with its fan facing upwards or downwards. The side sticker are also installed in such a way so that the sticker facing the left case panel will always be upright. However, the sticker facing the right panel will always be upside down. Typical cases very rarely have a transparent right panel but this could be a problem for custom designs. Still, an advanced user hypothetically can remove and reinstall the side stickers without voiding the unit’s warranty, as the warranty sticker is placed on one of the unit's screws. However, keeping the sticker in pristine condition while removing it could prove to be a challenge.

There is little of interest at the rear side of the chassis. Besides from the typical AC cord receptacle and the small on/off switch, we can only see a small badge with the unit's model printed on it. Moving to the front of the chassis, we find it littered with the numerous connectors for the modular cables. The large 24-pin ATX cable is split between two connectors. For the rest of the cables, there essentially are two connector groups: one for PCIe/CPU 12V 8-pin connectors and one for SATA/Molex 6-pin connectors. Both PCIe and CPU 12V cables can be inserted into any of the 8-pin connectors. The layout is a bit strange though, with one of the PCIe/CPU 12V 8-pin connectors placed amongst the 6-pin connectors, as the designer apparently figured that this is the most efficient way to organize the connectors on the vertical board.

Internal Design

Taking the chassis apart, we first lay eyes on the 135 mm fan responsible for the cooling of the RM850x. It is a Corsair NR135L, a fan we are used to seeing in the company’s medium-to-low output products. The presence of the medium speed fan hints that the efficiency of the RM850x has been significantly improved in comparison to the previous generation of units. It is a rifle bearing fan that may not be as prestigious as the fluid-dynamic bearing (FDB) fans that Corsair is using in some of their other models. However, the RM850x features full range fan control with a passive mode, meaning that the PSU’s thermal circuitry will not even start the fan unless the additional cooling becomes necessary.

Most of Corsair's middle range PSUs are made by Channel Well Technologies (CWT) and the RM850x is no exception. The previous versions of the RM850x/RM850 were also made by CWT. This platform is largely based on the previous model but there are several enhancements, with most of them done to improve the unit's efficiency. However, there are also a few modifications that improve the unit's long-term reliability and resilience to damage from external (AC) faults.

The heatsinks of the RM850x are adequate for a high efficiency PSU with that kind of output. Keeping its hybrid fan mode in mind, it is obvious that the designer wanted the RM850x to have a reasonably large operational range before the fan becomes necessary. The input filtering stage begins at the rear of the AC receptacle and continues onto the main PCB, with four Y capacitors, two X capacitors and two filtering inductors. No surprises here, that is pretty much the standard for high output consumer-grade PSUs. CWT added a relay here to bypass the NTC thermistor after the PSU starts, giving the RM850x a slight efficiency boost.

The RM850x has two input rectifier bridges with their own heatsink sandwiched between them, which heatsink is very large compared to what we are used to seeing in units with this kind of power output. The APFC circuitry is almost typical, with two MOSFETs and a diode. A fourth active component is present though, a SPN5003 Enhancement Mode FET, which minimizes the RM850x's current draw under no-load conditions. The passive components of the APFC circuitry are two Nichicon capacitors rated at 400V / 470μF each, plus a filtering inductor that is fully enclosed into insulation material. The insulation material is probably not intended for protection, but rather to prevent electromechanical vibrations that would result in noise (also known as “coil whine”). CWT extends this practice to nearly all the filtering inductors found inside their premium designs.

Two MOSFETs form an LLC resonant half-bridge inversion stage, whereas six MOSFETs on the secondary side generate the 12V line of the unit. The 3.3V and 5V lines are derived via DC to DC conversion circuits found on the horizontal daughterboard at the edge of the PSU. All of the electrolytic capacitors are supplied by Nippon Chemi-Con, as are nearly all of the polymer capacitors as well. A couple of the polymer capacitors are supplied by Nichicon, making the RM850x an all-Japanese affair.

Cold Test Results

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. For a thorough explanation of our testing methodology and more details on our equipment, please refer to our How We Test PSUs - 2014 Pipeline post.

The latest version of the RM850x easily honors its 80Plus Gold certification regardless of the input voltage. We recorded an average nominal load range (20% to 100% of the unit's capacity) efficiency of 91.7% with an input AC voltage of 230V, which drops down to 89.6% if the input AC voltage is 115V. The maximum efficiency appears at 50% load and is 92.7% and 90.6% with an input voltage of 230V AC and 115V AC respectively. Even though our ambient temperature is a little higher than the 25 °C recommended by the directive, the RM850x surpasses the minimum 80Plus Gold requirements across its entire nominal load range. At sub-20% loads, the efficiency degrades significantly but the RM850x is comparatively better than most competitive products.

