Insanity check: PWM fans

[ajones]

Quick question: I have some pwm fans and am trying to power them off the psu and not mobo (too many fans, or not enough fan headers, take your pick!)

Using http://www.specialtech.co.uk/spshop/...6648.html#tabs, it looks like the {12V + 2x ground} are through connected to the 3-pin adaptors, consequently forcing the pwm line to ground.

If I don't "break" the sensor line in the cable adaptor, will the pwm fan actually work?

[kalniel]

They work fine connected to three out of four pins, but the PWM won't work so it'll just be full speed all the time. Another thing to check is if they are open sided connectors, because if not you'll have to cut out a side in order to fit your 4-pin PWN fan plug onto them.

[scaryjim]

The alternative, if you don't mind multiple fans being controlled off one header, is to get a PWM splitter.

If your fan headers have high current ratings you could go for one that just draws power from the mobo like this: http://www.quietpc.com/gel-pwm-1-4-p...FasEwwodXroASA

Alternatively you can get splitters that run the power from a molex but take the control for a header: http://cpc.farnell.com/jsp/displayPr...FQcTwwodylYAyQ

[mikeo01]

I'll echo Scaryjim on this, a PWM splitter really is a cool way of PWMing your fans.

THIS or THIS.

I have the second link Scaryjim posted and also my first link (Flexa).

My fans draw 0.60A each and it works a charm, it's fine with it.


Alternatively you could buy a fan controller to voltage control it (if the channel can sustain enough amperage).

[Pob255]

Personally I'm not sure what they are doing with that connector, something rather odd that's for sure.

On a normal 3pin fan one wire is Ground (normally black on a fan), next is +12v (normally red on a fan) and the last is RPM sensor aka TAC (normally yellow on a fan)
A 4pin PWM fan has the same 3 wires with a 4th (normally blue) for the PWM signal

If you plug a 3pin fan on to a 4pin PWM header then it will work but will not receive any PWM signal so cannot be controlled via PWM
Same if you plug a 4pin PWM fan into a 3 pin connector, there's no PWM signal so no PWM control
Also as kalniel mentioned if the connector is the enclosed type rather than the open type (as found on motherboards) then you will need to cut away one side of the plastic housing.

Now what I don't like about that Nanoxia adaptor is that it's got wires from the Ground, +12v and +5v psu molex all wired up to the fan headers.
My guess is that the +12v from the molex is connected to the +12v on the fan, the +5v from the molex is connected to the Ground on the fan and the ground on the molex is connected to the TAC on the fan.
This means it's not just an adaptor but also reduces the fan speed to 7v by using current resistance and it's got a pointless and undeeded connection to the TAC
The whole thing seems to be more designed for looks, adding unnecessary sleeved wires because it looks pretty.

Specialtech has the nice 5fan PWM splitter cable that works off a sata power cable rather than a molex http://www.specialtech.co.uk/spshop/...pid-16447.html The advantage of this is if you're using a modular psu you don't need to add in a molex modular cable to the psu for the fans if you've got a spare sata power cable.

[herulach]

that adaptor looks like its wired weird, it may be using separate grounds for each fan to keep the amount of piggy backed cable to a minimum, if its grounding the RPM sensor thats just bad design. scaryjim's second suggestion is the best as you'll keep PWM control and RPM monitoring for all fans, but not risk overloading the headers (RPM sensor will only run off one, so if you care best to have all the ones on a run matched). You can also get ones that run of a SATA power connector incase a molex would mean plugging another run into your PSU>

[Pob255]

If your fan headers have high current ratings you could go for one that just draws power from the mobo like this: http://www.quietpc.com/gel-pwm-1-4-p...FasEwwodXroASA

Justwant to point out that that one does take power off the PSU via 4pin molex, it's only getting PWM signal from and sending TAC signal to the motherboard.

generally You don't want to pull more than two fans off a single motherboard header because motherboard headers are relatively low amperage, does depend on motherboard and fans though.

Specialtech has that Gelid cable too http://www.specialtech.co.uk/spshop/...pid-14673.html although it's out of stock atm
And others along the same lines http://www.specialtech.co.uk/spshop/...e.php?cat=2576

[Bonebreaker777]

It is really necessary to have that many case FANs on PWM? Would it be feasible to have them run on 7V or 5V constantly?

Many FANs on low RPM (should)= less FANs on high(er) RPM. Only if the thermal output of the case content is that wide ranged (from 80-120w in idle to 750-900w under load) would make a design like that sense to me (personally). Just an idea...

[ajones]

Thanks for the feedback guys. Maybe that cable is not for pwm fans, but for the 3-pin fans. If I understand the pinouts correctly though, a 3 pin fan header is +12V/ground/tacho; which still means the tacho signal is being grounded.

At the end of the day, I'll probably just wire the remaining 4 fans not connected to the mobo directly to molex, forgetting pwm totally. If it's too noisy, I think I've got some 7V converters in my toolbox.

[Pob255]

However a good PWM fan should be able to run at far lower speeds and with less chance of motor noise.
A lot will depend on the fan used, most fans at 5v run about 40-50% speed, most PWM fans run at a minimum of 20-30% speed
Also the fan speed control is automatic so they should increase in speed as heat build up occurs, but this is down to the PWM controller and sensors as to how well it would work.

In general if you've already got a whole bunch of PWM fans and PWM motherboard headers then why not run them off splitter cables?

[Pob255]

If I understand the pinouts correctly though, a 3 pin fan header is +12V/ground/tacho; which still means the tacho signal is being grounded.

Almost, it's ground/+12v/TAC
There's no need or advantage to ground the TAC it's just sending out an RPM counter tic, the only reason to connect it is to some device that can read/monitor the signal

Personally in your case I'd look at the Akasa Flexa FP5S that will let you power and control the speed of 5 fans off of one PWM header and a sata power connector for £6 which really isn't much

[Macadee]

I have the Akasa SATA PWM splitter and it works very well, check your mobo headers though - on my Asus Hero not all the 4pin connectors are PWM enabled.

[ajones]

I guess I should have mentioned that I'd already bought the phobya cable adapters; just preparing to install the blowers. I have found a problem with them though- they are wired incorrectly.
The molex +ve is wired through to the 3 pin sense line, and the 2xmolex ground to the 3 pin +ve and ground. Rewired with a test 3 pin fan, and voila, all is good.

[Pob255]

I've had something similar with a phobya cable, it's the Y cable with an extra lead, basically a Y splitter cable with an additional 3pin going to one header to provide extra power and rpm TAC of the 2nd fan however the +12v and ground where round the wrong way so when I plugged in the 2nd cable the wire went red hot and burnt off it's sleeving.
I killed the power fast, luckily no other damage was done and I just cut off the extra wire.

But all told Phobya is pretty cheap and low quality control I don't think I'd trust there stuff again, esp for anything major.