H-Bridge Circuitry and Flyback Diodes

How to make a microprocessor drive a motor?

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).

An H-bridge circuitry

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.

Flyback Diodes

Problems arise when switching inductive loads such as motors.

Fig(a)

Fig(b)

Fig(c) A flyback diode.

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 negative value. So, if Vb in fig(b) is much larger than Va in fig(b), we could have voltage spikes. These voltage spikes can cause the switch to arc over and blow up.

The solution to this problem is to putwhat is known as a flyback diode in the reverse direction across the inductive load, so that the voltage spike will forward bias the diode, creating a return path for the current. Fig(c) shows how a flyback diode is connected.

So, a complete H-bridge configuration will look like this:

Note: V- may be GND and the diodes may be implemented on the chip.

The H-bridge configuration shown above is fine but it looks like we can only spin the motor at one speed. To solve this problem, we have to pulse the H-bridge inputs. This leads to the discussion of Pulse Width Modulation. You can click here to read about details on Pulse Width Modulation.

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