H-bridge drive circuit schematic diagram and enable control and direction logic

The circuit is named after "H-bridge drive circuit" because its shape resembles the letter H. The four transistors form the four vertical legs of H, and the motor is the horizontal bar in H.

1. H-bridge drive circuit

Figure 1 shows a typical DC motor control circuit. The circuit is named after "H-bridge drive circuit" because its shape resembles the letter H. The four transistors form the four vertical legs of H, and the motor is the horizontal bar in H (note: Figure 1 and the following two figures are only schematic diagrams, not complete circuit diagrams, in which the drive circuit of the transistors is not shown).

As shown in the figure, the H-bridge motor drive circuit includes 4 transistors and a motor. For the motor to run, a pair of transistors on the diagonal must be turned on. Depending on the conduction status of different transistor pairs, current may flow through the motor from left to right or from right to left, thereby controlling the steering of the motor.

For the motor to run, a pair of transistors on the diagonal must be conducting. For example, as shown in Figure 2, when the Q1 tube and the Q4 tube are turned on, the current passes through the motor from the positive pole of the power supply through Q1 from left to right, and then returns to the negative pole of the power supply via Q4. As shown by the current arrow in the figure, the current flowing in this direction will drive the motor to rotate clockwise. When transistors Q1 and Q4 are turned on, current will flow through the motor from left to right, thereby driving the motor to rotate in a specific direction (the arrow around the motor indicates clockwise direction).

Figure 3 shows the situation when another pair of transistors Q2 and Q3 are turned on, and the current will flow through the motor from right to left. When transistors Q2 and Q3 are turned on, current will flow through the motor from right to left, driving the motor to rotate in the other direction (the arrow around the motor indicates the counterclockwise direction).

2. Enable control and direction logic

When driving a motor, it is very important to ensure that two transistors on the same side of the H-bridge do not conduct at the same time. If transistors Q1 and Q2 are turned on at the same time, the current will flow from the anode through the two transistors directly back to the cathode. At this time, there is no other load in the circuit except the triode, so the current on the circuit may reach the maximum value (this current is only limited by the power supply performance), or even burn out the triode. Based on the above reasons, in actual driving circuits, hardware circuits are usually used to conveniently control the switching of transistors.

The improved circuit adds 4 AND gates and 2 NOT gates to the basic H-bridge circuit. The four AND gates are connected to the same "enable" conduction signal, so that this signal can control the switch of the entire circuit. The two NOT gates provide a directional input, which can ensure that only one triode can be turned on on the same side leg of the H bridge at any time. (Like the schematic diagram earlier in this section, the circuit diagram shown in Figure 4 is not a complete circuit diagram. In particular, the direct connection between the AND gate and the transistor in the figure will not work properly.)

Using the above method, the operation of the motor only needs to be controlled by three signals: two direction signals and an enable signal. If the DIR-L signal is 0, the DIR-R signal is 1, and the enable signal is 1, then the transistors Q1 and Q4 are turned on, and the current flows through the motor from left to right (as shown in Figure 5); if DIR-L signal changes to 1 and the DIR-R signal changes to 0, then Q2 and Q3 will conduct and the current will flow through the motor in the opposite direction.

In actual use, it is very troublesome to use discrete components to make an H-bridge. Fortunately, there are many packaged H-bridge integrated circuits on the market, which can be used after connecting the power supply, motor and control signals. At the rated voltage and current Very convenient and reliable to use inside. For example, the commonly used L293D, L298N, TA7257P, SN754410, etc.

H-bridge drive circuit with two discrete components: