Using a microcontroller to do PWM to a MOSFET driving a PC fan, I'm curious if this is considered the proper and safe way to do it?
If I want to drive the fan very slowly, can I safely do that in this manner? Will it damage the motor if it receives a pulse so infrequently that it is not enough to keep it running, or perhaps barely enough to keep it turning? In general, is this something I should not be doing?
Hy halleffector,
This just answers your question and does not counter the information provided in the other posts.
No you will not damage the fan, so long as you do not exceed the voltage rating of the fan by more than about 25%.
Basically, there are two ways to damage any motor: over-temperature and over-voltage (gross simplification).
Over temperature changes the structure of the motor material: melts, burns, etc, and over voltage breaks down the insulation of the wire making up the field or armature. As a PC fan motor is brushless, it uses uses switching electronics which can also be damaged by over voltage.
There is a danger when switching a motor that the motor inductance will generate a damaging back EMF (voltage). To protect from this, simply use two catching (snubber) diodes (rectifier diodes). Take a motor being driven from 5V:
(1) Diode 1. Connect the anode to the output of the switching element and the cathode to the 5V supply line.
(2) Diode 2. Connect the cathode to the output of the switching element and the anode to the 0V supply line. As you are using a MOSFET this second diode will not be necessary because a MOSFET has a substantial Schotkky diode connected between its source and drain as a consequence of its fabrication process.
For a PC fan a 1Amp IN400x (x= any number) rectifier diode will be adequate.
Incidentally, if you would like to see the innards of a PC fan just peel the label off from the hub and remove the circlip and any washers. The fan assembly will then slide off the central shaft.
You can also quieten a noisy fan by lubricating the bearing (plain or ball bearing). First clean out the bearing with paraffin, lighter fluid, etc then lubricate with engine oil or, better still, lithium molly grease. And if you want to get really fancy you can add washers to remove any end float which also helps quieten a fan (if necessary file the washer to get a good engineering fit).
I had a Dell tower PC that was always a bit noisy, even from new and after three years it developed an annoying screech and the fan wasn't rotating at full speed. After the above treatment it was smooth and silent- better than when new.
spec