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Potentiometer to control DC motor speed Answered

I have a 12 volt fan in my RV. It has a 1.6 amp DC motor.
Would a 2 watt, 350 ohm potentiometer be suitable to use as a variable speed control?
If not, what potentiometer would I need?


Hi I've just joined because of this particular thread.

I have a small milling machine that has a 600W x 220v brushed DC motor. The instruction manual advises that if you run at a high speed , say 2000rpm and then stop the machine you must turn the speed control knob down to the lowest speed before restarting the machine. I suspect this is to prevent fusses blowing when trying to start at a high speed. This is a nuisance because if I forget to reduce the speed then I'm going to blow a fuse and not just the machine's!

What I would like to do is to replace the speed control potentiometer for another one that will switch off the machine at the low speed end. This will ensure that I always reduce the speed when stopping the machine. This could be wired into the machines own STOP button.

The speed control potentiometer is 4.7K. Any suggestions?

Some some kind of automatic soft start would be great but I think these only work on AC motors.

I suspect the control pot sets a PWM level and would admonish the circuit designer for not providing a low speed start up feature in the first place.. Then the seller probably copied a design and the seller may know components and wiring But does not understandreal electronics..

Anyway a schematic must be acquired or traced from the item.. Then it will be reasonable to add a low speed delay on power up as an option switch.

steveastrouk I made a significant error. My rheostat is 25 ohms, 3 watts; NOT 25K ohms. I apologize for that.

I performed a second test.; this time with the fan blade installed (which I should have done before).

I used the same two terminals as before.

Using the aforementioned pot, I could regulate the fan rpm from about 1200 rpm through 250 rpm.

This time, however, the pot heated up significantly at any speed except high. After 5 minutes it was too hot to touch. If I made the proper connections using all three terminals of the pot, it gets even hotter; even at max speed. So this looks like a no-go to me.

I am very familiar with basic electricity as I was a mechanic for 45 years, I have rebuilt alternators, generators and regulators, re-wired instrument panels and so on. So please don't take me for dummy.

iceng, you speak of "one to six 50 volt 3 amp 6A1-T diodes in series with the motor", but how are they switched to vary the motor speed?

What you will also find, is if you leave the motor at the lowest speed, with a pot, switch it off and restart, your motor is likely stalled, and burns out.

1.6 amps x 12 volts is 19.2 watts

and therefore at half speed, one could reckon on the pot dissipating 10W. I don't understand how the OP got results with a 25K pot !

Got it! Thank you very much.

steveastrouk I will order your suggested motor speed control from eBay as a backup.

Thank you to all

I will get back with results.

OK. So I need a rotary switch?

Yes, a 3 or 5 amp to handle the start current surge, be sure to plan for one more position which will be your OFF and should be next to the high speed..


3 years ago

Motor Resistance is R = V / I = 12 / 1.6 = 7.5 Ohms

Motor Power = V x I = 12 x 1.6 = 19.2 Watts

No you cannot use a mere 2 W 350 Ohm pot.

You could put one to six 50 volt 3 amp 6A1-T diodes in series with the motor


Each diode will lower the voltage about 0.7 volts slowing the fan in gradual steps and they cost $ 0.25 each.

Each diode reduces the voltage to the fan motor.

12v, 11.3v, 10.6v, 9.9v, 9.2v, 8.5v, 7.8v which reduces the speed

If you want a greater step change put two diodes in series for each step.


I appreciate your feedback. Thank you.

I tested the motor with an old 25k ohm rheostat (pot) that I had laying around. Using the center pin and pin #3, it varied the RPMs quite nicely. I let it run for a couple of minutes while at low speed and then many speeds. It seemed to be just fine. No heat at all that I could detect.

However, I need one that has a on/off switch in it. I have a very limited amount of space (the size of a 2" cube) in which to install a controller. Pretty sure I am going to need a pot of some kind, but I have read they may cause motor overheating at lower speed settings.

I have my eyes on a Radio Shack 10K-Ohm Audio Control Potentiometer with SPST Switch.

If I can't keep it really simple, I will just install an on/off switch.

25K ? Really ? Did you try starting it at minimum speed too ?

Using a pot won't generally work. You really need a little fan controller, like this one I found on Ebay