Small DC fan as a final control element


Thread Starter

Chris George

Does anyone know how to use a small DC fan (5V, 12V, or 24V) as a final control element in a 4-20 mA loop? Is there a simple circuit or an
off-the-shelf device that will vary the voltage to the fan based on a 4-20 mA signal?

Chris George

Curt Wuollet

Hi Chris

Somewhat. It's not hard to convert a 4-20 ma. signal to, for example 0-12 VDC. Fans are pretty non-linear and some don't like wide variations in input voltage. You can use a resistor to convert to voltage and an op amp to offset and scale the output. Then a driver for the fan. You might find the range for a particular fan is 3-12 or something like that and air delivery is exponential vs voltage. Many newer PC fans have a tach output that could be used for feedback if precise control is wanted. If you are controlling temperature with a closed loop this screwy transfer characteristic may not be a problem. email me at my work address. I have a circuit I did for a question a while back. Wouldn't take more than a couple bucks worth of components.



Kent L. Gerhardt

Chris, Velleman Electronics ( "" ) makes a 4-20 mA PWM kit (K8004) that will do exactly what you need. The PWM is much better than just voltage regulation as at the voltage to RPM curve for a DC motor is very non-linear. That makes it difficult to control the fan, especially at low speeds.::


Kent L. Gerhardt
Gerhardt & Associates
P.O. Box 2062, Hammond, LA 70404 USA
voice: 985.542.1963
fax: 985.345.5475
cell: 985.634.8804

Michael Griffon

I don't know if the specs of the device you mentioned are suited to the application, but why do you say a DC motor is very non-linear at low speed, and how would using PWM make it easier to control it?
The speed of a permanent magnet DC motor (likely what is used in the small fan mentioned) is linear with respect to voltage, and the torque is linear with respect to current. The performance curve of a DC permanent magnet motor is a straight line from no load to stall (neglecting saturation effects near stall).
PWM modulation of the supply voltage should produce the same result as a linear power supply. The PWM modulation is just more efficient than a linear power supply. This is essentially the same as using a switching power supply, except that the motor itself acts in the place of the output filter.

The load imposed by a fan is non-linear, but these effects are most noticable at high speed, not low speed. Open loop control of a DC fan motor is very simple. Could you please explain what you meant more clearly?

As a note to the original poster, "small" is a relative term and you will get a better answer if you can say how many amps, what type of DC motor (permanent magnet, series, etc.), what the operating environment is, and where you intend to draw your supply current from. A "small" 12VDC automotive fan motor could draw as much as 30 - 40 amps at full load, and would be better served by a different solution than what you might choose for a muffin fan.
A 4-20ma signal can be converted to a voltage signal by using a resistor into 1-5V, 2-10V, or some other suitable signal. Suppressing the elevated zero (making the 1 or 2 volt lower limit 0 volts) is a bit more work though. There isn't much point in going into more detail without further inforrmation from you.

Chris George


You're right, a "small" fan is vague. One particular fan I have that I would like to use is brushless, rated at 12VDC, 160mA, approx. 3"x 3" housing (essentially a PC fan).

Chris (original post)