# h-bridge circuitry

Posted on Feb 5, 2014

A microprocessor cannot drive a motor directly, since it cannot supply enough current. Instead, there must be some interface circuitry so that the motor power is supplied from another power source and only the control signals derive from the microprocessor. This interface circuitry can be implemented by a circuit known as the H-bridge. An H-bridge

merely consists of 4 switches connected in topology of an H, where the motor terminals form the crossbar of the H. The diagram on the right shows an H-bridge circuit. In an H-bridge, the switches are opened and closed in a manner so as to put a voltage of one polarity across the motor for current to flow through it in one direction or a voltage of the opposite polarity, causing current to flow through the motor in the opposite direction for reverse direction. In the circuit shown on the right, if switches S1 and S4 are closed while switches S2 and S3 are open, current will flow from left to right in the motor, or in other words, positive voltage across the terminals. When switches S2 and S3 are closed and switches S1 and S4 are open, current will flow from right to left, reversing the voltage polarity. If the terminals of the motor are open, the motor will freewheel (vicous braking) and if the terminals are short circuited, the motor will brake(dynamic brake). The switches in the H-bridge circuitry are implemented by power transistors, which can be run of TTL logic. In TTL logic, high means the switch is closed, and low means open. The H-bridge circuitry is being provided in IC chips like L293, L6202. Consider Fig(a). If the switch opens suddenly, then the current wants to go to zero quickly and dI/dt would be a large negative value. Since the motor is an inductor and V=LdI/dt, the voltage across the motor hass a large...

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