Can I use a DC PWM module (not a fan control unit) for a 4-pin fan (directly to 12v and GND)? something like this module wiring will be motor+ to red and motor- to black. does this method shorten the life of fan?
The answer depends on what DC fan controller is used in the four wire fan you have.
There are MCU controlled fans, and there are logic controlled fans. Let's look at one of each:
- Freescale have an MCU controller (MC9RS08KA2) that use an external regulator, and this type of fan would certainly not like it's 12VDC to be a PWM signal. The likelihood is that the fan may actually work where the PWM signal is close to 100%. Low PWM ratios of say 50% are more likely to result in erratic operation, though unlikely to damage the fan. The controller has a complex startup routine to set the fan direction, and stall protection implemented in firmware. If the external fan PWN used had a low frame rate (say less than 1 kHz) you'd likely be in real trouble if the controller started to reset often.
- On Semiconductor has a much more primitive fan controller, the LB8503 that is less likely to be upset by the 12V PWM modulation. But again, you have no idea what value C5 is and whether the controller will not enter it's softstart during the low period.
Your question therefore cannot be answered with any certainty at all, but you'd have to say modulating the 12V supply is likely to not deliver a good end result. It is unlikely to shorten the life of the fan (since they are stall proof), but may result in erratic operation.
You could however use the controller you bought and feed it into the CTL signal wire of the fan and reasonably expect good results (you might need to put a resistor of 1k Ohm or so across the output terminals). This way you let the in-fan controller do it's job and provide the speed control and other functions, while the external board provides the PWM to the 4 wire fan. Many of the fans I've seen are capable of supporting a 12V control signal (this gets somewhat complicated since most of the fan manufacturers recommend OC drivers).
If you have not bought the external PWM controller yet, then this may be a better type of controller for a 4 wire fan. Be aware this has no adjustment pot, it has a button to increase/decrease speed (it remembers the settings in the onboard MCU).
Another question that is asked quite regularly (do you need a diode across a computer type fan), and even those considered skilled get it completely wrong. If you look at this answer from some time ago, you can see the confusion this causes with folks saying with conviction that you even need to use Schottky diodes across the fans!! Let me re-iterate: You don't need to install diodes across BLDC fans with internal controllers EVER.