View Full Version : Reversing Fan Direction

05-13-2003, 05:54 PM
I have a Pabst 120mm fan. I want to reverse the direction the fan blades spin without turning the fan around.

The fan has two wires going to a molex connector. If I were to swap the position of the two wires, would the fan turn in the opposite direction? Would it hurt anything?

05-13-2003, 06:01 PM
To quote decodeddiesel:
Won't work on a DC Ball Bearing fan.

link (http://forums.procooling.com/vbb/showthread.php?s=&postid=36039&highlight=reverse+fan#post36039)

05-13-2003, 06:16 PM

05-13-2003, 10:17 PM
Originally posted by bigben2k
To quote decodeddiesel:

Won't work on a DC Ball Bearing fan.

link (http://forums.procooling.com/vbb/showthread.php?s=&postid=36039&highlight=reverse+fan#post36039)

Why? One way ball bearings?
Just have to ask because I don't think bearings have a thing to do with it. Could be wrong.

05-16-2003, 01:53 PM
I beg to differ. Reversing the polarity on a DC motor will change the rotation. Changing it on an A/C motor has no effect. Better check your electronics again on that one.

05-16-2003, 02:08 PM
Originally posted by TallTxnMo
I beg to differ. Reversing the polarity on a DC motor will change the rotation. Changing it on an A/C motor has no effect. Better check your electronics again on that one.

fan DC motors are diferent. when you conect them in reverse polarity, they wont spin.

05-16-2003, 02:42 PM
And tried it on sleeve bearing fans also, won't work.

05-16-2003, 03:07 PM
And also. The fanblades are not built to move air the other way so even if you managed to get the engine spinning the other way (wich I doubt) the fan would probably not perform as well as before and maybe also make a lot more noice.

05-16-2003, 03:27 PM
Well, I have accidentally connected a DC fan with reverse polarity and it did turn. I do agree that the fan blades aren't designed to work in that direction, so effeciency is very lacking. It would just be easier to remove the fan and turn it around.

05-16-2003, 04:21 PM
Originally posted by TallTxnMo
Well, I have accidentally connected a DC fan with reverse polarity and it did turn. I do agree that the fan blades aren't designed to work in that direction, so effeciency is very lacking. It would just be easier to remove the fan and turn it around.

funny, just did the same to 3 80mm, 1 60mm and 4 40mm and none has spinned :shrug:

05-16-2003, 05:08 PM
Some DC-engines spin the both ways...

05-23-2003, 05:58 PM

Just tried it on 4 other of my fans and they didn't turn either. Guess it was just the one fan. Unfortunatly, I can read the label to see who made it. Guess it was just a quirk. I stand corrected!!!!

05-23-2003, 06:02 PM
hey happens to everyone.
you should check my first post, about building my own watercooled system...

i only said BS lol :D

as a mater of fact i STILL only say BS :evilaugh:

Can O' Beans
06-10-2003, 03:38 AM
It completely depends on the fan. Some fans are nothing but a simple DC motor circuit. Other fans have a bit more circutry under there, like diodes & transistors - these would prevent a reversed polarity from working.

All my Adda fans work just fine reversed, but it's almost pointless to do so. Reversing polarity would cause opposite fan rotation, and less air flow. You could just flip the fan around for easier & better results.

06-10-2003, 06:30 AM
Well, it's no longer an issue. The fan is installed in the correct direction.

06-10-2003, 07:21 AM
what if the fans had a diode built in it to make sure it spins in one direction? I have noticed a small circuit on the fan. if so could it be bypassed?

06-10-2003, 05:45 PM
the fans have a circuit to control its 'movement'. dunno if this is the correct word for it.
DC computer fans use brushless motors that can last for long, but use this circuit to make then spin.

And those adda fans surely arent made to use with PC's.

06-18-2003, 11:05 AM
DC Axial fans, especially PAPST, will not work in reverse polarity:
o They are reverse polarity protected
o DC fans work by a control circuit
---- fan start circuit
---- overheating/locked-rotor/stall detection & shutdown
---- fan speed maintenance since it is a DC not AC fan

Eg, if you stop some fans, they will stay stopped for about
1sec as the circuit has detected the stop, then auto-restart.
Larger industrial fans use thermal systems instead.

There is no way to reverse this operation, since the blades
are non-reversible and the voltage certainly is.

o Fan blades are also tailored to work in one direction
---- leading edge design
---- trailing edge design - usually feather-thin
---- shape & profile of the blades
-------- eg, Tunnel Vortex (Panaflo) vs Tangential airflow (PAPST)

Q: Why can't you simply reverse the fan?

