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
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.
Transistorized H Bridge
In this circuit all transistors are wired as switches. An NPN transistor (Q3 and Q4) will be ON when we give HIGH to it and a PNP transistor (Q1 and Q2) will be ON when we give LOW to it. So when (A = LOW, B = HIGH, C = LOW, D = HIGH), transistors Q1 & Q4 will be ON and Q2 & Q3 will be OFF, so the motor rotates in clockwise direction. Similarly when (A = HIGH, B = LOW, C = HIGH, D = LOW), transistors Q2 & Q3 will be ON and transistor Q1 & Q4 will be OFF, thus the motor rotates in anticlockwise direction. 1N4148 (D1 ~ D4) 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.