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Small DC Motor Drive Circuit

This page will analytics several small DC motor drive circuits in automatic control and micro motor drive circuit.

Several small DC motor drive circuits in automatic control

In automatic control, computer control has always been the focus of attention, but the implementation of control has to be realized by electronic controllers, in which motor drive is the most common problem. The DC motor drive circuit collections given in this article are quite intuitive, but they have their own characteristics and can be used for different control needs.

DC motor drive is relatively simple, can be driven by relay or power transistor, or can be driven by thyristor or power MOS FET. In order to adapt to different control requirements (such as motor operating current, voltage, motor speed regulation, DC motor forward and reverse control, etc.), several circuits are described below to meet these requirements.

figure 1 darlington transistor to amplify the motor drive rurrent
figure 1 darlington transistor to amplify the motor drive rurrent

The circuit of Figure 1 utilizes a Darlington transistor to amplify the motor drive current. The illustrated circuit extends the 5A of the BG1 to the 30A of the Linton composite tube, and the input can be controlled with a low power logic level.

figure 2
figure 2

The driving mode of the above circuit belongs to the traditional single arm drive, which can only make the motor run in one direction, and the double arm bridge push-pull drive can make the control more flexible. Figure 2 is a bridge drive circuit controlled by a single terminal logic input, which controls the forward and reverse operation of the motor. Another feature of this circuit is that the control power supply and the motor drive power supply can be separated, so it can better adapt to the voltage requirements of the motor.

figure 3
figure 3

Figure 3 is also a single-ended positive and negative level drive bridge circuit, which is powered by a dual DC power supply, which is actually a combination of two inverting one-arm drive circuits. Figure 3 also controls the forward and reverse rotation of the motor.

The circuit of Figure 4 drives the motor in reverse rotation based on the Darlington tube. It consists of two parts that are completely symmetrical. When one of the two input terminals A and B is at the 髙 level and the other end is at the low level, the motor rotates forward or reverse; when both input terminals are high or low, the motor stops; if pulse width modulation is used, The speed of the motor can be controlled, so Figure 4 has four combined input states, and the motor can generate five operating states. Here, the addition of the clamp diodes D1 and D2 plays an important role, so that the Darlington tubes BG2 and BG3 are not out of control, which is safer when operated at high power. Another feature of this circuit is that the level of the input control logic is independent of the DC operating voltage of the motor and can be reliably controlled with TTL standard levels.

figure 4
figure 4

Compared with FIG. 4, the bridge driving circuit of FIG. 5 is more interesting, one of which is to trigger the motor operation with a low level; the second control terminals A and B have a trigger locking function; and the third has multiple protections, such as D1. , D2 trigger lock, D3 – D6 power tube collector protection. Therefore, this circuit has only three input states, and the motor still has five working states. The role of D1 and D2 is: If A is low level, BG1, BG2, and BG5 are turned on, and the 髙 level of BG2 collector will block the input of B terminal through D2 to ensure BG6 is cut off. If this circuit is triggered by TTL circuit, it must be Select the collector open circuit.

Because the motor has low requirements for power supply stability, the driving circuit of Figure 6 is an AC power supply scheme. After the AC power is rectified by the full bridge, the MOS field effect transistors Q1, Q2, R3 and C1 used in parallel are used for filtering; The flow diode D is used to prevent damage to the Q1 and Q2 by the high voltage.

figure 5
figure 5
figure 6
figure 6

Figure 7 uses the rectification characteristics of the thyristor to drive the DC motor. This circuit is only suitable for low-power motor speed regulation. The filter network of R2 and C3 can absorb the back-EMF protection of the motor to protect the SCR, C2 and L filters, which can restrain the grid. interference.

There are many cases of using integrated circuit to drive the motor, which is different from the general three terminal voltage regulator. Figure 8 circuit enables the motor to obtain the driving voltage from 0V to 7V, so it has the low-voltage speed regulation performance. IC1 is a fixed voltage regulator with positive output, IC2 is a four terminal voltage regulator with adjustable negative output, and adjusting R1 can make the motor obtain zero voltage. Because the inner part of the heat sink of IC2 is phase with the input end Therefore, IC1 and   IC2 can use public radiator to adapt to low-pressure operation.

figure 7
figure 7
figure 8
figure 8

Figure 9 uses a power-type op amp drive motor, which is a bridge drive circuit. The control signal is obtained from the Wheatstone bridge arm consisting of R1, R2, RP1, and RP2. If RP2 is used for signal detection, the motor feeds back RP1. Tracking adjustment can realize error proportional control. Here LM378 can provide driving current up to 1A. This circuit has a wide range of applications in servo systems.

figure 9
figure 9

Analysis of several micro motor drive circuits

The circuit described below is used for the driving of a 3V powered micro DC motor. This motor has two leads, replacing the polarity of the two leads and commutating the motor. This drive circuit requires positive and negative rotation and stop control.

