3 wire PC fans

[Bob Minchin]

I have an application where I want to control the speed of an instrument type axial fan to achieve the least noise and only turning on or speeding up the fan when the inside of a case starts warming up.

I've played with a few two wire brushless fans with series resistors and they don't start up reliably with enough series ohms to get low running speeds. It appears that the brushless switch does not like this - fair enough, I can understand that might happen.

I've also seen three wire fans online from places selling PC bits. Does Anyone know what the third wire does please?

Is it just an output back to the PC telling it that the fan is running? Is it a tacho output so the software can display fan speed for nerds that like that sort of thing? Is it a control input? Is is PWM to control the speed or even an analogue voltage input to vary the speed?

I have yet to find any design info on these fans as I suppose they are sold as fit and forget PC replacements.

TIA

Bob

[T i m]

Have you tried putting a reasonably size (capacity) across the resistor Bob? It would have to be of sufficient voltage to deal with the supply line (so 12+ V)

Typically yes So when you run something like SpeedFan you can see the fan revs.

Yes.

I think they can do that with 3 wire fans yes.

Cheers, T i m

[Andy Burns]

I think the 3-wire have GNV/12V/PWM-in and 4-wire additionally have TACHO-out.

To quieten my HTPC I used a variable fan controller which gives finer control than the old 12V-5V=7V trick, I can't remember where I got mine from, I should imagine eBay has several on offer, if not I can soon whip the lid off ...

[spuorgelgoog]

2-wire fans /can/ be controlled by PWM on the power wires but there is no feedback loop. 3rd wire is a tacho sensor reporting fan speed back to the PC. 4th wire is a PWM control wire.

Owain

[Graham.]

[Tim Watts]

There are 4 wire fans (rarer) with speed control.

[Bob Minchin]

Thanks Tim, I have indeed done this successfully on other jobs. The capacitor differentiates the rising edge of the supply voltage and kicks the fan into life and it then slows with the time constant of the parallel R-C network. down to a speed set by the resistor

However for this application, the fan will might well run 24-7 or for long periods and so won't get regular start up pulses.

I suppose I could do something clever with the tacho output to give it a kick if it stops.

Bob

[Thomas Prufer]

Try google or ebay for "fan temperature control", maybe?

For instance: 6 US dollars gets you "12V PWM PC CPU Fan Temperature Control Speed Controller Module High-Temp Alarm" , which used the 3rd wire for a stall alarm, adjustable temperature ranges, minimum PWM output and PWM characteristic.

Thomas Prufer

[Bob Minchin]

Thanks for all the replies chaps and no one mentioned Brexit

DIY is getting back to its old self again.

[Brian Gaff]

I'd like to know this as well, as speedfan seems to suggest my machine has the right chips to detect the temperatures and so one supposes with the right fans and connections the motherboard should be capable of it, but the current fans seem to be set to flat out and the thing runs mega cool, but sounds like a hovercraft. Brian

[The Natural Philosopher]

Check out model brushless motor controllers.

[Capitol]

Use a 2 wire fan with an ntc thermistor?

[T i m]

Yeah, that's what I meant. ;-)

But it only needs one doesn't it? As long as you set the minimum revs such that it doesn't stall it won't need to re-start?

Like a lot of the later CPU fans do in fact. You stop them, they sit there for a couple of seconds and re-start again. ;-)

Cheers, T i m

[Bob Minchin]

No this will not work as I have indicated above. Too high a resistance and the fan won't start. Selecting the resistor so it will start reliably (with out a capacitor differentiator) does not drop the speed by very much.

[Capitol]

How about a PTC and NTC parallel combination. The PTC uses the motor current for heating, the NTC increases the voltage available.

Alternatively use a 3 wire fan and use the tacho output to switch a fet across an NTC thermistor.

[Michael Chare]

Google will let you find out about 4 wire PC fans often used for CPUs. They appear to use PWM.

[Dave Liquorice]

Choose the type of thermistor that is low resistace cold but high hot. Low at start provides the required start umpf, once running the current draw warms the thermistor which increase it's resistance, drops more volts, slows the fan. That's a PTC type isn't it? Might be tricky to find one with suitable cold/hot resistances though

Failing that use the MIC502 PWM speed control chip, or some other PWM method.

As for the various forms of PC fans has google disappeared in your part of the world? "3 wire pc fan":

[T i m]

Or an Arduino Nano, a FET and thermistor and make it do exactly what you want. ;-)

Cheers, T i m

[Weatherlawyer]

Modern chips function quite well in clean cases even when a fan is not running (or fitted) they slow down their own working speed to cope. So if you have a fan and it is running don't f*ck around in the nebula of 'trons.

PS: I saw a video of this on Youtube just prior to all the Brexitstentialism.