I made some measurements on my DNS-320L, which has a AVC manufactured DS04010B12U fan, with c
3 pins, 12V, 0.14A, 25DBA, ball bearing, 5500 RPM, 40x40x10 mm, 28 (17-36) CFM
Measured DC voltages with the fan connected:
Set_Speed Applied_Voltage Fan_speed
Off ~0 3.7V
Low 7.8V 1.6V
Fast 9.8V 1.6V
So, some 12V fans might not be able to start rotating, even if correctly connected. I have some fans that have the wiring mixed, they will only work if I switch wiring.
Notice the warning on the Noctua fan site: Being a 5V fan, the NF-A4x10 5V can not be run at 12V and will be damaged when used with typical 12V power sources such as PC mainboard fan headers. So it might be adequate to be connected only to the USB connector. They also have a 12V fan.
The fan_speed wire probably just measures if the fan is turning or not (a rotation detector type wire, not a speed indicator (tachometer) type wire). When Set_speed if set to Low and the fan is mechanically forced to be stopped the Applied_voltage rises to 13.5V and the Fan_speed rises to 3.7V (DC voltages, no AC measuring was done).
Notice that the DNS-323-rev-A/B has proportional speed adjustment, so the third wire has to be of the tachometer type).
PS-The measuring voltmeter had a "low battery" indicator, so take the measured values with a grain of salt.