Multi-fan control

[Attila the Bun]

Morning Hexus,

As my motherboard is a bit shy of 3 pin fan connectors I thought one of these would be a good idea

http://www.scan.co.uk/Products/Akasa...he-motherboard

I was going to use it in conjuction with 2 of these

http://www.scan.co.uk/Products/140mm...e-emission-fan

My understanding is that I can connect the CPU cooler and the 2 case exhaust fans via this lead to the PWM 4 pin to the mobo. I thought that as the 2 case fans are directly above the CPU cooler they would act a bit like a pull fan attached directly to the heatsink array and being controlled by the mobo would speed up as and when required reather than being on full tilt all the time.

Then I thought what if I shouldn't connect the CPU fan along with the case fan.

As always any advice gladly accepted.

[Sputnik]

Well, that looks like the right thing and the way you described using it matches the labels on it. It draws the power from a Molex so no chance of melting the tracks on the board.

I cannot comment on the fans, The last time I looked at fans the Akasa Amber things got a good rating and I still have a few stock piled.

[Attila the Bun]

On the case I just wanted a couple of cheap and reliable quiet fans and at 20dBA the thermalrights fitted the bill.

My main concern is that by pairing the exhaust fans with the CPU cooler I might degrade the CPU fan performance.

[Funkstar]

Bare in mind that the fans are only 3 pin so won't benefit from any PWM signals coming from the motherboard.

My desktop system has two 80mm fans next to the CPU cooler, which is just a large passive block. The fans are both PWM and looking at them when the system is idle they barely turn.

[Attila the Bun]

Bare in mind that the fans are only 3 pin so won't benefit from any PWM signals coming from the motherboard.

Ah! I was thinking that there would be some sort of voltage control from the mobo that would manage the fan speed. Am I wrong in my assumption then?

[Funkstar]

It's very clever because motors need a minimum voltage and current to kick start them. If you just reduce the input voltage, a fan will slow down to a particular point (may still be spinning quite fast) then stop altogether even though voltage is still being applied. This can harm motors and end up drawing a lot of current, kind of like a short circuit.

PWM doesn't reduce the voltage, it sends pulses down the fourth wire turning the motor on and off. With a very slow pulse the fan is effectively just starting to spin up then being turned off very quickly. If you do this repeatedly the fan spins very slowly. A PWM system has a great deal more control of the motor than a system that just reduces the voltage.

This is exactly the same method they use to control the brightness of LEDs. If you reduce the voltage they just stop working, but if you flash them hundreds or even thousands of times a second the eye cannot tell it is flashing, the brightness just appears to be reduced. LEDs are also far more power efficient when on full brightness, so if you just flash them to full brightness very fast they can be very efficient even at low visible brightness

[Attila the Bun]

Well that gets the AtB award for "A damned good explanation" thanks very much.

I take it then I can either stick with the 2 fans I initially chose and accept that they will be running at full RPM or I can a couple of PWM fans at twice (at least) the price.

[Funkstar]

How about this one?

http://www.scan.co.uk/Products/Therm...ssion-fan-blad

Just a couple of quid more.

[Attila the Bun]

Hmmm...that looks a good possibility even if it's out of stock at the moment.

But as an extra question, how then, going by your previous explanation, do the 3 pin fans work with something like this?

http://www.scan.co.uk/product.aspx?ProductId=32349

Does it do the "stop/start" stuff at the controller end rather than the fan?

[Funkstar]

I don't know exactly. It does say there is voltage adjustment from 6-12 volts, so I presume it can use both techniques.

[Attila the Bun]

That would make sense if say that 6 volt was, say, just above the kickstart voltage for the fans.

[Sputnik]

It's very clever because motors need a minimum voltage and current to kick start them. If you just reduce the input voltage, a fan will slow down to a particular point (may still be spinning quite fast) then stop altogether even though voltage is still being applied. This can harm motors and end up drawing a lot of current, kind of like a short circuit.

That seem to be the opposite of the known characteristics of Inductive loads which generally draw more current at the initial point of powering up and then levels off.

[Funkstar]

That seem to be the opposite of the known characteristics of Inductive loads which generally draw more current at the initial point of powering up and then levels off.

Perhaps I have that wrong. But once the fan is actually turning all be it slowly (like it is initially spinning up or spinning down) it wouldn't need that much power to keep it spinning in that state, so just a little flick of power will do fine.

I do know that holding a motor just below its starting voltage, or stopping it from turning when at it's normal operating voltage is very bad. Causes a lot of heat and batteries to die out quickly. My years of playing with Lego, model trains and Scalextrics prove this

[Sputnik]

I do know that holding a motor just below its starting voltage, or stopping it from turning when at it's normal operating voltage is very bad. Causes a lot of heat and batteries to die out quickly. My years of playing with Lego, model trains and Scalextrics prove this

Indeed !

Computer fans are brushless and usually 2 phase, the frequency (and speed) is determined by the little circuit board built into the fan itself.
PWM fans appear to be are fans that have an external connection to this circuit board so that the frequency can be dictated from the motherboard.

Most of the fan controllers for PC's are just variable voltage units.

[Attila the Bun]

I'm a bit confused now...does this mean I can use those cheapy fans or not

If so I'll bang them on my order and get free postage because I've read the terms and conditions properly now

[DavidM]

Probably not what you're after - but I've got one of these myself for plugging my extra fans in

http://www.scan.co.uk/products/Fract...-from-x1-Input

(My case has 5 fans - and the mobo doesn't have so many outputs )