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Controlling an externally powered 12V PC fan by fan output PWM pin?

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To avoid breaking anything I'm trying to find out beforehand can I use a standard PC 4pin 12V PWM fan powered externally, most likely by a regular 12VDC wall charger, speed controlled by a motherboard 4pin 5V PWM fan output PWM pin alone? The reason why not to use only the connector is there at the end, as 5V PWM fans in required form factor seems to be practically nonexisting, aside from the original Delta AFB0705MC-00. So, 12V + and - would come from elsewhere and I would only connect the PWM and RPM pins from the fan connector.

According to white papers PWM control pin should always be the same 5V regardles of fan nominal voltage, be it 5, 12 or 24 V:

To provide more background here, I've taken up a project trying to replace a noisy, low quality and poorly performing original fan of a monitor with G-Sync module inside, which needs active cooling. I've already tried to replace the original fan with a Noctua NF-A8 5V PWM, but the 25mm thickness of the fan was too much to fit inside. Noctua doesn't have any slim 5V models, so that leaves standard 12V as only option. If speed control could be viable by the PWM pin alone, I would next try NF-A9x14 12V PWM, which should physically fit inside easily.

Here's a video showing the NF-A8 5V PWM running from that fan output:

In case you're interested I'm describing the project in more general terms here:

Is the concern the new fan will mess up the mobo when hooking up?
Two options:
1. Study the new fan in isolation, to see if the PWM output is indeed 5V, and the PWM in not pulling up to 12V, and it properly controls PWM @5V input PWM signal.

2. Build a current limiting/voltage limiting interconnect. A 1K resistor and a 5.1V zener on both lines would do.

4-pin fans have permanent power and adjust their speed from the pwm signal, so yes, it should work.
Just don't mess up when wiring, power on the wrong pin might easily kill your motherboard.
Remember to connect together the grounds from the PSU, mobo and fan.

Powering the fan externally is not a problem. But you may have to have Both the PWM(drive) and Tach(sense) wires connected to mother board. The PWM needs feedback from the fans tach sense to maintain "speed" control from the PWM.  And as DavidAlfa pointed out the grounds have to be at the same potential for the fan to work from an external power supply.

Nominal Animal:
Typical wall chargers in Finland are insulated ones, but I would personally use something like Mean Well IRM-02-12 (datasheet PDF) connected to the display mains input; TME and Mouser have the IRM-02-12S model (34mm × 22mm × 16mm, 34x27x16mm if including the legs; only four of the pins, two at each end are used) in stock for under 10€ apiece, and it seems to me soldering wires directly to it with suitable heatshrinking would make it easily placeable inside some nook or cranny in the enclosure.  (As it is 75% efficient, it'll generate at most 0.67W of waste heat.)

These are "class II" isolated, meaning there is no capacitor from either output to the inputs, so connecting the DC negative output to the motherboard ground should be safe.  IRM-02 only provides up to 2 watts or 167 mA at 12 V DC, but NF-A9x14 only consumes max. 1.3 watts.  If you want more headroom, look at IRM-03-12 (3 W) or IRM-05 (5 W).


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