H bridge is simply a circuit that allows a voltage to be applied across a load in either direction. They are commonly used for controlling DC motor in moving parts of robots. The advantage of using DC motor is that, we can reverse the polarity of applied voltage across the load without modifying the circuit.
Before going further, lets talk a little bit about DC motor.
A DC motor has two terminals through which we apply the potential difference. When we apply potential difference across them, it will rotate in a particular direction, say clock wise. When we reverse the direction of the current, it will rotate in the opposite direction, that is anti clockwise direction. In short, if you want to change the direction of the spin of motor, all you have to do is reverse the polarity.
Working of H Bridge
H-bridge is a very small circuit. A simple H Bridge can be build by using any switching element such as BJT or FET. Consider the following circuit.
Here we have 4 transistors of which two of them in either side always acts as a complimentary pair. Complementary pair means when one turns ON the other turns OFF and vice versa.
Now consider various cases
S1 and S2 are Open (Zero)
When S1 and S2 are at logic zero, S1′ and S2′ will be in logic 1 since S1′ is complement of S1 and S2′ is complement of S2.
So overall, the circuit will be open. There wont be any closed path for the current to travel.
When S1 is Open, S1′ will be closed and when S2 is closed, S2′ will be Open.
Now the Current Flows from +V > S2 > M > S1′ > GND . And the motor rotates in one direction (say clockwise).
Case 2 – S2 is Open and S1 is Closed
When S2 is Open, S2′ will be closed and when S1 is closed, S1′ will be Open.
Now the Current Flows from +V > S1 > M > S2′ > GND . And the motor rotates in one direction (anti clockwise).
S1 and S2 are Closed (1)
When S1 and S2 are at logic One, S1′ and S2′ will be in logic Zero since S1′ is complement of S1 and S2′ is complement of S2.
So overall, the circuit will be open. There wont be any closed path for the current to travel. So the motor wont rotate
S1 S2 S1′ S2′ Direction
1 1 0 0 x
0 1 1 0 Clockwise
1 0 0 1 Anti Clockwise
0 0 1 1 x
L293D Dual H-Bridge IC