potentiometer?

[Fabio]

I have a 12 volt case fan that I want to be able to regulate the speed. I bought a potentiometer from Radio shack but it does not appear to be sensitive enough. The fan either goes full speed or nearly nothing, there is no in between.

Can you suggest what and where I need? If you can send me a link that would be great.

Thanks!


PS

This is what I bought

 

[Navid]

If you look at the specs, you can see that is a 50k potentiometer.
That means that the maximum resistance is 50k. That is too large and turns the fan off.

The best way to control a fan is to use a fan controller.
This is an example.
http://www.svc.com/zazm1mufansp.html

Each channel is a voltage regulator, not a variable resistor (potentiometer). You can control multiple fans with this and it looks much nicer and it mounts as a drive and you will be able to control the fans without having to open the case.


However, if you want to do it with a resistor, you can too.

1- Find out the effective resistance of the fan you intend to control.
There are several ways to do that. One is to look at the label on the fan and see if it states the power consumption at 12V. If it does, divide 144 by the power. The result is the resistance.
It will be something in the approximate range of 20 to 100 Ohms.

Another way is to use a multimeter and measure the current through the fan when you apply 12V to it. Divide 12 by the measured current and that is the effective resistance.


2- Calculate the maximum resistance you need to place 5V across the fan. So, there will be 5V across the fan and 7V across the resistor (5V + 7V = 12V). Since you have the fan resistance, you can calculate the resistance value you need. R = 5 / 7 X Rfan.

Let's say the effective resistance of your fan is 40 Ohms (just an example). Then, R = 5 / 7 X 40 = 29 Ohms.


3- Calculate the maximum power consumed in the resistor.
That happens when the resistor value is set to be the same as the fan resistance. The power is V^2/R.
Or, 36/R.
For our example, the power is 36/29 = 1.24W.


4- Get a variable resistor (potentiometer) with a maximum value you calculated in 2 or less, and a maximum power capability you calculated in 3 or more.

 

[JackMDS]

50K is much too high, so when it is a total short it lets it run, but when you move it even a little the resistance it too high,

Example: Let say that a fan takes 100mA (0.1A)

12volt/0.1A=120ohm would be the resistance of the Fan to DC.

Therefore, if you use a 120 ohm potentiometer in serial to the fan at max resistance it would reduce the speed into half.

If you want less the half speed use 250 ohm or similar potentiometer.

Make sure that it is linear potentiometer and that it wattage rating is twice that of the Fan.

P.S. If you do not know the current consumption you need to know the wattage.

Then Current=Watt/Volt

Example: A 12 Volt fan that is rated 0.5watt

0.5/12=0.041A or 41mA

 

[Navid]

Originally posted by: JackMDS
12volt/0.1A=120ohm would be the resistance of the Fan to DC,

Therefore, if you use a 120 ohm potentiometer in serial to the fan at max resistance it would reduce the speed into half.

That would be true if the speed of a fan was linearly proportional to its voltage.
But, it is not.

Edit:
http://www.maxim-ic.com/appnot...fm/appnote_number/809/
As you can see from the first graph, halving the voltage could drop the rpm to a quarter.

 

[JackMDS]

Do you really think that post like this are the epitome of Engineering.

I was just trying to give the gist of it.

In addition, I indicated a serial connection (i.e. using the potentiometer as a variable resistance).

It would not be the same result as using the potentiometer as a potentiometer.

When using it as potentiometer part of the resistance become parallel, and part serial.

 

[Navid]

Originally posted by: JackMDS
Do you really think that post like this are the epitome of Engineering.

Telling someone that halving the voltage of a fan halves the speed is a mistake.
I just pointed out the mistake and posted a link. no big deal!

It would not be the same result as using the potentiometer as a potentiometer.

A potentiometer is just a device.
How you wire it determines what it really does.
You can wire it as a voltage divider, or you can wire it as a variable resistor.
The OP needs a variable resistor.

 

[Fabio]

So if the fan is 12volt and .45amp which one of these do I need?

 

[JackMDS]

You probably need about 50 ohm, but I do not see any.

Try this it would slow down the fan 35% to 40%.

http://www.radioshack.com/prod...p?productId=2062299&cp

 

[Navid]

Originally posted by: Fabio
So if the fan is 12volt and .45amp which one of these do I need?

12V / 0.45A = 27 Ohms

The best from that group is only the last one.
http://www.radioshack.com/prod...eter&parentPage=search
Its max resistance is 25 Ohms. When set to max, the voltage across the fan will be 27 / (27 + 25) X 12 = 6.2V.
This is enough to significantly drop the RPM of the fan.



Short the variable tap (in the middle? find out using a multimeter) of the rheostat to one of its other two connectors. Now, effectively, you have 2 ports (one is made up of two connectors shorted together). The resistance between these two ports varies as you turn the knob (variable resistor).

Connect the 12V positive wire to one side of the variable resistor. Connect the other side to the fan. Connect the other end of the fan to ground (black wire on Molex).