DC Motor Driving using H Bridge

dc motor

DC Motor Driving using H Bridge

H Bridge is a simple electronic circuit which enables us to apply voltage to load in either direction. It is commonly used in robotics application to control DC Motors. By using H Bridge we can run DC Motor in clockwise or anticlockwise directions.  This circuit is also used to produce alternating waveforms in inverters.

Basic H Bridge

Basic H Bridge Circuit

Basic H Bridge Circuit

When switches S1 and S4 are switched on, motor runs in clockwise direction.

Basic H Bridge Working

Basic H Bridge Working

When S2 and S3 are switched on,  motor runs in anticlockwise direction.

Basic H Bridge Working

Basic H Bridge Working

In actual practice these switches are replaced by transistors. Nowadays H Bridge ICs such as L293D and L293 are available.


Transistorized H Bridge

DC Motor Driving using H Bridge

DC Motor Driving using H Bridge

In this circuit all transistors are wired as switches.  When the input A is high transistors Q1, Q4 are turned ON and the motor runs in clockwise direction. When the input B is high transistors Q2, Q3 are turned ON and the motor runs in anticlockwise direction. 1N4148 is uses as freewheeling diode as it is a fast switching diode. It avoids problems due to negative voltage produced by the back emf the dc motor. Resistors R1 – R4 are used to limit the input current of transistors and are designed in such a way that transistor will work as a switch. Transistor should be chosen according to the current requirements of the DC Motor.

Design of Input Resistors

IL = Max. Motor Current
Ib = IL/β, Minimum input current required to flow current IL through the load (active region).
Ib’ = 10xIb, To make sure that transistor works in saturation region.
Rin = (Vin – Vbe)/Ib’
Take Vbe = 0.7V
This circuit is also used in inverters for producing alternate current across the load. In Square wave inverters, square wave is applied to inputs A, B and in sine wave inverters Pulse Width Modulated (PWM) square wave is applied to inputs A, B. Freewheeling diodes D1 – D4 can be avoided for resistive loads.

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