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I have a couple of server power supply units, namely HP DPS-1200FB. Those PSUs are working just fine (given their mileage) but the DC fans appear to be wearing out so I decided to replace them.

The original fan installed in those PSUs is a Nidec 40mm DC fan with four wires: GND, +12V, Temp and Rotation control.

When I was ordering replacement fans I explicitly asked for only two wires: GND and +12V because I prefer them to be running at full blast all the time.

When I install the replacement fans, the PSU runs for a couple of seconds and then turns itself off. My guess is it checks the RPM reading on the fan, does not get any data and thinks the fan is malfunctioning.

Is there a way to fool the PSU that it actually has the fan installed even though it only has two wires? I attached a picture of the original fan termination for reference.

enter image description here

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  • \$\begingroup\$ You can emulate the open drain RPM output. eg: with a 555 or Arduino. Or maybe remove the protection circuity. We need more info (eg: original fan type) But it's less of a hassle to just install the correct fans. \$\endgroup\$ – Jeroen3 Mar 12 '18 at 10:51
  • \$\begingroup\$ The thing is I don't want the PSU to manage the RPM. The PSUs are getting pretty hot and I want the fans to be running at 100% RPM all the time. \$\endgroup\$ – Pono Mar 12 '18 at 10:53
  • \$\begingroup\$ The original fan model is W40S12BS4A5-07 nidec.com/en-NA/product/fan/category/F010/G030/P2000076 \$\endgroup\$ – Pono Mar 12 '18 at 10:56
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    \$\begingroup\$ Is there an option in the BIOS to run the fans at 100% speed all the time? You'll still need a four-wire fan so that the fan monitor can detect that it is running. \$\endgroup\$ – Andrew Morton Mar 12 '18 at 11:33
  • \$\begingroup\$ As @AndrewMorton says, many of my UEFIs (BIOSs in olden days) have an option to turn off temperature controlled fan operation and run the fans flat out all the time. Does this control extend to the PSU fans? I know the PSU has a pseudo-I2C link to the motherboard, so the interface for such control is there. \$\endgroup\$ – TonyM Mar 12 '18 at 11:40
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four wires: GND, +12V, Temp and Rotation control.

From the datasheet I deduce that instead of Temp and Rotation control there are Tacho (pulses indicating the fan's speed) and Control which probably feeds a PWM signal to the fan to control the fan's speed.

This means that this fan uses quite "standard" signals. My bet is that the supply just wants to see the "Tacho" signal so that it knows that the Fan is spinning.

I also think that you could just a standard 4-wire CPU-cooler fan (and optionally ignore the PWM Control input or use a 3 -wire fan and then not connect the PWM control signal. That 3-wire fan will then just rotate at maximum speed but will provide a Tacho signal to the supply making it operate.

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I think that the best way is to use a Hall-effect-sensor-based fan, and the fan power terminals, as well as the sensor's, should be hooked up to the DC output of your power supply.

Also, use resistors with respect to the voltage and current ratings of both the fan and the sensor unit.

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