Question for my electronically minded friends: I'm trying to hook up a potentiometer to control the speed of a fan. (The fan is from a PC power supply, the pot is the volume control from a PC subwoofer, the power supply is from an old router.) when I hook up the fan side of the red wire to the right nubbin, and the power supply side wire to the center nubbin, I get almost the full speed that I do without the pot, but if I turn the knob on the pot as much as an eighth counter clockwise, I get nothing! The full range of motion is about 3/4 of a rotation. It's a 12v 0.19amp fan, and a 12v 1.25amp power supply, and the pot model number is B103, so it should be linear. Anyone know what's up? Do I just need more power?

 

Welcome to the forums.

A B103 potentiometer is 10,000 ohms.
You need a 100 ohm or 250 ohm pot at 5 watts.

 

Which is going to be a rather expensive wire-wound rheostat. Got any military surplus places nearby?

Also--I'm not sure you'll get effective speed control by limiting the power to a brushless DC "muffin" fan. Maybe someone who's done it will chime in...

 

Actually it may not need to be 5w but the size (resistance) is what would be needed if a variable control would work.

The most effective way to control DC motor speeds is with a pulse width modulation control. they are inexpensive and easy to use.

12V 36V Pulse Width PWM DC Motor Speed Regulator Controller Switch 12V 24V 3A | eBay

 

Thanks! That explains it. I was trying to do the whole build with nothing but salvaged parts, but it looks like I may just have to suck it up and buy a PWMC. It looks like the pot on the one you linked to is also a 10k pot; what makes it superior to just the pot itself?

 

Do DC brushless motors use an electronic circuit to get the rotor moving & control speed? Would PWM input damage the circuit?

How about a simple variable 3-pin regulator? Way cheaper than a rheostat that could handle the motor current.

I just hooked up a small PC fan to a power supply and was able to slow it down by reducing the input all the way to 3 volts--at which point the fan stopped and the current doubled.

 

The 10K pot that comes with the speed kit controls the on/off rate that the PWM operates at. The fan motor is not run thru that 10k pot.

A pwm speed control should work with any DC type motor. You could experiment with resistors to slow it down but when you get the speed down to a certain RPM with resistance.... it may not restart.