Rewinding a fan?

cavemanoc

cavemanoc

cavemanoc

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23 Mar 2005
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I need to get one of my fans running backwards and I'm stumped - I would have thought that swapping the earth and 12v terminals would do the job - but it didn't!

It's one of those front mounting 'lateral cross flow' jobs that bolt onto the front of the case and it's blowing out, where I need it to suck in!

Reversing the actual cylinder doesn't work as the blade are then completely in-effective - Anyone got a simple solution ? :confused:

(It's running off the standard 3pin header at the moment - taken from a molex connector through an adapter)
 
Negatron: looks like this:

pcac-1b.jpg


At the moment it vents out the bottom - but I need it to suck in there and blow into the case!
 
Are you sure you're swapping the right cables? Red and black should be the other way around. The yellow cable is for RPM monitoring.
 
Mmmm...

I have now tried the correct swap and it does not run the fan at all -I have swapped the Red and Black (12v and Gnd) and it does not run at all? I tried it on an Akasa Amber as well to see if there was something on the PCB stopping it but that doesn't work either - any other ideas?
 
cavemanoc said:
Mmmm...

I have now tried the correct swap and it does not run the fan at all -I have swapped the Red and Black (12v and Gnd) and it does not run at all? I tried it on an Akasa Amber as well to see if there was something on the PCB stopping it but that doesn't work either - any other ideas?
Click to expand...

Hmm, looks like there may be some sort of diode preventing the current going "backwards". I'm out of ideas.
 
Just for my own benefit I went back and re-learned the functioning of a basic DC motor, and yep - looks like it must be some sort of reverse polarity protection in the circuit!

Diode seems the most logical thought - guess this means I'm going to have to bypass it if I want the fan to go backwards - real shame - anyone know why they do this - no advantage to it - the motor shouldn't care which way it goes - I know for a regular fan it would kill the efficiency, but TBH given the idiot-proof nature of the 3 pin power connections, you really have to want to plug it in wrong!

So apart from finding and shorting the diode - anyone got any other genius ideas?
 
whoooo, no, if there is a diode probably means they dont want the voltage going the other direction. u mioght find there is circuit on the board tht helps with speed detection, if somthing in that breaks you might have just broken you fan. Also the way the fins are shapped is to spin in one direction with good output. If they spin the otherway they will not work to full potential. I know you sed u cudnt but try reli hard just to turn your fan around
 
I can flip the drum around but it has to be twinned with a change in direction in the rotation as well or it does nothing - The 'blades' are shaped like 'contact lenses' so if they run the wrong way they push almost no air. It has to be both - switch drum and reverse polarity!

Having played a bit more they actually do a reasonable job as they are so I might just leave it - but I do have a stripped down old fan in my draw that I'm going to beat! I'll let you know if I manage it ;)
 
cavemanoc said:
I can flip the drum around but it has to be twinned with a change in direction in the rotation as well or it does nothing - The 'blades' are shaped like 'contact lenses' so if they run the wrong way they push almost no air. It has to be both - switch drum and reverse polarity!

Having played a bit more they actually do a reasonable job as they are so I might just leave it - but I do have a stripped down old fan in my draw that I'm going to beat! I'll let you know if I manage it ;)
Click to expand...

Keep me posted, I'd like to see how it goes:).

I'm not sure why the fan won't run backwards, there's usually "split ring comutator" that changes the direction of the current every 180 degrees of rotation, or else the fan would spin 180deg and then stop. So if there is a diode, it'll be before or after the whole coil business. Although I may be talking crap, and it might be completely different for a brushless DC motor such as is used in a PC fan. :p
 
You've got it exactly right - not sure why it won't run as the motor side of it is easily reversible - can only assume that the board prevents it as the fan would be rubbish running backwards, and that way if you are hamfisted enough to force the power connection and set it up backwards you'll know about it - wouldn't have thought it would affect the speed sensor unit as that's just a very simple electronic pulse generator.

My scientific method is going to involve mainly applying 12v to various bits of the motor and seeing if it a) runs, or b) sets fire to the house. Must remember to set up 999 on speeddial :D
 
Curse their technology - the fans are running off a 4 coil setup instead of the simplistic bi-polar arrangement you see in the old DC electric motors - the timing is controlled by a 4 pin 'chip' that appears to sequence the flow and I'm guessing won't run backwards - the solution is simple - Dismantle the fan, de-solder the diode, resolder it in backwards - reverse the wiring and then reassemble the fan, take out the drum, flip it and reverse the mountings, re-attach... riiiiiight - How about I just say I did and we assume it all worked brilliantly and hand out smileys :p

Still - I'll claim a moral victory! I got to disassemble a fan and now have a broken fan - progress! :p
 
Is it a standard blower or a centrifugal (<sp?) blower?
A centri-blower will not function at all in 'reverse' due to it's very nature...
A DC motor can run in reverse, but are you sure it is a DC motor and not this crap they use in 'normal' fans?

I used to have a full tower case...
I drilled a 120mm hole in the top for a 120mm delta.... above the CPU I had an 80mm delta sucking air from the CPU heat sink and out the case...
At the front behind the plastic front panel at the bottom I had a 120mm hole with a filter made out of tights (god did I get a funnie look when I baught em). the rest of the case was sealed as best I could (draft excluder sales went up 50% :D ) but it was lovely.... absolutly freezing in my case....
Alas, the case had to go :mad: it twas too big....
 
It is the 'standard carp' they use in the normal fans, which is basically an improved (honest) version of the standard DC motor - instead of 2 coils they have 4 to prevent the non-start you could conceivably get with the normal bi-polar motors.

It will run backwards, but as it looks it will take far more effort than I'm prepared to give the project as I'd have to desolder the diode and reseat it backwards - The other option of course is to skip 2 of the loops and set it up as a standard motor, but again - soldering :( Really CBA'd to dig out my iron for the sake of a cheapo fan system - better to get a proper bay-fan and stick some real cooling in there - the bit that got me was the promise of 53-100CFM for 21-23dBa, yep - they lied like cheap Japanese watch :D
 
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