Electronics > Beginners

Is it a bad idea to underpower a DC fan?

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ebastler:

--- Quote from: atmfjstc on December 03, 2019, 06:26:06 am ---I for one was just thinking of doing this very thing with the fan cooling my Raspberry Pi 4. It's very loud and annoying at night when I'm trying to get some sleep :D

--- End quote ---

Have you considered switching it off?  :P

I wanted a rude username:

--- Quote from: NANDBlog on December 04, 2019, 01:58:08 pm ---You can plug it between the +5V and +12V, to run it from 7V, and that is just fine in a modern PC.

--- End quote ---

I did this in the Pentium II era. Felt so wrong ...



... but worked fine. As a bonus, when the (dual) CPUs were working hard, the 5 V line would dip, increasing the voltage across the fan, which would speed up. You could hear the computer straining.  ;D

Zero999:

--- Quote from: james_s on December 03, 2019, 11:16:06 pm ---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.

--- End quote ---
That was the point: he's not using a proper RC filter, just a capacitor in parallel with the fan, after the MOSFET.

If the speed doesn't need to vary, then it would be better to skip the PWM and add a resistor, with a small capacitor for decoupling the brushless motor driver inside the fan.

pcmad:
you should be find with lower voltage but you and get lower volage fans as well also you can over voltage these fans as will they are quite hardy

Nominal Animal:
There are two types of 12VDC PC fans of various sizes: 3-pin and 4-pin.

The 3-pin connector has +12 VDC, GND, and a +5V tachometer output (middle pin) with usually two pulses per revolution.  The +12 VDC is commonly reduced using diodes (e.g. Noctua), a voltage divider using a potentiometer (extremely common), or filtered PWM (most digital fan controllers), to reduce the fan speed.  Intelligent controllers use a higher starting voltage, to ensure the fan starts rotating, as the minimum voltage needed to start the fan varies; and essentially control the voltage to get the desired RPM (which can depend on the monitored temperature in more complex fan controllers).

The 4-pin connector has +12 VDC, GND, open drain tachometer output (controller expected to have a pull-up to 12VDC) with two pulses per revolution, and a 5V PWM control input at 25 kHz (21-28 kHz, less than 5mA).  For these, you use the duty cycle of the PWM control input to determine the speed.

I don't like the 4-pin ones, since I tend to "hear" the ultrasonic PWM whine, and prefer the 3-pin ones instead.   I've used e.g. Lamptronic FC5 V2 (I think), that controls four separate 3-pin fans to individually set RPMs (and if you use the same type of fans, you can put them in parallel, with only one providing the tach output), and has four thermocouples with Fahrenheit/Celsius display.  It's very useful when optimizing airflow and cooling for custom silent cases.

The widest variety of PC fans is currently in the 120mm×120mm×25mm category.  You have low-RPM quiet fans, high-RPM "hairdryer" fans (i.e., lots of airflow, but also a lot of noise), fans designed to work against a pressure differential (filters, restricted airflow), fans designed for unrestricted airflow, and so on.  At one point, I probably had three dozen different fans I'd tested, although only used about four of them in case builds...  The differences between them are surprisingly large, and it is very common that dropping the voltage (for the 3-pin fans) to 10-11 volts gives a significant reduction in the noise, with a comparatively smaller change in airflow.

Because larger fans need smaller RPMs for the same airflow, you should try and use the largest fan you can, to get the best airflow while keeping the noise to a minimum.  Baffles and filters are also very useful.

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