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Control Motors with the L293D or SN754410 H-Bridge driver

To turn a DC Motor in both directions uisng a microcontroller usually a H-Bridge is used. Already inegrated H-Bridges are the L293D and the pin compatble SN754410. A controller is normally not able to support the high current that is needed by a motor, So signals have to be amplified by transistor drivers. A H-Bridge is built by 4 transistors. The principle is shown on the following picture

Direction is controlled by switching the different transistors. Two transistors build a half bridge. Current flow is either on the red or the blue path. This curcuit isnt protected against shorts in any way. The integrated circuits have some more features. There are two complete H-Bridges in them and every bridge has an enable signal. By activating Enable, the signals on the inputs are switsched to the transistos of the H-Bridge. So you can put a PWM signal on the enable signals to control the speed of the motors. Without the enable signals you would need two PWM signals for each motor, which are synchror. Interfacing a motor to a microcontroller is shown at the following picture

The two enable signals can be used with normal digital outputs if you do not plan to change the speed of the motors. One important thing that logic supply ( VSS2 ) and motor supply ( VSS1 ) must be based on he same ground potential . So you have to connect ground from the controller to ground form the motor. If you use PWM take care of maximum freuencys. The L293D works up to 5 KHz and the SN754410 seems to work up to 10 KHz.


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