Fan Speed

Thread Starter

stilllearning

Joined Dec 9, 2013
18
Hello, I have a 12v fan that says 0.1200 amps

I want to be able to control the speed of the fan from slow-full speed, I asume i need a variable resister but how would I wire this and what values would I need

Also how would my numbers change if i was to use two fans with one speed controller

Thanks
 

wayneh

Joined Sep 9, 2010
16,102
...I asume i need a variable resister...
That's not likely a good solution. The motor experts around here will need more info about your fan motor - since there are different types and they require different methods to control them. Make and model? Photo?

Is this a compter cooling fan?
 

MaxHeadRoom

Joined Jul 18, 2013
19,053
Yes the fan technology needs to be known, if 12v DC such as a P.C. type, then it could be a modern BLDC motor that is hard to speed control because of the internal electronics.
Max.
 

#12

Joined Nov 30, 2010
18,076
You can buy fans that are smaller than 80 mm.
and, ps, it will not use .1200 amps. That is a labeling mistake.

While it is hard to interfere with the internal speed control, the inertia of the physical mass could be used to average out 1/10th of a second pulses you give it. The internal electronics will be up and running in less than 1/10th of a second and it will appear that nothing is wrong.
 

mcgyvr

Joined Oct 15, 2009
5,394
I tried PWM'ing some fans BLDC fans once before I knew any better and all they did was hum and would never start at anything but 90% duty cycle or greater.
 

Thread Starter

stilllearning

Joined Dec 9, 2013
18
sorry but i am really new to this and your terminology really lost me there, what is the current these fans use then if not 0.1200 amps, and can I not use a variable resistor to reduce the speed. Thanks for all the replys :)
 

MaxHeadRoom

Joined Jul 18, 2013
19,053
BLDC = Brushles DC (as per the link).
It essentially means that there are electronics internally that control the rpm of the fan.
so external control has little to no effect.
If you can get one of the older DC fans, these can be simply rpm regulated.
Alternatively if you can find a small shaded pole fan as used in bathroom fans etc, you can use a 'dimmer' type controller.
Max.
 

wayneh

Joined Sep 9, 2010
16,102
...can I not use a variable resistor to reduce the speed.
It won't hurt to try, as long as you don't mind a potential waste of time. The fan is nominally similar to a 100Ω resistor, in the sense that it drops 12V at 120mA of current. So put a 100Ω resistor (or another similar fan) in series with it and see how it looks. (It'll need to be rated to 2W or more.) Or, supply it with 9V or 6V, or even 3V with batteries.

Note that a variable resistor will be a poor choice as a control, since one rated to a high enough power rating will be too expensive.
 

mcgyvr

Joined Oct 15, 2009
5,394
.12 Amps is a reasonable/accurate number.. Not sure what #12 was referring to with that comment..

And those fans are brushless DC fans with internal commutation. They are NOT meant to be variable speed. Only On and Off.
 

#12

Joined Nov 30, 2010
18,076
.12 amps is a reasonable, accurate number, but .1200 amps is an unreasonably accurate number.

What I was referring to is that the label, ".1200 amps" means .1200 amps give or take .00005 amps under all conditions of voltage and load. It's simply a misprint, a typo. I don't think there has ever been a motor that uses the rated current +/- a twentieth of a percent. Much more likely it's .12 amps give or take 10% or .12 amps is the guaranteed maximum current and full load might be as much as 10% less for some specimens built on that day.
 
Last edited:

#12

Joined Nov 30, 2010
18,076
Much more likely .12 amps is the guaranteed maximum current and full load might be as much as 10% less for some specimens built on that day.
What I said^

Glad to see you're still learning. Learn this: Everything has a range of acceptable values. You are never going to buy a motor that is guaranteed to .0001 amp. They don't exist. You don't need them to exist. You are not going to build a circuit that can deliver .1200 amps safely and burst into flames at .1201 amps.
 

mcgyvr

Joined Oct 15, 2009
5,394
So what happens in my suggested experiment of supplying 3, 6 or 9V? Full speed at one voltage and off a few volts lower?
They typically have a voltage range.. like down to 5V or so for a 12V fan..then they just stall from what I've seen..

I intended to imply.. not intended for variable full 0 to XXXX RPM operation.
 

wayneh

Joined Sep 9, 2010
16,102
I'm just curious. Let's say it runs at 6V. If it's not drawing more current, it must be running slower? I'm not doubting that control by voltage is not useful, just trying to picture what happens when you try.
 
Top