The H-Bridge

 An H bridge is an electronic circuit that enables a voltage to be applied across a motor in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. Most DC-to-AC converters (power inverters), most AC/AC converters, the DC-to-DC push–pull converter, most motor controllers, and many other kinds of power electronics use H bridges. In particular, a bipolar stepper motor is almost invariably driven by a motor controller containing two H bridges.Most of the H Bridge Circuits are made using 4 transistors.

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Working of H Bridge 

 

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration. The switching elements are usually bi-polar or FET transistors. In the Figure, above, the two transistors in the left are in complimentary pairs. Similarly the two trasistors in the right are complimentary pair. Complimentary pair means when one turns on, the other turns off, similarly when one turns off, the other turns on. A DC Motor has two terminals. When we apply a potential difference across the two, it will rotate in one direction. If we reverse the polarity, it will rotate in the opposite direction. To reverse a DC motor, you need to be able to reverse the direction of the current in the motor. The easiest way to do this is using an H-bridge circuit. Now lets take an example. Take a look at the Circuit shown below. Here, S1′ is complement of S1 and S2′ is complement of S2. Now what happens when S1 and S2 are in the OFF state (Logic Zero)? S1′ and S2′ will be in the ON state but there wont be any closed path for the current to travel from +ve to gnd. What if S1 is OFF and S2 is ON? S1′ Turns on and S2′ turns off. Since there is a closed path, the current flow from +ve terminal, S2, M, S1′ and to the GND. Here, the motor rotates in a particular direction, say clockwise. When S2 is OFF and S1 is ON? S2′ Turns ON and S1′ turns OFF. Here the current flows from +ve terminal, S1, M, S2′ and to the GND. Here, we noticed that the polarity at the terminals of the motor is reversed and the motor rotates in the opposite direction. The Truthtable S1     S2     S1′     S2′     Motor 1         1       0       0         No Rotation 0         1       1       0         Clockwise 1         0       0       1         Anti Clockwise 0         0       1       1         No Rotation This is how we control the direction of DC motor Using H-Bridge

L293D Dual H-Bridge IC