Driving Motors

I dug out a couple of small motors to try to drive. I have a 4 channel motor driver, L293D too. Micro controllers (MCU) cannot power a motor directly, the motor draws far too much current and would quickly destroy a micro controller in a puff of acrid smoke. Some sort of interface is needed that is controlled by the output of a MCU and switches the current needed by a motor. Motors come in a variety of voltages too, again the interface could handle this. There are a few options for the interface. One is to build your own using transistors to handle the difference in current and voltage but another is to use a dedicated chip, such as the L293D.

The L393D allows 2 motors to be connected, which can operate at up to 36V. They can each draw up to 600mA, which is not very much. There are motor drivers available that handle much bigger currents, but 600mA should do for now. The controller allows the motor to be driven in either direction too, which is useful.

The L293D has three control connections for each motor: enable, A and B. Enable lets the motor run, so if it is low the motor will not run. A and B determine the way the motor turns, if A or B is high and the other is low the motor will run in one direction depending which is high and which way round the motor is connected. If A and B are both low or both high the motor is braked to a halt, whereas if enable is made low when A or B (not both) is high then the motor coasts to a halt.

The chip has two supply pins, one logic supply should be the same as the micro controller and one to supply the motors. Keeping the two supplies separate is important, not only to prevent damage to the micro controller, but also because motors are very electrically noisy beasts. Some capacitors are often required across the motor connections to smooth out the spikes.

The L293D can respond to switching on and off at up to about 5kHz. If the enable pin is switched on and off quickly this will give the motor bursts of power with coasting in between. By varying the length of the pulse and the pause in between the speed the motor runs at can be controlled. This is ideal for Pulse Width Modulation from a MCU.