Some questions on motor speed control

[Charles Linquis]

A lot of motors (most of the brushless ones) don't really like it when you simply "chop" the voltage. While, they will vary their speed, but it is awfully hard on them, and you will probably shorten their lifetime. Remember, there is an IC inside the fan, and there isn't enough capacitance inside that little motor to smooth the voltage to it. What you are attempting to do isn't the same as using an H-bridge to drive motor windings directly!

A far better approach is to use a more sophisticated circuit using a FET driver, a FET, an inductor, a Schottky diode and a capacitor. You will also have to write to the PIC registers directly, because you should increase the PWM frequency to above 100Khz as well.
The result is basically pure DC, with very little ripple. A true switching power supply with
variable output voltage.

Let me know if you really want to try the approach I outlined, and I'll send you a simple schematic.

[joseph Degorio]

Thanks Charles Linquis to the quick reply, please do send me your schematic where I can learn how to work on these motor. I'm willing to try what you are suggesting.

regards,
joe

[Charles Linquis]

This circuit is good for up to about 5 amps. Make certain you choose an inductor that can handle 7 Amps or so without saturation.