Possible issue and solution when using standard PC fans in your devices
Intro
When you need active air cooling the best choice is PC-type BLDC fan with standard connector:
- MFR: MOLEX
- MPN: 470531000
Here is fan extracted from real PC SUNON KDE1209PTVX DC12V 4.4W:
 |  |
This connector support both 3P or 4P fans:
| 2-pin | GND | VCC | ||
| 3-pin | GND | VCC | RPM | |
| 4-pin | GND | VCC | RPM | PWM |
🔺 Issue Description
| PWM pin FLOATING (UNCONNECTED) | PWM pin GROUNDED (U=0V; f=0Hz) |
12.01V 0.356A 4.28W | 11.99V 0.033A 0.40W |
 |  |
| If you connect rated voltage (in my case 12V) to the VCC and GND and leave PWM floating (unconnected) it will work at maximum speed (see power 4.3W) | Then if you connect PWM control pin to the ground you can expect no rotation at all, but in real it continues work with low speed (xW) |
❔ Causing
Looks like most of PC-fans designed to maintain minimum cooling even if PWM=0V; 0Hz.
🔧 Issue Fix
For our cases we want to fully control fan behavior, so just add load switch on positive side, the simplest solution is two transistors:
✔ Fixed
Now when fan is not need to work just cut-off the 12V using LOAD SWITCH:
Conclusions
- Standard PC fan doesn't fully stop rotation even if control PWM signal is 0V (connected to GND)
- Additional simple load switch give c chance to fully control rotation by cut-off the power
- For some rare cases even this 0.038W (12V; 0.032A) small power is significant in sleep modes, so load switch is required
- When active cooling not required fan could be fully disabled by power load switch