How to stop PWM noise?

[tompson]

pwm noise

Hey there guys,

So I have a new water cooled computer with three very big 12" fans. I want these to be temperature controlled, so I figgured the best way was off the motherboard fan header (since this can be set of the CPU temp).

The problem is that the output off the motherboard is PWM (pulse width modulated) and these big slow RPM (but very high air flow) fans make a huge amount of PWM noise at slow fan speeds. You can hear the 60Hz switching noise, which makes the fans louder at slow speeds then they otherwise are at medium speeds.

Does anyone know a really nice trick to take PWM signals and turn them back into analog voltage modulated signals.... Besides just using an active low pass filter?

Thanks, much appreciated,
Jonno

 

[PaulHolland]

Hi, Put a large inductor in series with your FAN +Line. and a diode needs to be placed from the +line to ground before the inductor. Like a step-down SMPS.

regards,

Paul.