What happens if I put a non-PWM fan on a PWM header and set the BIOS to PWM?

[Joshua D]

Just wondering. I had a fan RPM display erratically earlier after setting it to PWM controlled in BIOS. Then I remembered it was a 3 pin fan....

 

[clutchc]

No harm. Just poor to non existent speed control. PWM (pulse width modulation) controls speed by varying the DC pulses at the full +12v. But it only does that with the presence of the 4th wire. I suspect the 3-pin fan was running at full speed all the time.

DC control varies the speed by varying the voltage from +12V to +5V.

 

[ThatVietGuy]

MERGED QUESTION
Question from Joshua D : "Bad Motherboard? Fan question..."

I'm checking my BIOS on my motherboard (z370 gaming 5) for something and I saw the RPMs of a couple of the case fans going from normal speed 1300ish to N/A. They are 3 pin on 4 pin headers. I took a video. The fans seem to be doing fine and not stopping, but is this a bad sign for my new motherboard? Should I send 'er back?

Thanks!

Noctua 120s

https://youtu.be/WbE7lIGqiKw

You had your answer here - http://www.tomshardware.com/answers/id-3652456/put-pwm-fan-pwm-header-set-bios-pwm.html

Normal behavior.

 

[Joshua D]

MERGED QUESTION
Question from Joshua D : "Bad Motherboard? Fan question..."

I'm checking my BIOS on my motherboard (z370 gaming 5) for something and I saw the RPMs of a couple of the case fans going from normal speed 1300ish to N/A. They are 3 pin on 4 pin headers. I took a video. The fans seem to be doing fine and not stopping, but is this a bad sign for my new motherboard? Should I send 'er back?

Thanks!

Noctua 120s

https://youtu.be/WbE7lIGqiKw

You had your answer here - http://www.tomshardware.com/answers/id-3652456/put-pwm-fan-pwm-header-set-bios-pwm.html

Normal behavior.

I don't know if I'm just not understanding it... (and, seriously, I appreciate any help), but if my fans are 3 pin non-pwm, and they are on 4 pin headers, how does that make the RPM display in the BIOS show the RPMs going from 1350, 1347, N/A, 1360, N/A, etc.? Isn't the 3rd wire the tach one? I'm not worried if it's some BIOS bugginess or something like that. It's just a new mobo, and I want to be sure nothing's wrong before the return period is over from the dealer.