PWM circuit doesn't vary much.

[Michael Daly]

I want to control the speed of a pair of 12V computer fans in parallel. I put together a circuit based on **broken link removed**. It works, however, all the variation occurs in the first third of the potentiometer's rotation. After that, the fans are at full speed for the remaining two thirds of the pot's rotation. I've experimented with a few other fans and different pot ranges ( 10k, 25k and 100k, definitely linear and not log). I even dropped the voltage by about 20% to see if that made a difference.

What can I do to get a wider range of adjustment (IOW, less sensitivity to the pot position in the beginning of the range). I don't know much about analog circuits.

 

[colin55]

The fan you are using may not alter frequency in a linear way to the energy being delivered to it. It may behave differently to a motor such as a 3-pole motor.

 

[MikeMl]

Most box fans have a constant-speed controller built-in to them. By varying the external voltage (with your external PWM), you are trying to overcome the fan's natural tendency to run at a constant speed. Boxer fans are not meant to be varied. Read the data sheet.

 

[crutschow]

If all the variation occurs in the first third of the pot rotation, then just put a resistor ins series with the high side of the pot element, equal to about twice the pot resistance. That way 100% of the pot rotation will give about the same setting as 33% of the rotation does without the resistor.

 

[Michael Daly]

Actually, the first thing I thought of was using a resistor to offset the range of the pot. However, this doesn't do anything! I just tried it again to make sure. I still get only 1/3 causing variation! I can't explain this at all.

 

[MikeMl]

... I can't explain this at all.

I can, and did. Read my post again. Box fans are designed to keep their speed constant regardless of what voltage you feed them (up to a point when they suddenly stall).

 

[crutschow]

Actually, the first thing I thought of was using a resistor to offset the range of the pot. However, this doesn't do anything! I just tried it again to make sure. I still get only 1/3 causing variation! I can't explain this at all.

What terminal of the pot did you put the resistor?

 

[Michael Daly]

Box fans are designed to keep their speed constant regardless of what voltage you feed them (up to a point when they suddenly stall).

I ignored that comment since I am getting a full variation of the speed from zero to maximum. The problem is not that it is running at a constant speed, but that it is varying speed with only one third of the pot's rotation.

These PWM circuits are often recommended on various web sites for controlling fan speeds so I can't see why they would be recommended if they can't work. The circuit does not vary the voltage but produces a train of full-voltage pulses. It's the total power that varies due to the amount of current that is passed through, not the voltage that varies.

 

[Michael Daly]

What terminal of the pot did you put the resistor?

I put it on the high end of the pot. I also put it on the low end just to verify. As expected, on the low end I got full speed, since it it varied from two-thirds to full resistance. When attached to the high end, I got zero to max.