Slowing down a computer fan

[stephenmodel]

Hello All,
I am completely new do anything electronic but i want to slow down an 80mm computer fan and power it with a 9v battery.

A bit of background of the project. I am making some terrain piece for a wargame (warhammer 40k) and i want these to spin SUPER SLOW. My friend said i would need to put a resister in so they would do that and not kill the battery. What sort of resistor would i need for the 12v 80mm fan?

Also i plan i putting a switch in so i am assuming it would be batterr --> switch ---> resistor ---Fan

is that correct?

thanks for the help
Steve

 

[audioguru]

A 12V fan does run slower when the voltage is only 9V from a brand new 9V alkaline battery. The battery develops an internal series resistance as it runs down which also slows down the fan.
If you add a series resistor then the fan might not start running, especially when the battery is running down.

You need gears or a pulley to make an electric motor run SUPER SLOW.

 

[stephenmodel]

A 12V fan does run slower when the voltage is only 9V from a brand new 9V alkaline battery. The battery develops an internal series resistance as it runs down which also slows down the fan.
If you add a series resistor then the fan might not start running, especially when the battery is running down.

You need gears or a pulley to make an electric motor run SUPER SLOW.

Hmm ok ithink i got it. It might be more complicated than i can handle. How long do you think i would get with just a 9v?

 

[MrAl]

Hi,

A 9v alkaline battery can put out about 500mAh, so if your fan draws 500mA that means it will run for an hour. It probably draws a little less than that, like 400mA or maybe even 300mA, so it may last an hour or a little longer.

9v batteries dont hold much energy. It's always better to use AA's or lead acid for longer run time. AA's hold about 4 times as much energy with the same number of cells as a 9v has.

 

[stephenmodel]

ok will do.

what if i use potentiometer to slow down the fan?

 

[audioguru]

The current of the fan is much too high for a potentiometer. The pot will get much too hot then fail.
A pot is a resistor and will not work anyway.
A pot can slow down the fan if the pot adjusts the pulse widths in a Pulse Width Modulation motor speed control circuit. Then the pulses are full power but the reduced duty-cycle slows the speed.

I agree that a little 9V battery is too weak to power a fan.

 

[audioguru]

How long do you think i would get with just a 9v?

We don't know how much current the fan needs. Its current draw is less at 9V than it is at 12V.
A battery is rated to deliver a certain amount of current. Its voltage drops as it runs down.
I showed that if the fan draws 400mA then a brand new 9V alkaline battery voltage drops to less than 7V in 15 minutes when the fan might stop or will not start running.

 

[alec_t]

Post deleted

 

[Mr RB]

...
A bit of background of the project.

I am making some terrain piece for a wargame (warhammer 40k) and i want these to spin SUPER SLOW.
...

I think it's better to fix the problem at the source, by using a more suitable motor for your "SUPER SLOW" rotation.

Ebay has small stepper motors with driver PCBs that will rotate at a fixed speed. All you really need to do is hook up the power.

Or if you must have a DC motor solution, you can get cheap DC toy gearmotors with plastic gearboxes quite cheap. They will probably make more noise than the stepper motor due to the cheap plastic gears.

 

[MrAl]

Hi,

What i should have asked was, just how slow do you want this to go, as in RPM's or turns per second?

 

[Mr RB]

And how much load does it drive, and what limits for size and shape, and what is the power source...

It makes it difficult to help people when they only give incomplete information.