Electrical Question (regarding 12v DC fans)

[loudsubz]

I am doing a project for school, and am using an 80mm case fan I have in my basement. I was trying to swith the polarity to see if the fan would spin the other direction but it only goes one way. anyone know why this happens? Do all DC fans do this, or are there some case fans that spin both ways, if so how can you tell without trying them out.

 

[Nick_the_'Nole]

AFAIK all computer fans have circuitry inside 'em that correct polarity. You could bypass it, I suppose, but that'd likely just fry the fan motor since they're usually brushless designs that use hall effect sensors and switching transistors, and they can't do reversed motion. (Trying not to get too technical here since I don't completely understand that crap either...) Dunno where you'd find one that isn't made like that, but just out of curiosity why do you need to do that anyway? You can't just turn the fan around?

Even if you did get one to spin the oposite direction, the blades are designed to push air in one direction only, and it wouldn't work very well having it push air the opposite way.

 

[kc8adu]

they are not electricly reverseable
to reverse airflow just flip it over

I am doing a project for school, and am using an 80mm case fan I have in my basement. I was trying to swith the polarity to see if the fan would spin the other direction but it only goes one way. anyone know why this happens? Do all DC fans do this, or are there some case fans that spin both ways, if so how can you tell without trying them out.

 

[loudsubz]

thanks for the response guys.

Reason I asked is:

I am making a Fan speed controller using a Pulse Width modulator and one of the requirements is that I am able to reverse the direction of the motor via a switch, so in *theory* the fan could be controlled remotely at another location

Everything works fine except for the fan actually going in the other direction. I guess I will look into a regular brushless DC motor then, instead of a fan?

 

[Yamaha B-2]

Most any good hobby shop that sells R/C planes, boats, models, etc., should carry a line of small DC motors that are reversible. A cheap one is probably $10. Or try a model train store. HO gauge is DC, IIRC.

 

[ilimzn]

DC fans are a brushless designs in 99% of the cases nowadays and are not reversible. In fact, you are lucky if it still works after you applied 12V with the wrong polarity - in most cases this results in fan electronics failure, sometimes with various other special effects (like smoke, fire, smell...)