Try adding a LC filter so the fan  only gets fet the DC component -
not the raw PWM.  With raw PWM the fan is still seeing the same peak
voltage which may (or may not) correspond to the switching point of
thefan driver.

RP

On 13/01/07, Marcel Buijs <Marcel.Buijs@wxs.nl> wrote:
>
> Rob,
>
> Perhaps it's better to controll the DC current throug the motor by the PIC,
> instead of using PWM.
>
> pic controlling DAC with (DC current) powerdriver.
>
> Best regards,
> marcel.
> (haven`t starting programming pic's yet.)
>
> -----Oorspronkelijk bericht-----
> Van: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]Namens Rob
> Hamerling
> Verzonden: zaterdag 13 januari 2007 10:15
> Aan: PIClist
> Onderwerp: [EE] Fan noise with PWM power
>
>
> I wanted to smooth noise of the airflow produced by the case-fan of my PC.
> I made a simple circuit with a PIC12F683, a temperature sensor as
> inpout and a PWM signal as output to control the speed of the fan.
> That works, but now the fan produces a mechanical noise, apparently
> with the PWM frequency. I have tried PWM frequencies between 60 and
> 500 to see if I just hit a resonance frequency of the fan, but the
> noise remains.
> Any suggestion (where to find info) on how to smooth this noise?
>
> Regards, Rob.
>
> --
> Rob Hamerling, Vianen NL (http://www.robh.nl/)
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