DC Fan noise issue

Thread Starter

nickagian

Joined Mar 12, 2010
34
In order to cool a telcom device I am using 4x 12Vdc fans from Delta Electronics (the exact type is FFB0412VHN-F00). The fans have a 3-wire interface and I operate them by sending PWM pulses at their power supply wire. The pulses are generated from a PIC-microcontroller and are "converted" to 12V pulses through two FETs (see attached schematic).

The issue is that the fans seem to disturbe the 12V power supply and as a result they generate several different problems, like bit errors at the transmissions etc, which seem to disappear when I remove the fans. A logical thought was that the pulse current drawn periodically from the fans is causing small dips in the power supply and I was able indeed to verify that with an oscilloscope. Therefore I built an LC low-pass filter in order to make the 12V quieter. And it seems to work when the duty cycle of the PWM pulses is low.

What however I still not understand is that when I give full power to the fans (the duty cycle is 100%) then I still have the same problems as before. And that actually still puzzles me because at full power there are no pulse currents drawn from the 12V... I have a feeling that the filter has worked at low duty cycle values because it creates an additional voltage drop and thus the fans receive a lower rms power supply.

Does anyone have any idea on what to do?
 

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Sensacell

Joined Jun 19, 2012
2,518
While applying PWM to a Brushless DC fan can create some additional noise, be aware that the internal commutation circuitry also generates pulses of current even when running from steady DC.
 

Thread Starter

nickagian

Joined Mar 12, 2010
34
I agree with @Sensacell you may be applying PWM to a fan that already has internal PWM such as TC653.
Max.
This is definitely not the case! This fan (FFB0412VHN) doesn't have internal PWM.

While applying PWM to a Brushless DC fan can create some additional noise, be aware that the internal commutation circuitry also generates pulses of current even when running from steady DC.
Well, I guess that's why I still have disturbances when the fans run from steady DC. Any idea on how to block these pulses disturbe my power supply? What would the frequency of these pulses be in that case?
 

ronv

Joined Nov 12, 2008
3,770
This is definitely not the case! This fan (FFB0412VHN) doesn't have internal PWM.



Well, I guess that's why I still have disturbances when the fans run from steady DC. Any idea on how to block these pulses disturbe my power supply? What would the frequency of these pulses be in that case?
A picture of your scope may be useful.
 

Alec_t

Joined Sep 17, 2013
10,598
Even better, can you put a small current-sense resistor in the ground leg of the motor and show us a scope shot of the voltage developed across the resistor, so that we have some idea of the current pulses produced during commutation?
 
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