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2022.05.25 motor

Driving Brushed DC Motors with an
H-Bridge: Principles

Brushed DC Motor

From this article, we explain driving of brushed DC motors using H-bridge circuits. Here, before entering into the specifics of driving methods, we begin by explaining the principles of H-bridge circuits.

About H-Bridge Circuits

A brushed DC motor essentially has two power supply terminals, and the motor is driven by applying a voltage to the two terminals. The following are the four possible combinations for connections of these terminals (there are no other possible combinations).

  • ① Neither of the two terminals are connected to anything. (This is the same as when one is connected but the other is not)
  • ② One terminal is connected to the positive (+) terminal of a DC power supply, the other is connected to the negative (-) terminal.
  • ③ The motor is connected to the DC power supply with the polarity opposite that of ②.
  • ④ The two terminals are connected to each other.

These four states can be realized by the circuit shown below, which uses four switches. Because the shape of the circuit resembles the capital letter "H", it is called an H-bridge circuit (or a full-bridge circuit).

The state ① in which the power supply terminals are not connected to anything is obtained by turning off all of the switches SW1 to SW4.

In order to obtain ②, in which one terminal is connected to the DC power supply positive (+) terminal and the other to the negative (-) terminal, switches SW1 and SW4 are turned on, and switches SW2 and SW3 are turned off. In this state, the motor rotates in a certain direction.

The connections for ③ are the opposite of those for ②. Switches SW1 and SW4 are turned off, while SW2 and SW3 are turned on. In this state, the motor rotates in the direction opposite of that for ②.

In ④, the power supply terminals are connected together. When switches SW1 and SW3 are turned off and switches SW2 and SW4 are turned on, the terminals are connected to each other.

Actual H-Bridge Circuit Example

In the above diagram, switches are used to facilitate an easy-to-understand explanation of combinations of connections. But in an actual electronic circuit, power semiconductor transistors are used for the four switches. The configuration shown on the right is an actual H-bridge circuit. In this circuit, P channel MOSFETs are used for the transistors (Q1, Q3) on the positive (+) side of the power supply, and N channel MOSFETs are used for the transistors (Q2, Q4) on the negative (-) side.

Example of an Actual Driving Circuit Configuration

In order to change the rotation rate of a motor, the voltage applied to the motor must be changed. To this end, a voltage control circuit is inserted between the power supply and the motor, to directly or indirectly control the voltage applied to the motor. There are methods in which the power supply negative (-) side is directly connected to the motor, and methods in which the negative (-) side is also controlled with the control circuit inserted. Actual driving circuits are configured by combining an H-bridge circuit, which changes the connections, and a voltage control circuit.

Key Points:

・An H-bridge circuit is a circuit for changing the state of connection of a motor.

・Four switches (transistors) are used to switch between connection states.

・An actual brushed DC motor driving circuit combines an H-bridge circuit and a voltage control circuit.

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