What Potentiometer should I get to control the speed of a computer fan?

[DiamondsareMine]

Hi, I am trying to control the speed of a fan I have with a potentiometer but I don't know what to get. I don't know much about electronics and I don't know if I will pick the right one. What kind potentiometer do I have to buy? It would be awesome if you also had a link too. Thank You!

Fan Specs:
12V DC
0.32A
3.84W

 

[AnalogKid]

This type of control is the most basic, and the most inefficient. Important question - do you want to just slow down the fan a bit, or dial it all the way down to 0 speed?

ak

 

[DiamondsareMine]

I would like to slow the fan down a bit if it a "bit" could take it down to about half speed.

 

[Colin Mitchell]

For a start, the pot must be able to take 350mA without burning out.
Next, you want to drop about 6v to 8v to reduce the speed.
The effective resistance of the fan is about 38 ohms.
If you add about 100R pot, the fan will reduce in speed.
A 100R pot capable of handling 350mA is 12 watts.
A 10 watt wire-wound pot will just be acceptable.

 

[Minder]

There are many PWM on ebay for $5.00 pp 291341338166
Circuit here http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm
You might have a problem with current fans as although they are DC fed, they have an internal PWM controller IC.
M.

 

[DiamondsareMine]

For a start, the pot must be able to take 350mA without burning out.
Next, you want to drop about 6v to 8v to reduce the speed.
The effective resistance of the fan is about 38 ohms.
If you add about 100R pot, the fan will reduce in speed.
A 100R pot capable of handling 350mA is 12 watts.
A 10 watt wire-wound pot will just be acceptable.

Do you have a suggestion on what you would get that would match all these requirements?

 

[cjdelphi]

a 555 under astable to supply pwm to the fan, a pot to adjust the 555, the 555 controls the fan..

but, as stated the fan may already be pwm controlled from internal curcuutry

 

[Alec_t]

Digikey's price for a 100Ω 12W wirewound pot is £77 . So PWM is the way to go!
If a conventional PWM control conflicts with the internal commutation of the fan you could try my 'Synchronous Pulse Delay' method as described in this article.

 

[Kiwi]

Are you using the fan in a computer, or are you using it for another application?

 

[cjdelphi]

Not bad price for a 5w pot...

 

[cjdelphi]

anyway, a 20c 10-100k pot which controls a 555's astable mode will give you a varied on/off pulse the pot will adjust the firing time, the output pin3 could be then connected to the base via a 1k resistor which can power the fan...

you may need a flyback diode across the motor in reverse to suppress high voltages from the motor (unless it's brushless and then i'm not entirely sure, do brushless have inductive kick backs? i don't think they do?)

 

[AnalogKid]

A BLDC (Brush-Less DC) fan is a wierd combination of a constant resistance and constant current device. As the voltage to the fan is decreased, the current does decrease, suggesting constant resistance, but not linearly as it would with a resistor. Still a constant resistance is a reasonable starting point for this type if question.

I = E / R
P = I^2 x R

For a nominally 38 ohm fan, let's start with a 50 ohm pot. At full speed the pot resistance is zero so it dissipates 0 power. At full rotation the total resistance of the pot plus fan is 88 ohms, so the current is only 0.137 mA, so the power dissipated in the pot is 0.92 W. If you limit the pot's rotation to only 38 ohms, the power is 0.86 W. Digikey and Mouser have 2 W critters for $9.

http://www.digikey.com/product-detail/en/RV4NAYSD500A/RV4N500C-ND/222956
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtC25l1F4XBU7rZyhV5cpJRj5zllmXo7sI=

ak

 

[Minder]

A BLDC motor operates from a DC supply but requires commutation to the 3 stator winding, so There has to be embedded electronics of some kind.
Picmicro TC653 etc
M.

 

[DiamondsareMine]

A BLDC motor operates from a DC supply but requires commutation to the 3 stator winding, so There has to be embedded electronics of some kind.
Picmicro TC653 etc
M.

Would it be easier just to change my fan? If I were, what would i be looking for?

 

[Minder]

If you want to guarantee a simple form of control, then DC brushed are generally the simplest, the older PC fans were practically all this type.
M.

 

[DiamondsareMine]

If you want to guarantee a simple form of control, then DC brushed are generally the simplest, the older PC fans were practically all this type.
M.

http://comingsoon.radioshack.com/25-ohm-3-watt-rheostat/2710265.html#.VavSRflViko
Ok then, would this work? Sorry,noob question: Would my fan not be able to spin as fast because it is a 3 watt potentiometer or would the potentiometer break?

 

[Minder]

A potentiometer is not a really recommended way to control a motor directly, it is very wastefull in power also.
M.

 

[DiamondsareMine]

A potentiometer is not a really recommended way to control a motor directly, it is very wastefull in power also.
M.

I have never done anything with embedded electronics before. Do you know of a guide then?

 

[Minder]

You mean apart from post #5?
M.

 

[DiamondsareMine]

You mean apart from post #5?
M.

Never mind, thanks for helping me. I'm sorry that I am so bad at this.