Dorothy Bradbury
www.stores.ebay.co.uk/panaflofan (Ebay)
http://homepage.ntlworld.com/dorothy.bradbury/panaflo.htm (Direct Prices)

06-26-2003, 10:13 PM
I have apart right now and I can garuntee it will not run in reverse. The first thing in the curcuit is a diode.

If your wondering why its apart it because I'm trying to figure out how maintain a 12v signal on the RPM line because MBM5 shows 0 rpm once i get below a certain voltage when a turn my fans down through my rheobus. So if you knwo how to mod the fan tell me. Thanks.

06-26-2003, 10:20 PM
try to bypass the diode. does it spin in either direction?

06-27-2003, 06:23 AM
I've disected a few dozen fans in my day I can say say each one has had some electronics inside of it. These fans are not just a motor and blades. Electronics do not like having ground being vdd and vdd being ground. I wouldn't risk doing it let alone plugging the rpom sensor into anything.

06-27-2003, 08:25 AM
hook it up to a 12v transformer from a scanner or something. thats how i usually play and test out circuits for my comp. i'd try this out (bypassing the diode) put i dont have a fan to salvage.

06-27-2003, 11:25 AM
A brushless fan will not spin in the opposite direction if you reverse polarity. If you bypass the diode, you will likely just fry it.

I have yet to see a DC operated computer grade fan that is not a brushless design. These fans basically depend on four components to spin - 1.) A hall effect sensor 2.) a ring magnet glued to the inside of the hub, 3.) Two coils (electromagnets) and 4.) Two switching transistors.

The ring magnet is just a flexible strip magnet glued inside. There are two coils on a circuit board inside the hub. When current is passed through one coil and the other is switched off, the magnetic field created by the coil attracts one pole of the ring magnet and fan hub starts to rotate. Once the fan starts to move, the hall effect sensor - which is basically a magnet operated switch - turns one coil on and the other off (through the switching transistors) and continues to alternate turning one coil on and the other off as one pole of the magnet passes by.

Removing the diode and reversing polarity won't work because the hall sensor and the transistors can only operate when the polarity is correct.

Brush motors all eventually suffer performance problems as brushes wear out, get dirty, etc. Plus dirty brushes create very "noisy" arcs.

The brushless fan is a very elegant little design that accomplishes the same thing as a brush motor with a minimum of moving parts and much more reliability.

06-27-2003, 12:42 PM
Didn't anyone pickup on the purpose of the arrow, on the fan's frame?:p (where applicable)

06-27-2003, 05:08 PM
OK, since I now have 3 fans apart and scattered across my desk I've come up with an idea. PWM and RPM Sensing.

Heres the idea.

This circuit I'm looking at is from the BLORBs fan but it should be similar in most fans. The 12V line enters the fan, its split between the HES(Hall Effect Switch) and 2 windings. Now If I cut the trace between the HES and windings I can run 12V to the HES. Now will I be able to solder on a new lead to the windings and send whatever voltage/pwm signal to the windings and still get an RPM signal.

Forgive me if this is totally crazy, I'm not a big electronics Guru.

06-27-2003, 08:45 PM
If you do that you will basically get one square wave 12v signal per revolution. I don't think the standard tach signal is 12v and I am pretty sure that most tach outputs for fans are two square waves per revolution.

08-30-2003, 06:40 PM
o Correct re 12V
---- it will generally blow the fan tacho monitoring circuitry
---- which can be by smoke, and may affect the chipset

o You can have 1 pulse per revolution on some fans
---- however far more usually it is 2

Note some motherboards have an option to set a divider
for the tacho pulses. It depends on how much the BIOS
writer abstracted from assumed to user-configurable.

11-21-2003, 06:53 PM
Umm... why dont you just flip the fan over if you want to reverse the airflow?

08-30-2004, 11:10 PM
Hi this is my first post =')

I was searching for this topic on google and found this thread, I hope its ok to raise this thread after so long. Reason I post is because I would like to figure out if it is at all possible to reverse the direction of airflow through a PC fan. I have 2 power supply fans I have recovered from junk computers, and I would like to experiment with counter rotating fans. My idea is similar to the way a jet turbine works, having 2 fans counter rotating but the direction of airflow being the same.
To my disapoiment I have realized that simply reversing the polarity on the fans will not work. So I hope someone can give me a hint as to what might work. Thanks for taking the time to read.

