H-bridge using transistor

This project and circuit finds a huge application in the area of robotics to control motor direction. This circuit uses the basic concept of transistors as a switch. Transistor with proper biasing can be used as switch, i.e. it can be used to toggle between the two states of a switch on or off. This configuration of transistor has been used in H-bridge to drive a motor in both clockwise and anticlockwise direction. This circuit derives its name from the English alphabet “H”, which closely resembles the circuit pictorially.

The description of the circuit based project can be divided into two parts. The first part explains the concepts behind the working transistor as switch and the second part the function and operation of H Bridge.

1. Transistor as switch:-

The circuit diagram in the figure 1 shows the use application of transistor as a switch. When there is no input on the base, the transistor is in the cut off region and acts as an open switch. As a result the LED does not glow. When the base is given an input through a resistance

R1, the transistor gets biased and enters into saturation state, hence behaving like a closed switch. As a result the current starts flowing through the LED and it starts glowing.

2. H-bridge:-

In H-bridge four npn transistors (BC547) are used in the switch mode configuration. The emitter and collector of transistors T1 & T2 and of T3 & T4 are connected. The collector of T1& T3 is connected to VCC and emitter of T2& T4 is connected to ground. The base of T1 & T4 and T2 & T3are connected through resistors. The motor is connected as shown in the figure2. When high signal (logic 1) is given at A and low (logic 0) at B, the transistor T1 & T4 will behave as closed switch andT2 & T3 will behave as open switch. As a result of which the current will start flowing from Vcc to T1 to motor to T4 to ground and motor will start running in one direction.

Similarly if B is given high signal and A low, then the transistors T1& T4 are in the cut off state and T2 and T3 are in the saturation state i.e., T1 & T4behave as open switch and T2 & T3behave as closed switch and motor rotates in other direction. We should avoid giving either high or low to both A & B simultaneously.