Corsair's engineers managed to design the RM850x so as to operate without using its fan even when the load is considerable. The fan of our test sample started when the load exceeded 220 Watts, a very significant power figure, meaning that the cooling fan will most likely never operate while the system is idling or while performing basic tasks. Regardless of the fan staying still, the operating temperatures of the RM850x remain relatively low. When the fan starts, it remains practically silent until the load is greater than 450 Watts, at which point the fan will speed up significantly. Though even then, the sound pressure levels are very low, far lower than what an average graphics card outputs under load.

Hot Test Results

Even though Channel-Well Technologies is better known for their cost-effective designs, the platform that they developed for the RM850x has rather outstanding electrical performance. Even when subjected to high ambient temperatures, the power quality of the RM850x is fantastic, with a maximum ripple of just 12 mV on the 12V line under maximum load. Furthermore the maximum ripple recorded on the 3.3V and 5V lines does not exceed 12 mV either. The voltage regulation is fantastic too, with the all of the voltage lines staying within 0.6-0.7% across the nominal load range.

These figures are extremely low for a unit with this kind of output, regardless of its retail price. We have only tested a handful of products that can compete with the new RM850x in terms of power quality and they all were significantly more expensive.

The impact of high ambient temperature is not apparent when looking at the power quality figures but the conversion efficiency drop is significant, especially under heavy loads. With an ambient temperature between 45 °C and 50 °C, the average nominal load (20-100%) efficiency drops down to 90.2%/88.1% during this test with an input voltage of 230V/115V AC respectively. The drop is not consistent across the entire load range, with the RM850x performing significantly worse when heavily loaded for extended periods of time, suggesting that its active components are struggling under such adverse operating conditions.

Strangely, the thermal profile of the RM850x hardly changed at all while the unit was operating inside our hotbox. The fan did not start before the load was greater than 200 Watts but it was noticeably faster when it did, yet barely audible. This changes quickly when the load surpasses 450 Watts, where the fan's speed increases abruptly and reaches clearly audible noise levels. This is necessary to maintain reasonable and reliable operating temperatures, which the RM850x does, yet the figures are relatively high when the unit is operating at or near its maximum capacity. The RM850x should be able to maintain reliable operation under such operating conditions, yet the high temperatures clearly do have an impact on its energy conversion efficiency.

Conclusion

Corsair's engineers designed the RMx series to offer a great balance between quiet operation, high quality electrical performance, and reasonable pricing. In practice, the series is bridging the gap between the value and the top-tier performance products that Corsair produces. In that respect it serves as an upgrade over the relatively simple but highly reliable TXM series, offering quieter operation without the high price tag of an AXi series unit.

The primary marketing point of the RMx series is quiet operation and the RM850x did not fail to deliver. Its fan does not even start with loads lower than 200 Watts, meaning that the RM850x will operate fanless while any modern PC is idling or performing simple everyday tasks, like browsing or media playback. For most users, the fan of the RM850x will only start when a performance graphics card is put to use, i.e. while gaming.

Although the primary selling point of the RM850x is its quiet operation, we cannot help it but point out that the electrical performance of the particular platform is exceptional. The RM850x greatly surpassed our initial expectations and delivered astonishing power quality during our testing, with figures that we usually see on products retailing for twice the price. We have no complaints regarding the electrical efficiency of the platform either, as it easily surpassed the 80Plus Gold certification requirements regardless of the input AC voltage.

The Achille's heel of the RM850x is its thermal profile design, which favors quiet operation over low operating temperatures. This works perfectly well for most scenarios, as the vast majority of users would never fully load a 850 Watt PSU for extended periods of time. As such, the RM850x rewards its owner with excellent acoustical performance, generating no noise when lightly loaded and retaining comfortable noise levels even when the load is high. However, if the load is excessive and the ambient temperature is high, the efficiency of the RM850x will degrade slightly, as the active components will start becoming stressed. But the only real-world scenario that matches these requirements is cryptocurrency mining, for which the RM850x clearly is not the best choice.

Corsair currently sells the new version of the RM850x for $130. The price is admittedly a bit high for an 80Plus Gold certified PSU, yet it's more than reasonable when taking its performance and the lengthy 10-year warranty into account. And if anything, Corsair is probably creating internal competition against themselves, as the RM850x offers astonishing electrical performance for a significantly lower price than the company's respective HX and AX units. So if you don't mind dropping one efficiency certification level – and merely to 80Plus Gold at that – then the RMx series ends up being an extremely attractive power supply even for high-end users.