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I bought two fans for my PC case, but I accidentally ordered the 5V version of the fans. The box states that CAN'T be run on 12v of regular Motherboards or the'll die.

I happen to have a step-down voltage converter that can handle 12v and I can regulate it to 5v. So, with a little soldering I can solve my problem. My doubt is about that the fans are PWM and I intend to preserve that functionality.

I thought of connecting the +12 and GND to the buck converter and the PWM and TACH directly to the motherboard. I'm almost certain that it's OK to do it, but I want to be a 100% confident. I'm not sure if the TACH and PWM from a regular motherboard will have higher voltages than this fans actually supports.

Am I safe doing this?

PS: If you're wondering why I don't return the fans the answer is: Argentina.. shipping and custom taxes are more expensive than the fans itself and I bought the fans overseas..

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The PWM output of the mainboard is usually a open drain configuration and it works with 3.3 V or 5 V, so it should work with the lower voltage fans as well.

The TACH signal is also a open drain configuration inside the fan. The mainboard however will have a pull-up to 12 V. I haven't found the specified maximum current for motherboards on that pin, but I guess it will be okay for the 5 V fan as well.

Noctua has a PWM specification which tells the maximum current is the same for 12 V and 5 V fans, so it should work fine (the current being independent because the pull-up is on the 12 V line of the motherboard).

You don't actually need a separate step down converter. 5 V is readily available inside your PC already - the red wires from an ATX power supply carry 5 V.

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  • \$\begingroup\$ the red wires from an ATX power supply carry 5 V wow... Why I didn't think about that on the first place haha. Thanks man! \$\endgroup\$
    – KnF
    May 13 '19 at 15:42

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