This article introduces the principle of overcurrent protection and details the design method of a DC motor power supply with overcurrent protection. A small-power DC motor power supply designed using the LM317 integrated voltage regulator features a simple circuit structure, high efficiency, low cost, and good output voltage performance, showing promising application prospects. Introduction Currently, various DC power supply products flood the market, and power supply technology is relatively mature. However, for applications where power supply performance requirements are not very high, integrated voltage regulator circuits with overcurrent protection can be used to meet the product requirements, based on cost considerations. Overcurrent protection circuits are an indispensable component of power supply circuits. Based on their control methods, they can be broadly divided into shutdown and current limiting methods, with the shutdown method being more suitable for DC motor power supplies. An overcurrent protection circuit first needs a current sampling stage. A common practice is to obtain the current signal by connecting a small resistor or a Hall element in series. Since Hall elements are relatively large and expensive, the method of connecting a small resistor in series is considered. 1. Working Principle The LM317 voltage regulator circuit with overcurrent protection is shown in Figure 1. Integrated voltage regulator circuits generally consist of five parts: AC step-down circuit, rectifier circuit, filter circuit, voltage regulator circuit, and protection circuit. The 220V AC voltage is stepped down and rectified by the power transformer to obtain a DC voltage Vin. This voltage is then input to the input terminal of the integrated voltage regulator through the filter circuit, resulting in a DC voltage of 1.25–37V at the output terminal. The working principle diagram and voltage waveforms of each part are shown in Figure 2. Figure 1: LM317 voltage regulator circuit with protection Figure 2: Block diagram of DC regulated power supply The working process of the protection circuit is analyzed below. 1.1 Protection of the integrated voltage regulator To obtain a higher output voltage value, the resistance R2 between the adjustment terminal of the LM317 voltage regulator and ground is often relatively large, and its voltage drop is also relatively large. Connecting a capacitor C3 of less than 10μF in parallel across R2 can effectively suppress the output ripple. When a short circuit occurs at the input or output, the discharge of capacitor C3 will generate a surge voltage across R1, which could endanger the reference voltage circuit of the voltage regulator. Therefore, diode D3 needs to be connected in parallel across R1 to protect the voltage regulator. The voltage regulator can operate without a capacitor at the output. However, because the voltage regulator operates with a 1:1 deep negative feedback, it may experience self-oscillation when the output load is capacitive. Therefore, a 0.1μF capacitor C1 is connected at the input, and a 1000μF electrolytic capacitor C5 is connected at the output to provide sufficient current supply, prevent potential self-oscillation, reduce high-frequency noise, and improve the transient response of the load. When a short circuit occurs at the input, C5 discharges through the regulator's regulating transistor. If C5 is large, the surge current during discharge will be significant, and the voltage will discharge through the output transistor inside the voltage regulator, potentially causing reverse breakdown of the output transistor's emitter junction. Therefore, diode D2 is connected in parallel across the voltage regulator. When the input is short-circuited, C5 discharges through D2, protecting the voltage regulator. 1.2 Overcurrent Protection The overcurrent protection circuit principle is shown in Figure 3, where R5 is a small sampling resistor. When the power supply is working, the regulator outputs a positive DC voltage, and the motor starts. Since the instantaneous starting current iout of the DC motor is relatively large (approximately 8 to 10 times the rated current), iout flows through the small resistor R5 and charges C4 through R4. By setting the values ​​of R4 and C4, the charging time Y is made greater than the motor starting time δ, and V2 (9013) is in the off state. After the motor starts to a stable state, the current returns to the operating current. Once the motor experiences a short circuit or stall, causing the voltage across capacitor C4 to reach the conduction voltage of V2, V2 conducts, forcing the regulator's output voltage to drop to the reference voltage of 1.25V. Figure 3 Schematic diagram of the protection circuit 2. Circuit Design 2.1 Selection of Integrated Regulator When selecting an integrated regulator, performance, usability, and price should be considered. Performance indicators are mainly selected based on the magnitude of the load voltage and current, the level of regulation, and the width of the operating stability range. The LM317 series has been widely used due to its adjustable output voltage, high voltage regulation accuracy, high ripple rejection ratio and good output voltage temperature characteristics. Assuming the total output power of the power supply is P0 and the rated load voltage is U0, the rated output current is I0=P0/U0. In order to make the circuit operate stably, a certain design margin (generally more than 10%) needs to be considered. The main parameters of the LM317 series voltage regulator are listed in Table 1. Select the appropriate voltage regulator according to the calculated current value. Table 1 Main members of the LM317 family 2.2 Rectifier and filter circuit design The bridge rectifier and filter circuit needs to determine the rectifier diode and filter capacitor values. (1) Selection of rectifier diode The diode selection should