S1 S2 S3 S4 Result 1 0 0 1 Motor moves right 0 1 1 0 Motor moves left 0 0 0 0 Motor coasts 1 0 0 0 Motor coasts 0 1 0 0 Motor coasts 0 0 1 0 Motor coasts 0 0 0 1 Motor coasts 0 1 0 1 Motor brakes 1 0 1 0 Motor brakes 1 1 0 0 Short circuit 0 0 1 1 Short circuit 0 1 1 1 Short circuit 1 0 1 1 Short circuit 1 1 0 1 Short circuit 1 1 1 0 Short circuit 1 1 1 1 Short circuit Construction. The following table summarises operation, with S1-S4 corresponding to the diagram above. The two basic states of an H bridge The H-bridge arrangement is generally used to reverse the polarity/direction of the motor, but can also be used to 'brake' the motor, where the motor comes to a sudden stop, as the motor's terminals are shorted, or to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit. This condition is known as shoot-through. The same applies to the switches S3 and S4. Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor. When the switches S1 and S4 (according to the first figure) are closed (and S2 and S3 are open) a positive voltage will be applied across the motor.
An H bridge is built with four switches (solid-state or mechanical). The term H bridge is derived from the typical graphical representation of such a circuit. Structure of an H bridge (highlighted in red) H bridges are available as, or can be built from. › ▲▲▲ L293d Motor Driver Circuit Wikipedia ▲▲▲
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