Help! Potentiometer as motor speed controller question

goodterling · Sep 4, 2012

Hi, I've got a little 3 volt DC motor that I'm trying to speed control. I bought 5 potentiometers at Frys ranging from 5K Ohms to 50 k Ohms hoping one would do the job. They can all control the speed, but only within a tiny fraction of their knob travel distance. So the motor sits still until I get to 95% of a knob turn, then I've got speed control in that 5% of knob travel. Sounds like too much resistance, but all the 5 pots I've tried have different resistances and they all behave the same . Can someone clue me in on what pot would give me a greater range of control? Thanks.

(*steve*) · Sep 5, 2012

Yeah, sure. Potentiometers make really bad speed controllers for motors (as you've discovered).

You would be better off with using one of these potentiometers (and several other components) to vary the duty cycle of a 555 oscillator and use that to operate your motor.

However, at 3V it's going to be a little tricky.

You could run the 555 from 9V and use it to switch a mosfet which connects your motor to the 3V supply.

But there's quite a bit more electronics there than just a pot

CocaCola · Sep 5, 2012

As Steve said it's not the best way to do it, but if you insist on doing it with a pot...

Take your 5K pot, fire up the motor and turn the dial until the motor is as slow as you can get it, or as slow as you will ever use it... Now take a multimeter and measure the resistance of that pot... Take that measured value and get a common pot value as close to that value (round up) as you can... Say you measure 91 Ohms, then get a 100 Ohms pot... If you measure 358 Ohms, get a 500 Ohms pot and so on... This will increase your range of travel on the pot for the speed control... To get even more travel range get a multi-turn pot...

Another hack way to do this if you just want some 'harsh' speed adjustments is to use a series of diodes... You are not going to get much range and the steps will be significant @ 3V max... But, diodes do have some advantages over doing it with resistance in that you will generally experience less 'stalls' at slower start speeds... But, they are still no replacement for what Steve suggested that is a much better solution to speed control...

goodterling · Sep 5, 2012

Thanks for the quick replies. You guys are awesome.
So is it a safe guess that I need to go with a lower resistance pot? These things are just 2 bucks each, and I'll go buy 5 more before I shell out for a voltmeter. If I can get even a quarter of the knob working, I'll call it good. It's just part of a science fare project that I'm trying to get out the door.

iknowvator · Sep 5, 2012

Actually if you are driving the motor with any load then to control your motor speed only with a pot is not very good idea as you will find it practically. The speed variation will not be same. You should do something more then only using a single pot.

CocaCola · Sep 5, 2012

goodterling said:

These things are just 2 bucks each, and I'll go buy 5 more before I shell out for a voltmeter.
Click to expand...

Investing in a tool is never a waste, while 5 pots that you won't use probably is... You can get a cheap multimeter for $5 from Harbor Freight right now and have a tool that you can use to expand your knowledge and help test and fix things down the road...

Help! Potentiometer as motor speed controller question

goodterling

(steve) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

CocaCola

goodterling

iknowvator

CocaCola

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Help! Potentiometer as motor speed controller question

(*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

(steve) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator