# Controlling the speed of a small DC Fan

Discussion in 'General Electronics Discussion' started by gazj, Sep 23, 2012.

2
0
Sep 23, 2012
2. ### GreenGiant

842
6
Feb 9, 2012
with such a narrow voltage tolerance and the extreme RPM's (12,000... wow) on this fan you would probably be better off using PWM, using a pot you will go from full speed to stopped in 1/8 of a turn, not even, I mean you can obviously fiddle around with it but thats going to take an expensive pot (multiturn) and a lot of calibrating
a 555 timer should do it fairly well with a simple pot/cap setup so you can adjust it, though they do make PWM IC's so you could go that route as well

if Im doing my math right here, you are going to want 18%... make it a round 20%... duty cycle for this:

(0.16/0.90)*100=17.77777777 ... aka 18%, better to go a little high than low so 20% full speed aka 20% duty cycle

I know guys (*cough* CocaCola *cough*) that this is not an exact science but I am merely pointing in the right direction here, so before you start ranting about airflow not being a linear slope based on fan speed, I know, I'm just simplifying

3. ### MrEE

84
0
Apr 13, 2012
This looks like we are dealing with small power. Based on the fan datasheet, a resistor of a few tens of ohms will do the trick. the 500 ohm pot has too high a value (like the previous reply suggested). I'd try a 100 ohm pot or get a bunch of 100 ohm resistors and start by make a parallel combination of about 3 or 4 resistors and then use that combination in series with the fan. Now you can add or remove resistors by trial and error. If you already have the 500 ohm pot, you can add a 100 to 200 ohms in parallel with the pot effectively reducing its value.

4. ### gazj

2
0
Sep 23, 2012
Thank you both for your help. I will do as suggested and get a 100ohm pot. (I have got one with 10 turns so hopefully that will help). My electronics capabilities isn't up to more complex solutions unfortunately. I will let you know how it pans out.

Thanks Guys