Hi Wouter,

[RobH]
> I made a simple circuit with a PIC12F683, a temperature sensor as
> input 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.

Wouter van Ooijen wrote:
>> Any suggestion (where to find info) on how to smooth this noise?
> 
> Is it a fan with electronics?

Don't know. I tried different (pretty old) fans, all behave similarly.

> maybe you can use >20 kHz PWM? 

Must be possible. In the meantime I found an example in a Predko book 
which uses a 10 KHz PWM.

> Or: why PWM at all? 

Well, I used as example the circuit of Jos van Eijndhoven
(http://jos.vaneijndhoven.net/fancntl/index.html).
I liked its simplicity.

 > Smooth the PWM output with an RC and feed a current
 > source (BD139 or similar with R in emitter-to-ground).

I'll do some tests.....

Thanks, Rob.

-- 
Rob Hamerling, Vianen, NL (http://www.robh.nl/)
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