In the circuit shown below terminals X and Y are connected to the digital output

Question

In the circuit shown below terminals X and Y are connected to the digital outputs of a microcontroller. Given that the motor runs CW if terminal A is connected to +V and terminal B is connected to ground. a) What is the purpose of this circuit and what is it called? R1 R4 R2 R5 R7 G1 Motor Q3 OC R6 R8 OC G2 b) In the following table, for each set of output values of X and Y, give output of gates Gl and G2, the status of each of transistors Q1, Q2, Q3 and Q4 (saturated or cutoff and the status of motor (turns clockwise, turns counter-clockwise, or does not turn) G1 G2 Motor 0 0

Explanation / Answer

The circuit is called as H-Bridge, it is used for Bi-directional control of DC-Motors.When the transistors and the motor alone are considered the shape of the circuit resembles alphabet 'H'.

The purpose of the circuit is to control the direction of rotation of the DC Motor with digital Inputs.

The input can be of TTL Level or the input can be derived from the circuit containing digital ICs which operate at low voltages or even a microcontroller.

This circuit can be used to control the speed of the motor by employing PWM Technique.

The upper transistors are PNP type and lower transistors are of NPN type.The bridge can be viewed as a two-legged configuration (vertical orientation) each leg containing one PNP and one NPN transistor.This configuration makes it possible to turn ON the desired transistor of a Leg which simultaneously turns OFF the complementary transistor in that leg. This eliminates the chance of short-circuit due to turn ON of both the transistors in a leg.

X Y G1 G2 Q1 Q2 Q3 Q4 Motor 0 0 0 0 Saturated Saturated Cut-Off Cut-Off Does not run. 1 0 1 0 Cut-Off Saturated Saturated Cut-Off Runs Counter-Clockwise. 1 1 0 1 Saturated Cut-Off Cut-Off Saturated Runs Clockwise. 0 1 0 0 Saturated Saturated Cut-Off Cut-Off Does not run.

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