Circuit one:

As shown in the figure below, this circuit is the circuit originally designed by the author. P1.3, P2.2 and P2.4 are the IO pins of the 51 MCU, respectively. The working principle of the design is: When P1.3 high level, P2.2 and P2.4 are both low level, the motor rotates forward. At this time, Q1 and Q4 are turned on, Q2 and Q3 are turned off, and the current is directed to +5VàR1àQ1àMàQ4; when P1.3 is low, P2.2 and P2.4 are both high, the motor is reversed. At this time, Q2 and Q3 are turned on, and Q1 and Q4 are turned off. When P2.2 is high and P2.4 is low, the circuit is completely disabled and the motor stops.

In the figure, the resistance is: R1=20Ω, R2=R3=R4=510Ω

circuit 1
circuit 1

However, the actual experimental situation is unexpected, that is, the motor does not turn forward and reverse. After measurement, when P1.3 is high, P2.2 and P2.4 are both low, Q4 is turned on, but Q1 is not turned on, and the level of P1.3 is only about 0.67V, so Q1 cannot be turned on.

The reasons for the analysis are as follows: 51 P1, P2, P3 pins are internally pulled through the resistor, connected to the ground MOSFET, the so-called high level, is the MOSFET cut-off, the pin pull-up resistor is pulled high. If the internal pull-up resistor is large, such as 20K, when the circuit above is connected, the current flowing through the b-pole of Q1 is (5-0.7)/20mA=0.22mA, and it is difficult to turn Q1 on. So this circuit is not working.

Summary: The pin pull-up capability of the 51 MCU is weak, which is not enough to drive the triode to conduct.

Circuit two:

As shown in the figure below: The four transistors in this circuit are all PNP type. In this way, the conduction drive is the control pin output low level, while the 51 low level is grounded through the MOSFET, so the pull-down capability is extremely strong.

However, Q1 and Q3 of this circuit need to be separately controlled, and more control pins are required. If you want to use an IO pin control, you can add an inverter.

However, Q1 and Q3 of this circuit need to be separately controlled, and more control pins are required. If you want to use an IO pin control, you can add an inverter. As shown in Figure 3. The measured voltage values ​​at each point are marked in the figure.

circuit 2
circuit 2

Circuit three:

In circuit 2, since the emitters of Q2 and Q4 are 0.7V higher than the base and the base is 0V, the emitter of Q2 and Q4 is not lower than the actual MOSFET tube voltage drop inside the CPU pin. 1V, this reduces the effective voltage range across M.

To solve this problem, Q2 and Q4 need to be replaced with NPN tubes. However, the driving of the NPN tube is as shown in circuit 1. The pull-up of the CPU pin is not enough, so it is necessary to add a pull-up resistor, as shown in the figure below.

circuit 3
circuit 3

In the above figure, unlike the circuit one, the two NPN tubes are moved below, and the PNP is above, so that the potential of the collectors of Q1 and Q3 can reach a tube voltage drop (0.3V). This increases the pressure drop range of M.

However, in order to ensure sufficient driving of the NPN tube, P1.3 and P2.2 must be added with pull-up resistors as shown. In the figure, R2, R5, and R6 are indispensable. Therefore, the components of such a circuit are used in a relatively large amount.

Also, R5 should be several times larger than R6, such as 10 times. Thus, when Q1 is turned on, the voltage at P1.3 can be divided so much that Q2 is not turned on. If R5 is too small or 0, then when Q1 is turned on, since the voltage drop at P1.3 is only about 0.7V, Q2 will also be turned on.

After testing, R2, R6, R3, R4 can take 510Ω, and R5 takes 5.1kΩ. The voltages for this value are as follows (R1 is 20 ohms):

U1:4.04        U2:2.99        U3:3.87        U4:4.00        U5:0.06        U7:0.79

Circuit four:

This circuit is modified from the circuit, as shown in Figure 5, which shows the measured voltage values ​​at each point:

circuit 4
circuit 4

The current-limiting resistors in this figure are removed because the circuit designed by the author requires less components. From the circuit analysis, don’t matter, R1 plays the role of total current limiting, and there is a pull-up resistor inside the pin, so that the circuit will not pass too much current.

This circuit allows the motor to run.

However, in the choice of R2, it is more stressful, because the pull-up effect of R2 not only has an effect on Q1, but also on the conduction of Q2. If R2 is selected too small, although it is favorable for the conduction of Q1, it is effective for the conduction of Q2, because the smaller the R2 is, the stronger the pull-up effect is, and the conduction of Q2 is the higher the P1.3 potential. The lower the better, so this is contradictory. That is to say, the conduction condition of Q1 and the conduction condition of Q2 are contradictory.

After experiment, R2 is suitable for 5.1k ohms. It can be seen that although this circuit saves components and CPU pins, the driving capability has a maximum limit. That is, the driving of Q1 and Q2 is mutually constrained, and only a compromise solution is obtained. Otherwise, if one magnification is large, the other will become smaller.

Summary: The above circuits have their own advantages and disadvantages, depending on the application.

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