Server PSU Hack – Dell N870P / NPS-885AB. 12V 71A 870W

The Dell N870P power supply origins from the redundant PSU for the Dell Poweredge R710 / T610 rack servers. This article and video shows how to make it run standalone.

Pictures of the Dell N870P / NPS-885AB unit and its specifications.

The list of hacked server power supplies gets longer and longer. Use my guides on how to get cheap and powerful 12V power supply for your projects. They can even be put in series for 24V or 36V.

Inside the power supply

The printed circuit boards / PCB of this power supply is well documented with its silkscreen and modularly built with PFC front end, DC-DC converter and driver boards as extensions.

The backside of the main PCB reveals some specific areas of interest, for further analysis and hacking. Especially the OVP / Over Voltage Protection would be interesting in order to get this power supply up to 13.8V. Besides that, there is markings for UVP / Under Voltage Protection, SB / Stand By, OTP / Over Temperature Protection for Main, Stand By and Ambient.

The driver of the DC-DC output stage is a UCC3895. It is placed on its own extension board with 3 potentiometers for adjust output voltage and possible current limits.

Hacking the Dell N870P interface

Firstly I map out all the ground connected pins. Secondly I power up the power supply to look for standby voltages and housekeeping power supply rails. Thirdly I start out with a 100K resistor from ground to all unknown control pins, then move down to 10K and last 1K resistors to look for external power on signals. Repeat with +3.3, +5 or +12V auxiliary power supplies if nothing is found with the ground reference to input pins.

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