Squeezing a few extra volts from a Dell N870P-S0 (NPS-885AB) power supply

Today I’m back down the familiar rabbit hole of playing around with high powered 12V power supplies from retired IT equipment. This time it’s a popular model from the Dell PowerEdge server line.

Increasing the voltage externally

Somebody else already did a good guide on this power supply, including pinout and how to increase the output voltage to 12.5V.

A few more volts (12.5-14V)

But what if you want a little bit more? Well. We’re going to have to open it up.

You can increase the voltage up to 14V by adding a resistor between pin 1 of the controller and GND as shown above.

220K should yield around 14V. As low as 190K may be required on some units. It’s not possible to go any higher with this modification alone, as the overvoltage protection will kick in. It’s set at 14V.

Cranking it a bit higher (14-15V)

By strapping a 22K resistor (I used 20K) on top of the resistor circled, we can push the overvoltage protection to about 15.8V. Don’t go any higher as the components on the output are rated for 16V max. No need to worry about the fan – it’s on the auxiliary 12V rail which we’re not touching here.

With this modification I was able to reduce the resistor on the controller down to 100K, giving an output of 15V. It didn’t seem to want to go any higher.

I haven’t tested any of this this with a 110V supply. Do drop me a comment if you have!

Don’t f— with it while it’s powered on

While I was experimenting with the output voltage, I had a through-hole resistor soldered between the controller IC pin 1 and GND. I made the mistake of touching the controller end of the resistor while the supply was powered up and subsequently blew all four MOSFETs on the primary. Apparently it’s quite sensitive ๐Ÿ˜‰

Posted in Repair / modification

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