be based on the reverse withstand voltage VRM and forward current IF of the diode. Since the larger the capacitance of the filter capacitor, the smaller the diode conduction angle, and the larger the amplitude of the pulse current through the diode, the amplitude current of the rectifier diode must be considered. The average current flowing through the rectifier diode is ID = Ii/2, where Ii = IR2 + I0, IR2 = IR1 + Iadj ≈ 0.01A (where Ii is the input current of the regulator, and IR1, IR2, and Iadj are the currents flowing through R1, R2, and the adjustment terminal, respectively). Therefore, ID = (0.01 + I0) / 2. Considering the impact of the capacitor charging current, the forward current is generally taken as 2 to 3 times the average current. The maximum reverse voltage of the diode is given by the formula, where U2 is the effective value of the secondary voltage of the power transformer, and Ui is the rectified output voltage (i.e., the regulator input voltage). To ensure the stable operation of the LM317 regulator, the difference between the input voltage Ui and the output voltage U0 is generally in the range of 5 to 15V. Taking Ui - U0 = 10V, we get Udmax = 1.2Ui = 1.2(U0 + 10) = 12 + 1.2U0. A certain margin can be considered in the design. (2) Filter Capacitor Design The selection principle for the filter electrolytic capacitor C1 is: its discharge time constant RLC1 should be greater than 3 to 5 times the charging cycle, and its withstand voltage Uc must be greater than the peak value of the pulsating voltage. For the bridge rectifier circuit, the peak value of the pulsating voltage is 2U2, and the charging cycle of C1 is equal to half of the AC power supply cycle T. That is, RL is the equivalent load resistance after rectification, and RL=Ui/Ii=（10+U0）/（0.0l+I0）. Substituting into the formula, the value of C1 can be determined. 2.3 Power Transformer Design In the series voltage regulator circuit, it is very important to determine the secondary voltage of the transformer. If the secondary voltage is made higher in order to have a margin, it will increase the loss of the regulating tube, and the heat sink will need to be increased accordingly. Therefore, to design a power supply with excellent performance, the parameter values ​​of the transformer often need to be adjusted many times. The relevant literature comprehensively explains the key points of power transformer design, which will not be repeated in this article. Here, the approximate calculation method is used to determine U2 and I2. U2=Ui/1.2=0.83（10+Uo） I2=（1.5～2）Ii=（1.5～2）×（0.01+Io） 2.4 Integrated voltage regulator circuit design To ensure that the voltage regulator can work normally under no-load, the current flowing through resistor R1 cannot be too small. Generally, IR1=5～10mA is taken, so R1=VREF/IR1=1.25/（5～10）×10-3≈120～240Ω, where VREF is the reference voltage of the voltage regulator. The output voltage U0 has the following relationship with VREF, R1, and R2: Un=VREF+（IR1+Iadj）R2=（1+R2/R1）VREF+IadjR2 （1） Adjusting resistor R2 can change the magnitude of the output voltage. Since Iadj is very small (only 50μA), equation (1) can be written as Uo=（1+R2/R1）VREF=1.25（1+R2/R1） (2) From equation (2), we can obtain R2=（0.8U0-1）R1. 2.5 Protection circuit design Source: http://tede.cn The selection of protection diodes in the circuit is relatively simple. As long as the reverse withstand voltage and surge current requirements are met, it is sufficient. The main function of R3 is to limit the base current of the transistor. Generally, it is taken as 1～2kΩ. The design of the overcurrent protection circuit is discussed below. (1) Start-up state When the motor starts, the charging time Υ must be greater than the start-up time δ, and V2 must not be conducted so that the motor can start normally. Since the starting current is very large, it is generally 4～7 times the rated current. It can be regarded as constant and set as I=5I0. According to Figure 4, we can see that. Figure 4 Instantaneous short-circuit response of protection circuit (2) Protection state Assume that the motor load is running under rated conditions and the motor current I0 has stabilized. After a motor short circuit or stall, the current suddenly increases to the short-circuit current Is, and capacitor C4 begins to charge. Considering a certain design margin, the protection current setting value IG is taken. 2.6 Heat Dissipation Design The maximum allowable power consumption of the voltage regulator depends on the highest junction temperature TjM of the chip. After obtaining Rθd, the range of surface area can be obtained by referring to the relevant manual on the relationship between the equivalent thermal resistance of the heat sink and the material thickness and surface area. Table 2 lists the thermal resistance of several commonly used package forms. 3. Experimental Results Under the conditions of AC power supply voltage 220 (1±10%) V, rated output voltage U0=24V, rated power P0=15W, rated current I0=0.625A, motor start time δ=50ms, allowable short-circuit time ts=500ms, protection current setting value Ic=2A, and maximum ambient temperature TAM=+45℃, the circuit parameters are designed as shown in Table 3. After assembly according to the parameters selected in Table 3, the following technical indicators were achieved through experiments: Output characteristics Un=24V, In=0～1A; Voltage stability Sv≤5×10-6; Load stability S1≤5×10-5; Temperature coefficient α≤1×10-5/℃. The motor can start normally, and the overcurrent protection functions normally when the motor is stalled. Table 2 Thermal resistance of several package types Table 3 Design parameter values ​​4. Conclusion In summary, the small-power DC motor power supply designed using the LM317 integrated voltage regulator has a simple circuit structure, high efficiency, low cost, and good output voltage performance, showing great application prospects.