Simple adjustable voltage stabilized power supply circuit with current limiting function

The maximum output voltage of this adjustable regulated power supply is about 20V, and the maximum output current can reach 2A. It has three current limiting gears of 200mA, 300mA, and 600mA and a pass-through gear. It has output indication and overcurrent limit indication. It is easy to use and can meet the needs of general maintenance. 　　The power supply is composed of a three-terminal adjustable voltage stabilizing integrated circuit LM317 as the core. The circuit is shown in Figure 1. Since the maximum output current of LM317 is 1.5A and power consumption occurs when the voltage difference between the input and output terminals is too large, Q1 high-power transistor is used to expand the output current. RP is a wire-wound potentiometer that can accurately adjust the output voltage. Q2 is set to prevent the output voltage from being higher than the set voltage when the RP contact is in poor contact. Under normal circumstances, Q2 is cut off. Once the RP contact is open, the Q2 is turned on through the bias voltage provided by RP, causing the voltage at the adjustment terminal to drop, thereby making the output voltage lower. R1, R2, R3, Q3 and K2 form a current range detection circuit. When the voltage drop generated by the load current on resistor R1, R2 or R3 reaches 0.3V, Q3 turns on, triggering Q4 to turn on, JK pulls in, and the output is cut off, LED2 goes out, and LED1 turns on, indicating that it is the overcurrent limit state at this time. Press K1 to restore the normal output state. C6 of the G pole of the thyristor acts as an anti-interference function, which can reduce false triggering of the thyristor. In addition to indicating the working status, LED2 is also the load when the power supply is no-load, so that the output voltage is not much different between the load and the no-load state. The voltmeter in the circuit can be replaced by a multimeter. The component models and values ​​of the power supply are marked in the figure, and it can be used without debugging after assembly. It should be noted that Q1 should choose a high-power triode and install a heat sink. Rectifier bridge D1 should be greater than 3A. LED1 and LED2 use different colors of light-emitting diodes. The resistance values ​​of R1, R2 and R3 can be determined according to your own needs. The transfer switch K2 should be in good contact, otherwise it will affect the use.