Simple pot control for dc motor

[Stonecoldbrew]

I am building a stir plate for yeast propagation. I have a dc pc fan with the blades removed and rare earth magnets attached. The fan is rated 24 vDC and 140 mA. The power supply output is 26 vDC and 210 mA. I want to use a pot as a simple variable resistor to control the fan speed. Can I use a simple pot or should I use a little more sophisticated design to protect the circuit? What values are needed?

Thanks ahead of time!

 

[ccurtis]

As a simple solution, a 2 watt wirewound potentiometer should work okay, but a .5 watt carbon type cannot handle the heat dissipation. Use a 500 ohm pot, though you can probably use a 250 ohm pot since the fan probably won't run below 10 volts. A 500 ohm pot will allow adjustment below 10 volts.

 

[Stonecoldbrew]

Thanks ccurtis. Will this give me the ability to slow it down really low? I don't need it to stop, but lower speeds can help to prevent it from kicking the stir bar I'll be using. I still want it to go fairly fast on the high end once the stirrer has a good vortex. Fine tuning would be nice.

What is the method of figuring the calculations for a circuit like this. I may be building another one in the near future, but with a different fan and power supply. I am handy with a soldering iron and have background in electronic product servicing, but that was over 15 years ago and all the circuit design has left my head.

 

[ccurtis]

Thanks ccurtis. Will this give me the ability to slow it down really low? I don't need it to stop, but lower speeds can help to prevent it from kicking the stir bar I'll be using. I still want it to go fairly fast on the high end once the stirrer has a good vortex. Fine tuning would be nice.

Use a 10-turn pot for finer tuning. I don't know the voltage at which your fan stops rotating, but typical brushless fans (which are what you find in typical PCs) do not run at a third or less of the rated voltage, judging from info I researched. The 500 ohm pot will adjust the fan to a stop, but you will also have some unused turns on the pot over which the fan is stopped over that adjustment range. With the pot set to minimum resistance, the fan will run at full power.

I suspect you could probably get the fan to run at the lowest possible speeds using a more complex PWM circuit that applies full voltage to the fan but at varying on/off pulse duty cycle. So, if you want to run the fan at the lowest possible speeds, a simple pot will not do the job. Probably the best you could do with a pot is get the fan to run at half the maximum speed, approximately. Below that it will just stop.

What is the method of figuring the calculations for a circuit like this. I may be building another one in the near future, but with a different fan and power supply. I am handy with a soldering iron and have background in electronic product servicing, but that was over 15 years ago and all the circuit design has left my head.

Since the plot of current vs voltage for a brushless fan is fairly proportional (based on information I saw), I substituted a resistor for the fan, using ohm's law. 24V/.140A= 170 ohms. With the pot added, you now have a simple circuit composed of a 26v power source and two resistors in series. Calculations from there I used ohm's law, Voltage=Current * Resistance and the properties of electrical circuits that current is the same at every point in the circuit, series resistances and series voltage drops across series resistors are additive, . Power=Voltage * Current.

 

[Stonecoldbrew]

Thanks again ccurtis! I will dedicate a beer in your honor.

If anything else comes to mind for this from anyone, please post. I know this isn't a very in depth problem, but I'm always up for projects and fine tuning things. I want to gadgetize my whole brew process.

Next up will be rewiring a side by side fridge for dual temp control.

 

[ccurtis]

If you want to kick the complexity up a notch and run the fan at lowest possible speeds you can do a PWM type control, like this:

**broken link removed**