Rewinding a fan?

cavemanoc · 4 Nov 2006 at 15:44

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)

cavemanoc · 4 Nov 2006 at 16:37

Negatron: looks like this:

At the moment it vents out the bottom - but I need it to suck in there and blow into the case!

cavemanoc · 4 Nov 2006 at 20:33

Edit:
Ooops - yep according to This I was swapping the wrong ones - D'oh - assumed it would be the outer ones!

cavemanoc · 4 Nov 2006 at 20:44

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 · 5 Nov 2006 at 09:23

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?

cavemanoc · 5 Nov 2006 at 14:42

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 · 5 Nov 2006 at 15:40

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

cavemanoc · 5 Nov 2006 at 16:10

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

Still - I'll claim a moral victory! I got to disassemble a fan and now have a broken fan - progress!

cavemanoc · 6 Nov 2006 at 15:40

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