Is changing the supply voltage to a 12V, 80mm brushless fan effective and safe?

 

I would say probably not effective, and if you go above 12V, after certain threshold not safe either.
Just get a 12V PWM module, those that cost 5-10$ would do just fine for ordinary speed control.. depending on your application of course...

 

I would say probably not effective, and if you go above 12V, after certain threshold not safe either.

I am thinking mostly of reducing the speed.

 

According to me PWM is the way to go. But as I said, depending on your application, if you need to constantly reduce it for 15% maybe lower voltage would be OK. With PWM you can fine tune the speed with a potentiometer.

 

I am using one now. It is for a fan that is supposed to run 24/7. It is rated 12v but I power it up with 9v. Speed is lower, but I think the fan's gonna live longer.

 

Yes, it works, and it's safe as long as you never exceed the fan's rated voltage. Most 12 V fans will run at reduced speed on 7 to 9 V and many keep turning down somewhere around 5 to 7 V, but every model has a minimum start voltage that is higher than the voltage it will keep running at. So test yours: ramp up from low until it starts reliably from a standstill, then add a little margin.

Easy ways to drop the voltage are a small adjustable linear regulator with a pot, or a couple of series diodes (about 0.6 to 0.7 V each) for a fixed reduction. A plain series resistor is less predictable because the fan current changes with speed.

One exception: if it's a 4-wire fan, leave the supply at a steady 12 V and drive the PWM control wire instead, that's what it's designed for. Chopping the supply of a 3-wire fan with PWM can make it chirp or hunt, and it garbles the tach signal.

 

According to me PWM is the way to go. But as I said, depending on your application, if you need to constantly reduce it for 15% maybe lower voltage would be OK. With PWM you can fine tune the speed with a potentiometer.

From google AI:
2-Pin or 3-Pin Fans (No Dedicated PWM Pin): Applying PWM directly to the power supply (chopping the DC voltage) is not recommended for standard brushless DC (BLDC) fans. Because BLDC motors have built-in commutation circuitry and capacitors, external power-pulsing can starve the internal controller, cause audible whining, increase ripple current, and damage internal components over time. Instead, use a variable DC voltage regulator or a proper 4-pin PWM-controlled fan.

 

well if we only knew the actual fan...

 

well if we only knew the actual fan...

This discussion will lead my choice of fan.

 

Regardless of what you decide, I would first start the fan at full 12V, and after a couple of seconds reduce the voltage.