08-31-2004, 12:34 AM
Yeah, your totally crazy. The hall effect switch is used to sense the position of the rotor so the electronic control circuit knows when to turn the windings on/off at the right time to keep the motor running at the right speed, and to sense the speed of the rotor. It supplies one of the functions of the commutator of a DC brush motor. The other function of the commutator, turning the windings on and off is supplied by the electronic conrtol IC. Attempting to bypass the hall effect switch and supply the windings directly will not work unless you want to manually turn the voltage to the windings on and off at the right time. Of course this would drastically reduce the speed at which the rotor turns. There may be a pin on the electronic control IC to reverse the direction. That would depend on the make and manufacturer of the IC.

08-31-2004, 12:49 AM
would reversing the polarity of the magnet inside the fan hub work? Just an Idea, I would try it but I only have the 2 fans and im afraid i would ruin one needlessly in the process. Thanks for replying, I know this is kinda silly. Just something to keep my mind busy for the time being.

08-31-2004, 07:29 AM
You don't need counter-rotating blades to do what you want.

When you put 2 fans serially in-line (intake-2 to exhaust-1), you
generally effectively boost static pressure but not additively airflow.
o Technically you can occasionally boost cfm beyond that of a single fan
o More usually, you boost static pressure and so realise closer to free air cfm

For example, your enclosure has a static resistance 5Pa and you fit a single
fan of 25cfm in free-air, and you find your in-enclosure free air is 18cfm. If you
fit another 25cfm fan serially in-line, your in-enclosure free air is 24cfm.

This doesn't come at a cost however - noise :-)
o Additionally you need to vary the separation between 2 inline fans
o At some separation you will minimise noise for the given benefits

In reality this method is only used for redundant fans:
o You use tier-1 fans, NMB, Panaflo or EBM-Papst, some Nidec
o You fit them serially inline for redundancy
o Reason is not so much re risk of failure - but risk of service personnel
---- it allows YOU to choose when to replace the fan, not when the server does
---- it reduces the risk of disrupting other rack servers (common coloco downtime cause)

Many fans produce air in a Tunnel Vortex (eg, Panaflo) others more Radial.
The ideal fan type for making it work is Tunnel Vortex where noise is not so
critical, or fans which use an airflow straightener on the outlet - eg some Delta.

In general, you will suffer more turbulence noise, as the airflow impacting on
the leading edge of the foil will be at an angle it wasn't designed for. Thus the
blade will suffer more stall, moving it down the P-Q (Pressure-Airflow) efficiency
curve, with the resulting increase in noise from that inefficient blade turbulence.

Axial fans produce very little static pressure, but high airflow:
o Typical fan produces 0.1-0.2" elevation in a H2O (water) manometer
o Human beings can produce 16"+
Radial fans produce lots of static pressure, but low airflow:
o Typical fan produces 1-4" elevation in a H2O (water) manometer
o So whilst the cfm is less, *more* cfm is realised in high resistance applications

The only real use for radial fans (blowers) on PCs is for extremely high resistance
heatsinks comprising a very long, very dense, very deep, skived copper method.
Even here, axial fans can perform better where they are designed for such, an
example is the 40x28mm Sanyo high-velocity axial flow fan - and 40dB(A) too.

1U rack cases will be moving to a dual-motor, concentric shaft fan eventually.
It will give the cfm of an axial fan, but the static pressure of a radial fan - with
the result you get >25cfm at 1.2" of H2O for dense 1U applications, and 56dB(A).

Better to engineer a design to utilise the airflow in m/sec over a large surface
area, at least for lower noise, than rely on a pressure-orientated solution (noisy).
Dorothy Bradbury
www.stores.ebay.co.uk/panaflofan for quiet Panaflo fans & other items
www.dorothybradbury.co.uk (free delivery)

08-31-2004, 02:43 PM
Wow nice reply. I appricate your time and value your knowledge. I may try 2 fans serially in-line, I'm sure it would be a much more effective configuration than my idea. Your knowledge on this subject dwarfs mine, while I understood the majority of what you said some of it was lost to me. My experiance with electronics and airflow are all on a hobby level, as I have no formal education in either subjects. Again thanks for your reply, I really appricate your time.

08-31-2004, 02:48 PM
My pleasure.

As always:
o Change 1 variable at a time
o Measure temperatures after it

That way you can best establish cause & effect.

If you can, always vary fan distance from a heatsink or such high airflow
resistance object - over a certain range of distances there will be little
variation in temperature, but considerable variation in noise.