I have a 5v usb fan and I want to reduce the speed.

@Rayjay

What needs to be understood here is that a potentiometer will work, but it is not built to handle the load of something like a fan, so its lifespan would be reduced. A good simple method is to put a diode in series with the fan, a common diode has a voltage drop of about 0.7v so this will give you 4.3v from your 5, to drop it more simply add another diode Just make sure you buy (salvage) one that can handle 500mA (milliamp) or more such as 9564993RL. VT2095's circuit would allow you to actually vary the voltage. However if your fan is noisy, it may be worth looking at the fan itself for damage/wear.

5v-4.7v

5v-3.6v

I think a good potentiometer would be fine. We are not talking huge amounts of current, plus it give you an easier way to manually control speed. For the matter of current, you may even look into a varistor if a pot wouldn't be robust enough. I seriously doubt you are running big steel bladed fans that need amps of current.

If you wanted to get really crazy, you could mount a temperature switch to the coolong coils in series with the fan power and have the fans kick off and on at your own predetermined temperature window.

As for fan noise, I would look into mechanically isolating the fans from their mounting with a neoprene or foam rubber gasket. This would virtually eliminate mechanical vibration noise. If it is motor humm you are hearing, then the fans are loading and you may have cheapo crap motors.

Food for thought.

Reduce air flow may result in temperature increasing and your pc may turn off. Better way i can suggest is pwm driver dependind on the temperature but also change your fan with a new one low-noise and good quality.

That's plain wrong .

A 12v BLDC can work down to 1.5V .

PWM with a TL494 into a N-channel mid power mosfet will do .

Hi Dave,

You are right.  I was thinking of a brushless DC motor that requires 3 phase AC drive (a controller module), not the cheap brushless fans that just provide electronic commutation.

Thanks guys.

but I solved this awhile ago, I reduces the speed by soldering 4 150ohm resistors together. Not sure how much that droped the voltage to.

They are 5v laptop cooler fans.

I tried puttibg some grease on the fans, and it didn't help with the noise, so I felt I needed to reduce the voltage.

You should put a big size resistor on the 5 Volt side. It would be helpful I think. And if you need more details or any technical assistance about laptop cooler then contact to Lenovo Support.

A little late, but here's a simple solution:

    https://www.amazon.com/stepless-Controller-Regulator-Switch-Module/dp/B07GN5BJVQ/

This way you can adjust the speed as you see fit.