12V fans

Hi there, I was thinking about replacing the current fans in my Mini with 12V computer case fans. I realize that the 80mm and 40mm fans in the mini run at 24V, but if the fans tolerate 24V, will I risk damaging the electronics board by using 12V fans?

I’m using Noctua fans:

I have found 12 volt fans do not like to be supplied with a 24 volt PWM. They tend to not turn at all, or make a LOT of noise from the pulses going through them. YMMV.

Plus I would worry they could catch fire while under operation and not be noticed soon enough.

Would using the low-noise adapters (essentially simple resistors) alleviate the issue?

E.g. Using a 7V adapter would bring the voltage across the fan to 14V.

One can make a voltage divider easily with two same value resistors…

GND o---------WWWW---------o---------WWWW-------o 24V

The middle ‘o’ would measure 12V

Something like 2x 1k ohm resistors, 1 watt should do the trick.

If you look at the specs for the fan(V*I) it works out to about 130 ohms. so a 1000 resistor with a 130 ohm fan would be about 3.5 volts. :ugeek:

The fan is an inductive load and not a straight resistor load.

Let me clarify my horrible ascii art…

Since the fan is an inductive load, I decided to add inline resistors to halve the voltage.

The fan is 12V, 0.05A, which works out to an impedance of 240. I put a 250 ohm inline resistor (1x100 ohm and 1x 150ohm) in the way and the fan is spinning at a reasonable speed now.

I’m curious though, can I just take off the inline resistors and run the cooling fan at 50%? If the fans are voltage controlled that should give 12V, right?

Edit: Teddy, since the fan is a load, the second half of your voltage divider will have an overall impedance much lower than 1k ohm.

It is more 24 volts then 0 volts with a 50% ON-OFF duty cycle rate. The average would be 12 volts with a ‘nice’ buzzzzzz from the PWM, as the fan tries to go really fast then stop spinning at some KHZ rate.