DC Motor Driving using H BridgeLigo George
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
When switches S1 and S4 are switched on, motor runs in clockwise direction.
When S2 and S3 are switched on, motor runs in anticlockwise direction.
In actual practice these switches are replaced by transistors. Nowadays H Bridge ICs such as L293D and L293 are available.
Transistorized H Bridge
In this circuit all transistors are wired as switches. When the input A is HIGH and B is LOW, transistors Q2, Q3 are turned ON and the motor runs in anti-clockwise direction. When the input B is HIGH and A is LOW transistors Q1, Q4 are turned ON and the motor runs in clockwise 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.