Stepper motors, also known as pulse motors or step motors, are among the earliest and most typical mechatronic components. For example, in mechanical devices, lead screws can convert angles into linear displacement, and stepper motors can drive potentiometers to adjust voltage or current, thereby controlling actuators. Stepper motors can directly receive digital signals without analog-to-digital conversion, making them very convenient to use. They are widely used in valve control, CNC machine tools, plotters, printers, and optical instruments. Stepper motors and stepper motor drivers constitute two inseparable parts of a stepper motor system. This article introduces the design of a practical three-phase reactive stepper motor drive circuit. 1. Application Device Introduction 1.1 PMM8713 Chip The PMM8713 is a pulse distributor (also known as a logic converter) for stepper motor control, manufactured by Sanyo Electric Co., Ltd. of Japan. It is a 16-pin dual-in-line package (DIP) monolithic CMOS integrated chip. The PMM8713 can be used for both 3-phase and 4-phase control. The PMM8713 offers three excitation modes: 1-phase, 2-phase, and 1-2 phase, allowing for selection of any excitation mode through circuit design. Furthermore, it features single-clock or dual-clock operation, forward/reverse control, and initialization/reset functions. Internally, it includes circuits for clock selection, excitation mode control, reversible ring counting, and excitation mode determination. Because all inputs of the PMM8713 utilize Schmitt triggering circuits, it exhibits strong anti-interference capabilities. With an output current greater than 20 mA, it can directly drive a micro stepper motor. The logic block diagram is shown in Figure 1. 1.2 LM331 Chip The LM331 is a high-performance, cost-effective integrated circuit manufactured by NS Microsystems. The LM331 can be used as a precision frequency-to-voltage (F/V) converter, A/D converter, linear frequency modulator/demodulator, long-time integrator, and other related devices. The LM331 is an 8-pin dual-in-line package (DIP) chip, and its logic block diagram is shown in Figure 2. The LM331 internally includes an input comparator circuit, a timing comparator circuit, an RS trigger circuit, a zero-reset transistor, an output driver transistor, a bandgap reference circuit, a precision current source circuit, a current switch, and an output protection circuit. The output transistor uses an open-collector configuration, allowing for flexible changes in the logic level of the output pulse by selecting the logic current and external resistors, thus adapting to different logic circuits such as TTL, DTL, and CMOS. Furthermore, the LM331 can be powered by a single/dual power supply, with a voltage range of 4–40 V and an output voltage as high as 40 V. 1.3 Voltage-Frequency Conversion The LM331 has a simple external circuit; only a few external components are needed to easily construct a voltage/frequency (V/F) or frequency/voltage (F/V) conversion circuit. This paper selects the voltage/frequency (V/F) conversion function of the LM331. The structure is shown in Figure 3. The external resistors and capacitors Rt and Ct, along with the internal circuitry, constitute a monostable circuit. When a positive voltage is input to the input terminal Vi+, Vi+ is greater than Vi-, the input comparator outputs a high level, the RS flip-flop is set, and the high output level turns on the output driver transistor, causing pin 3 (f0) to output a logic low level. Simultaneously, the current source IR charges the capacitor CL. Since the base of the reset transistor is connected to the inverting output of the RS flip-flop, the reset transistor is cut off, and the power supply Vcc charges the capacitor Ct through resistor Rt. When Uct is greater than 2/3 of Vcc, the timing comparator input (pin 5) is positive, thus outputting a logic high level to the reset terminal of the RS flip-flop, resetting it. The non-inverting output of the RS flip-flop outputs a low level, turning off the output driver transistor, and Vdd, through pull-up resistor R0, causes pin 3 (f0) of the LM331 to output a logic high level. At this time, the inverting output of the RS flip-flop outputs a high level, turning on the reset transistor, and the capacitor Ct discharges to ground through the reset transistor. The current switch is switched to the left, and the capacitor CL discharges to ground through resistor RL. When the discharge voltage of capacitor CL equals the voltage Vi at the positive input terminal of the input comparator, the input comparator outputs a high level again, setting the RS flip-flop, turning on the output driver transistor, and causing f0 to output a logic low level. This cycle repeats, thus outputting a pulse signal of a certain frequency at the f0 terminal. Based on the principle of charge balance on the capacitor and related electrical knowledge, let the charging time of the capacitor be t1 and the discharging time be t2. From C=Q/U, I=Q/t, Q_discharge=Q_charge, we can get I_discharget₂=I_charget<sub>1→t₂UL/RL=（IR-UL/RL）t<sub>1→（t<sub>1+t₂）=（IRt_1RL）/UL; and f=1/T, where T=t<sub>1+t₂, so: f₀=1/（t<sub>1+t₂）=UL/（IRt_1RL） UL is the voltage across capacitor C. Since UL fluctuates within a range of approximately 10 mV, UL=Vi, hence: f₀=Vi/（IRt_1RL） (1) From equation (1), it can be seen that the output frequency f₀ of LM331 is proportional to the input voltage Vi, thus realizing the conversion between input voltage and output frequency. t₁ is determined by the external