# Controlling speed of 12VDC fan

#### caje

Joined May 2, 2020
9
I have a fan (for cooling Peltier device) that runs on 12VDC 0.28A with impedance protection (KunPenz FS8025HD1). I would like to reduce the speed of the fan about half, in order to reduce sound noise generated. I have tried both adjusting the supplied voltage and current (using a 10kOhm potentiometer). It seems to me that either way it is really sensitive in that either it runs 100% or nothing at all no matter how careful I am in turning the potentiometer. Why is this? Do I need a smaller potentiometer, is there a way to calculate how much I need? Or am I out of luck because the fan has some internal IC ensuring it runs 100%?

#### ericgibbs

Joined Jan 29, 2010
13,208
hi caje,
A 10K is much to high a value.
Is the fan 2 or 3 wire.?
E

#### caje

Joined May 2, 2020
9
hi caje,
A 10K is much to high a value.
Is the fan 2 or 3 wire.?
E
The fan is 2 wire. But which is the correct/preferred way to control the speed; by reducing voltage or current?

#### caje

Joined May 2, 2020
9
Right. So I guess either that or a fan that supplies a greater supply voltage range.

#### ericgibbs

Joined Jan 29, 2010
13,208
hi,
A greater voltage would not help.
Do you have some power resistors , say about 5R thru 50R at 5Watt you could try.?
E

#### ElectricSpidey

Joined Dec 2, 2017
1,753
For that kind of wattage a simple emitter follower circuit would work just fine.
NPN
Pot
Small heatsink

Joined Jan 15, 2015
5,992
On average small 12 VDC fans, depending on fan of course, at 12 volts draw about 157 mA of current so lets call it at 200 mA. You really want, as mentioned, about a 50 Ohm pot. In the interest of keeping things cool I would look towards a 5 Watt pot. I have also noticed that when reducing the fan voltage the fan current drops proportionally. Just make sure that with whatever reduced voltage you come up with the fan starts turning.

Ron

Joined Jul 18, 2013
23,342
Now it is hard to detect the actual form of small DC fans, at one point they were almost always DC motors, now although fans are supplied with two conductors and state DC, internally, they are actually BLDC fans with a small IC that contains a commutation sensor on board the IC.
So in some cases dropping the voltage works, it is not always a positive way of control.
You may still find a true DC version, if lucky.
Max.

#### andrewmm

Joined Feb 25, 2011
1,612

Joined Jul 18, 2013
23,342
Measuring the resistance of the fan the way mentioned is of no use in BLDC fans of the type that I mentioned .
Max.

#### Alec_t

Joined Sep 17, 2013
12,148
I have a fan (for cooling Peltier device)
You do realise that when the fan speed is reduced, the cooling effect will be reduced too? Will that be a problem?

#### caje

Joined May 2, 2020
9
You do realise that when the fan speed is reduced, the cooling effect will be reduced too? Will that be a problem?
Yes. Currently the cooler performs way better than what I need, so I am okay with reduced cooling effect.

hi,
A greater voltage would not help.
Do you have some power resistors , say about 5R thru 50R at 5Watt you could try.?
E
I was thinking a greater operating range. So for instance this SUNON fan (link; sorry for the danish website) has a rated voltage on 12V but operating voltage ranging 4.5-13.8V. I guess this is intended for being able to control the speed?

On average small 12 VDC fans, depending on fan of course, at 12 volts draw about 157 mA of current so lets call it at 200 mA. You really want, as mentioned, about a 50 Ohm pot. In the interest of keeping things cool I would look towards a 5 Watt pot. I have also noticed that when reducing the fan voltage the fan current drops proportionally. Just make sure that with whatever reduced voltage you come up with the fan starts turning.
Sounds like this should be my next move; trying out a smaller potentiometer.

Joined Jul 18, 2013
23,342
According to the manuf. site they are 4 pole BLDC motors, there is a rider in the spec sheet that states:-

"DO NOT use power or ground PWM to control speed, if the fan speed needs
Max..

#### caje

Joined May 2, 2020
9
According to the manuf. site they are 4 pole BLDC motors, there is a rider in the spec sheet that states:-

"DO NOT use power or ground PWM to control speed, if the fan speed needs
Max..
Looking at this Sunon DC brushless fan, the data sheet clearly states operating supply voltage 4.5-13.8Vdc. So is it safe to assume that by adjusting the voltage (i.e. voltage divider) I will be able to control the speed/output of the fan?

Alternatively a fan lige e.g. Arctic F8 PWM has out-of-the-box support for PWM. Then I would also need a PWM IC to control it.

Is this correctly understood?

Joined Jan 15, 2015
5,992
It's just a simple 80 mm 12 volt 1.1 watt fan. Just place a resistor in series with it or use a pot. The maximum current is only 104 mA with an average of 90 ma. It will start as low as 4.5 volts so slowing it down is not difficult. Start with about a 120 Ohm 2 Watt resistor or get a 100 Ohm 2 watt pot and find a speed you like.

Ron

#### MisterBill2

Joined Jan 23, 2018
8,990
If the fan speed change is linear with voltage then at 6 volts the current would be 70mA, since fan power changes with the square of fan speed. So you would use a resistor dropping 6 volts at 70 mA, and the resistor power would be 420 milliwatts. So use a half watt resistor, 82 ohms is the closest standard value. And 68 ohms is probably close enough. No, it is not at all efficient, but cheap, simple, and effective.
Of course, the reduced air flow will greatly reduce the cooling effect that is available.