user59170: I know there are fans out there that have a dedicated PWM input, but I like to know if I can PWM control a fan that does not have such a dedicated PWM input. So...
If I have a 3 pin 12v fan like the Noctua NF-A20 with connections:

GND
12 V
tach out (RPM measurement)

Can I control its speed by PWM on the 12V supply voltage (using a MOSFET most probably)?
Would the tach out still work?

user4143: Yes, and yes.
You have to modulate the +12V rather than GND because the output is referenced to ground. That means you need to use the classic N+P channel FET arrangement:

simulate this circuit – Schematic created using CircuitLab

59170: <p>I know there are fans out there that have a dedicated PWM input, but I like to know if I can PWM control a fan that does not have such a dedicated PWM input. So...</p>
<p>If I have a 3 pin 12v fan like the <a href="https://noctua.at/en/products/fan/nf-a20-flx" rel="nofollow noreferrer">Noctua NF-A20</a> with connections:</p>
<ul>
<li>GND</li>
<li>12 V</li>
<li>tach out (RPM measurement)</li>
</ul>
<p>Can I control its speed by PWM on the 12V supply voltage (using a MOSFET most probably)?
Would the tach out still work?</p>
 4143: <p>Yes, and yes.</p>
<p>You have to modulate the +12V rather than GND because the output is referenced to ground. That means you need to use the classic N+P channel FET arrangement:</p>

<p><img src="https://i.stack.imgur.com/HOe8H.png" alt="schematic" /></p>

<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fHOe8H.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>