timing elements Rt and Ct, and their relationship is t₁=1.1RtCt, so the values ​​of Rt and Ct can be selected accordingly according to the requirements of the circuit design. Provided by an internal precision current source. IR=1.9 V/Rs. Equation (1) can be changed to f0= ViRs/(2.09RLRtCt) (2) The input resistor Ri makes the bias current of pin 7 cancel the influence of the bias current of pin 6, thereby reducing the frequency deviation. Rs is an adjustable resistor, which is used to adjust the gain deviation of LM331. Ci is a filter capacitor, generally 0.01～0.1 uF. When the filtering effect is good, a 1uF capacitor can be used. When the RC time constants of pins 6 and 7 are matched, the step change of the input voltage will cause the step change of the output frequency. In order to improve accuracy and stability, low temperature coefficient devices are selected for the resistor and capacitor components. 2 Driver Circuit Design The driver circuit is shown in Figure 4. The external resistor Rt and capacitor Ct, internal timing comparator, zero-reset transistor, RS flip-flop, etc. constitute a monostable circuit. When the voltage of the input terminal Vi+ is greater than the voltage of the input terminal Vi-, f0 outputs a logic low level. At the same time, the current source IR charges the capacitor CL. The power supply Vcc also charges capacitor Ct through resistor Rt. When the charging voltage across capacitor Ct is greater than 2/3 of Vcc, the output terminal f0 outputs a logic high level. The f0 signal is output to the clock terminal of the PMM8713 chip. After processing by the PMM8713, this frequency outputs a certain frequency drive signal at pins A, B, and C to control the conduction time of the power transistor, thereby controlling the speed of the stepper motor. The direction control circuit is composed of an LM348 four-circuit general-purpose operational amplifier. The external direction control signal forms a voltage comparator circuit through the LM348 and the reference voltage. When Vdi is greater than the reference voltage VH, the output of U3A is positive, connected to pin 4 of the PMM8713, controlling the output terminal to output a positive phase pulse sequence. When Vdi is less than the reference voltage VH, the output terminal is negative, connected to pin 4 of the PMM8713, controlling the output terminal to output a negative phase pulse sequence. The corresponding phase drive output terminal outputs positive and negative pulse sequences, thereby controlling the forward and reverse rotation of the stepper motor. The input instructions from the LM331 are the input clock f0 and the direction instruction DIR. These two instructions are converted into the timing logic signals for the on/off states of each phase in the PMM8713 through logic combination. The drive current of the phase drive output terminals (PIN10~PIN13) of the PMM8713 is over 20 mA, which can directly drive a micro stepper motor. R1 and C1 are the automatic initialization circuit at power-on. Within tens of milliseconds of initial power-on, the R terminal is at a low level, thus the A~D terminals are automatically reset to the initial state. If the external stepper motor has a large power, the drive capability of the PMM8713 output drive terminal is insufficient. In this case, a power amplifier drive circuit should be designed before driving the stepper motor. The conduction sequence logic signals of each phase output terminal of the PMM8713 are sent to the power drive section and converted into the base (or gate) drive signals of the internal power switch. Stepper motor drive methods can be divided into unipolar and bipolar drives according to whether the current flowing through the phase winding is unidirectional or bidirectional. Usually, three-phase stepper motors use unipolar drives. From the perspective of power drive stage circuits, there are voltage drive and current drive methods. This design uses a series resistor voltage drive method. A resistor of a certain resistance and power is connected in series in the phase winding, which reduces the time constant of the winding circuit and limits the current during low-frequency and static operation. Based on this principle, an automatic gate valve controller was designed. The up and down positions of the gate valve are controlled by limit switches. The corresponding circuit changes the voltage at the comparison voltage input terminal of the LM348 shown in Figure 5 by adjusting the action of the limit switches, thereby controlling whether the stepper motor runs or stops. Its working principle is as follows: The non-inverting input terminal of the LM348 is the reference voltage terminal, and its inverting input terminal is the comparison voltage input terminal. When the voltage at the comparison voltage input terminal is less than the reference voltage, a high level is output on pin 1 of the LM348, turning on BD237, thus enabling the stepper motor to rotate forward or reverse; when the voltage at the comparison voltage input terminal is higher than the reference voltage, a low level is output on pin 1 of the LM348, turning off BD237, and stopping the stepper motor. 3. Conclusion This design represents the main body of a stepper motor driver, featuring a simple structure, low cost, and stable performance. The three-phase reactive stepper motor driver designed using this system drives a 55BF004 type three-phase reactive stepper motor. It has been successfully applied in an automatic gate valve control system with excellent operational results.