Help for DC fan controller in PC

[leonstafford]

Hi all, first post for a newbie.

I am trying to stick a potentiometer (I think trimpot type at 10khm: ) into a PC's fan circuit (12VDC).

I have tried 2 methods:

1. connecting one end of the trimpot to the 12V+ coming from the computer, the other to the fan's input and trying to ground the middle pin of the trimpot to the computer's ground.

&

2. connecting one end of the trimpot to the 12V+ coming from the computer, connecting the middle pin to the fan's input and then grounding the other pin of the trimpot to the computer's ground.

Both to no avail. I seem to be able to only get power to the fan when I turn it fully clockwise or almost fully, but I can't get any difference in fan speed, it's all or nothing. Do I have the wrong type of pot? The fan is rated at 12VDC 0.44A

Questions (I'm still 1 day into electronics!):

Is there a specific input and output end of the potentiometer or either is the same?

How do I work out the hm: 's of the pot I should be using?

Is there an obvious dumb newbie thing I am doing?

Any help will be greatly appreciated

 

[leonstafford]

oops!

Found my mistake - 10,000 vs 10hm: s resistance will make a difference

 

[dannix]

you need to check what power the pot can take I doubt it can take enough for a fan, you may just fry it.

Have a look at this site http://www.cpemma.co.uk/rheo.html that explains reostats which are a like pots but carry more current, it will explain whats good and bad ultimatly then look at http://www.cpemma.co.uk/thermal.html, it shows different methods and full circuit designs for stripboard builds of other fan control methods that are prob better suited to what you wish to do.

 

[jpanhalt]

The fan is most likely a brushless DC motor. You cannot control it with a pot, as you have found out. John

 

[bryan]

http://www.cpemma.co.uk/pwm.html

 

[leonstafford]

Thank you both for your help!

I will do a little more shopping at the geek store down the road (lucky I'm in Japan!)

And will read up a bit/lot more before killing my spare fans!

Cheers

 

[shaneshane1]

The fan is rated at 12VDC 0.44A

Just so you don't get confused, 0.44A is 440mA and not 44mA, i am not sure if you already knew that, i am just pointing it out for you

 

[electronic spark]

Thank you both for your help!

I will do a little more shopping at the geek store down the road (lucky I'm in Japan!)

And will read up a bit/lot more before killing my spare fans!

Cheers

You wouldn't burn your fan, but the pot. Why do you want to lower the fan, I wonder?

R=E/I, so if you wanted to reduce the voltage down to half (6V to get half the speed) then R=12-6/.44, would be ~13hm:
P=E*I, so power would be 6*.44, ~3 Watts.

That should allow you to adjust the rpm from 1/2-full.

For full range of speed, you'd need ~26hm: at about 5 Watts.

Using the middle and either outside pin. Hope that helps.

 

[leonstafford]

First project achieved!

Thanks everybody for your help.

There were enough pointers along the way for me to get lucky and finish the project.

I went back to the geek shop and ended up buying a 30hm: resistor rated at 3W.

I calculated the stock fan's output:

12v/.44mA = 27hm: (about 5W)

Then to approx. halve my speed, I calculated what another 30hm: s would give me:

12v/60 = .2mA

&

12v x .2mA = 2.4W

So I went ballpark and bought a 30hm: 3W cement resistor for 64yen.

It was only a bit more for a pack of 2 pots, but the resistor looked nice and easy to solder and other people had reduced their same model computer's fans to 50% speed with a big noise reduction and minimal thermal gain.

This machine had been used to salvage parts for others, so after the fan mod, I did a bit of drilling and cut a piece of the PCB off to accommodate a cool little red push button for power (about 200yen I think at the same electronics parts store).

Still a total electronics newbie, but it feels really cool to have fixed a machine I would have just sold as junk a year ago

Thanks for all the advice, I'm sure my solution is nowhere near perfect but I will be studying up more and with the help of electro-tech-online.com will be challenging myself more soon.

Cheers,

Leon

 

[dannix]

Hi, you could have halfed the fan speed with the 7volt trick details here http://www.cpemma.co.uk/7volt.html, would have cost you nothing, but then you have still acheived what you want. well done.

 

[leonstafford]

Hi, you could have halfed the fan speed with the 7volt trick details here http://www.cpemma.co.uk/7volt.html, would have cost you nothing, but then you have still acheived what you want. well done.

Hi dannix,

That's a cool trick - thanks!

The machine I was playing with had only 3 wires - a 12v+, Gnd and a speed sensor (no fan control onboard though, and running with the fan disconnected doesn't stop the computer, so seems a redundant feature).

I will definitely play with that method in normal PCs though

Cheers

 

[dannix]

I think you missunderstand. If you supplied your fan with 12v to the fan positive and 5v to the fans negative it would slow the fan, that page is talking about the voltage rails from the computer power supply, so your fan can still have the 3 wires you refered to and work.

 

[leonstafford]

I think you missunderstand. If you supplied your fan with 12v to the fan positive and 5v to the fans negative it would slow the fan, that page is talking about the voltage rails from the computer power supply, so your fan can still have the 3 wires you refered to and work.

Thanks dannix,

I didn't want to out myself as a Mac user , but the computer was an eMac which doesn't have any 5v's anywhere (or not near enough to the fan anyway).