Having trouble with circuit to adjust speed of electric motor

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I'm building a device powered by an electric motor, it needs to travel much slower than the RPM output the motor has... I've gotten into some trouble with my circuit trying slow the thing down, hoping for some suggestions...


at the moment my circuit goes: battery, switch, potentiometer, motor, (back to battery). - All connected in series.

Without the potentiometer there's no problems, but I'm finding the once I put one in it overheats within a few seconds...

it's a 12v, 2.3Ah battery
dont know the switch's specifications but i havent had any problems with it
The motor's specifications I dont know (still in a testing/playing around to see what works phase - I could buy a different motor later if needed)... I took it out of a golf buggy though, so maybe you could make an educated guess about the info you need to help me? :/ sorry...

The potentiometer I tried was 3W, 200Ohm...

Sooo.... What can I change to make the circuit function better?

Any help would be greatly appreciated.

justinreeves

If the resisitance of 200ohms is what you need try using one with a larger value say 50w, if you dont know if you need a lower resistance try like a 35ohm or something.

justinreeves

also thinking about building or buying a PWM circuit to turn your motor, its more efficient.

berkeman

Yeah, I don't think you can effectively slow down your motor by dropping the voltage with a resistor. No wonder stuff is overheating. I believe that PWM is the right way to slow down the motor.

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what's a PWM? :(

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nevermind, i looked it up... thanks guys you've helped a lot, I'm going to replace the potentiometer with this: http://www.dse.com.au/cgi-bin/dse.st...3fc0a87f9c06b4

sound good?

Ouabache

If you're interested in learning more, here is a spiffy application of PWM (pulse width modulation) generated by a PC, used to control the speed of a DC motor (on a fan). Temperature is also sensed by the PC. Combining the two pieces of information, the speed of the motor is changed (increased/decreased) relative to the temperature in the environment, to maintain a constant temperature.

Cliff_J

The radio control market has many different speed controllers for remote control vehicles and your battery is within the range of most of those applications.
http://www2.towerhobbies.com/cgi-bin/wti0093p?&C=MJA

There are also variable resistor style speed controls for the small cars (that want to use an inexpensive method with a standard servo) and these are much closer to a rheostat that is designed to handle current (instead of a potentiometer that is NOT designed to handle current). But a PWM motor driving circuit is the way to go.

If you're linking to K3070 on that site, it'll work to a certain extent but is only good to 1A, and with a 12V battery that's 12W. Not a lot of power.

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was trying to link to K3072... dunno why that didnt work... but anyway, that one is good to 15A, i'd imagine that'll be enough?

Cliff_J

15A should be plenty enough for the size of battery you're running.

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eeeeeeeeeexcellent.

thank you very much

Bob S

The simplest way to generate a variable width PWM (pulse width modulation) signal is with a triangle wave (from an op amp oscillator) going into a second op amp used as a comparator (with potentiometer for voltage level input). FETs can be used for outputs up to about 15 amps, and IGBTs for currents up to 100's of amps.