Controlling a geared motor

Question

I've used small high speed dc motors by just wiring them to a battery and potentiometer, but i've heard a lot of talk in these forums about motor controllers and h-bridges, and I don't fully understand when they are needed.

My question is: I want to run a geared motor like one of these:

http://www.surpluscenter.com/item.asp?item=5-1571&catname=electric

http://www.hobbyengineering.com/H2013.html

Do I need a motor controller, arduino motor shield, or h-bridge to do so? Or can I just wire up a 6/12 volt power supply and use a pot to control the speed? I realize I won't be able to run it in reverse this way. Would I be able to just use a switch to reverse the polarity and run it forwards/backwards?

Answer 1

You can use a potentiometer, but this will give you greatly reduced torque at lower speeds, and you'll probably need a pot with a high power rating.

You'll have much better results using PWM, increasing the duty cycle will increase the speed. The best implementation of this would be to use a microcontroller with a pot on an analogue input, and a MOSFET to power the motor via an output pin. Program the microcontroller to change the duty cycle depending on the value of the voltage divided by the pot.

You can use a switch to control the direction if you're controlling it manually, but if you want your microcontroller to automatically control direction then you'll need an H bridge.

Answer 2

Potentiometers are terribly inefficient for controlling power loads, since significant power is burned up in the potentiometer itself. Under the right load conditions, most of the power sourced by the power supply will be burnt in the pot.

This doesn't matter too much if you're only dealing with small loads and have a mains supply, and can deal with the heat from the potentiometer. But if your power source is a battery, this inefficiency translates directly to shortened run time between battery charging.