DIY power supply with current limiting gear and adjustable voltage

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, causing Q4 to trigger conduction, 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 plays an anti-interference role, which can reduce false triggering of the thyristor.

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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.