Electronics > Beginners

Is it a bad idea to underpower a DC fan?

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james_s:
If that box is the switch then you're missing the R component of the RC filter entirely, of course that's not going to work. Also 10R is quite a large load.

SparkyFX:
It is just a 22µF electrolytic and surely not recommended to approach the same problem with larger loads.

--- Quote from: james_s on December 03, 2019, 08:20:36 pm ---PC motherboards drive the fan directly with PWM and it works fine. I have had a few fans that squeal at very low PWM values but no other issues.
--- End quote ---
I suspected this noise to come from the fan-internal BLDC controller suddenly driving the rotor reverse (or enough out of sync to make the rotor wiggle in it's bearing, or maybe just coil whine), because of restarts due to the supply being underpowered until it catches up. The point is the fan or the output might not be designed to handle the rush-in current at PWM frequency over long periods of time, next to the noise it makes.

This might be interesting in this regard and regarding the OPs question, at least for this example:


tszaboo:
You can plug it between the +5V and +12V, to run it from 7V, and that is just fine in a modern PC.
Or buy a 2 EUR fan controller from aliexpress.

David Hess:

--- Quote from: andy3055 on December 03, 2019, 04:46:53 am ---In the first place, why would you want to do that? Did you consider the air displacement/cooling that will suffer due to the low speed?
--- End quote ---

It might be done for lower noise or longer operating life.  I have often done it for variable cooling with a temperature sensor and feedback loop.  One of my homemade power supplies is configured this way and at low power, the fan does not even run.


--- Quote from: Circlotron on December 03, 2019, 06:43:46 am ---
--- Quote from: SparkyFX on December 02, 2019, 06:25:49 am ---I also run some on a PWM output on a fan controller, which makes them occasionally emit some noise. An electrolytic cap parallel to the fan kind of solved the problem.
--- End quote ---

A largish cap across a load that is being PWM'd is going to eventually break either the cap or the switching device. Large currents will flow at the start of every switching cycle. Much better to leave the cap in place and feed the combination via a series inductor and have a reverse biased catch diode between the switching device and + volts.
--- End quote ---

Driving the fan directly with PWM is definitely a bad idea.  The better circuits include an LC filter in the form of a buck inductor and output capacitor so the fan just receives low ripple DC.

Alfons:
A buck converter module costs one euro. Of these, I have five in my 3D printer, two for fans and three for other things. I have now installed one with a 12cm Sunon fan in my Infiniium, because this fan is much too strong. With that I could easily operate a cooker hood. Even regulated to 6V, now the highest temperature inside is just 30 degrees. Anyway, the converters have the advantage that you can regulate how much you need. The disadvantage is: they can break, so a source of error more.

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