Board-only fan controller

kaku · 16 Oct 2015 at 20:33

I'm looking for a compact fan controller for adjusting the speed of 6 non-PWM (3 pin) fans. I'm thinking something like this: https://www.overclockers.co.uk/showproduct.php?prodid=CA-009-PT only with a single knob to adjust the fan speed, rather than using the PWM signal from the motherboard.

Does such a device exist? Is it something that would be feasible to build yourself?

I already have one of these cables lying around: https://www.overclockers.co.uk/showproduct.php?prodid=CB-070-AK
I did toy with the idea of putting a variable resistor in-line at the 12v molex to get the adjustment I need, but after some research online I've seen a lot of people saying it's a bad idea and to be honest I don't know enough about electronics to spec it up properly.

Tealc · 17 Oct 2015 at 18:29

A variable resistor, or potentiometer, is a solid way of reducing voltage to a load but unfortunately most potentiometers are very low power handling. Rheostats are the high power brothers of potentiometers and are much larger in comparison and come with fairly hefty price tags. This is why it is a poor idea.

What you need to do is create a circuit that has a low power potentiometer that passes it's output voltage to a transistor that'll take the load you give it. You'll experience a small drop out of voltage at the top end with such a simple circuit. I'm sure there are dozens of circuit examples for such a circuit.

I'm sure I've built them from time to time but prefer the automatic control myself.

I use the circuit in my signature to control my fans and that has a potentiometer and can handle 3-4 with ease. It is adjustable but in a slightly different way.

Dirk Diggler · 17 Oct 2015 at 21:00

Have you tried Speedfan?

kaku · 20 Oct 2015 at 00:59

Tealc said:
A variable resistor, or potentiometer, is a solid way of reducing voltage to a load but unfortunately most potentiometers are very low power handling. Rheostats are the high power brothers of potentiometers and are much larger in comparison and come with fairly hefty price tags. This is why it is a poor idea.

What you need to do is create a circuit that has a low power potentiometer that passes it's output voltage to a transistor that'll take the load you give it. You'll experience a small drop out of voltage at the top end with such a simple circuit. I'm sure there are dozens of circuit examples for such a circuit.

I'm sure I've built them from time to time but prefer the automatic control myself.

I use the circuit in my signature to control my fans and that has a potentiometer and can handle 3-4 with ease. It is adjustable but in a slightly different way.

Thank you, this has given me a lot to look in to.

A quick follow-up question:
Considering the absolute worst-case scenario my fans draw a maximum of 0.25a; meaning ~18w total power draw when running at 12v (18 = (0.25 * 6) * 12). However, am I correct in thinking the power rating for a transistor applies to the power it is dissipating - as in, when the fans are running at maximum speed @ 12v no voltage adjustment is occurring, thus I don't require a transistor that's rated at the full 18w? From looking at a few spec sheets most I've seen are over-specced for this kind of application, so I'm probably worrying about nothing.

I found an article here that should be a good start: http://www.pcbheaven.com/circuitpages/Simple_Linear_Fan_Controller/

String said:
Have you tried Speedfan?

This would be by far the easiest option, but the motherboard headers on this machine don't support voltage control.

ErikN · 20 Oct 2015 at 07:50

There are ones that fit in a case PCI slot with just a knob. Google image search for pci slot fan speed controller to get an idea of what I mean. If you find one, it should be a simple amtter of removing the PCI metal plate should you wish to place it somewhere else.

Mik3 · 20 Oct 2015 at 11:14

You could use one of these, remove the pot and attach another one with wires so you can adjust the speed easier. I use one of these to control the speed of my H100i pump, very simple and cheap.

Tealc · 20 Oct 2015 at 12:59

kaku said:
I found an article here that should be a good start: http://www.pcbheaven.com/circuitpages/Simple_Linear_Fan_Controller/

In that example the BD243 is certainly able to handle the energy dissipation but as Giorgos mentions you'd need a heatsink.

As you reduce the voltage the current the fans use will also drop so the transistor will never have to dissipate full power.

One drawback is that the transistor will always drop 0.6-0.7v no matter what you do so don't expect the full 12v operational speed.

A DC-DC buck converter will suffer less with heat dissipation as they are fairly efficient but again will have a 'drop out' voltage, which may or may not be an issue for you.

kaku · 20 Oct 2015 at 17:44

Thank you for your helpful input everybody.

This should make for a fun project.