LM317T voltage regulation adjustable circuit diagram explanation

LM317T voltage regulation adjustable circuit diagram (I)

When using LM317T to make an adjustable voltage-stabilized power supply, the output voltage often increases and burns the load due to poor contact of the potentiometer. If a transistor is added (as shown in the figure below), under normal circumstances, the base potential of T1 is 0, T1 is cut off, and there is no effect on the circuit; when W1 has poor contact, the base potential of T1 rises, and when it rises to 0.7V, T1 is turned on, the adjustment terminal voltage of LM317T is reduced, and the output voltage is also reduced, thereby protecting the load. If the transistor is removed and the connection line of the center point of W1 is disconnected, the 3.8V small electric bead will burn out immediately, and the measured output voltage is as high as 21V. When T1 is added, the brightness of the small electric bead decreases, and the output voltage of LM317T is only 2V, which effectively protects the load.

LM317T voltage regulation adjustable circuit diagram (II)

LM317T voltage regulator with adjustable current limiting protection circuit is composed of three-terminal adjustable voltage regulator integrated circuit LM317 as the core. The maximum output voltage is about 20V, the maximum output current can reach 2A, and it has three current limiting gears of 200mA, 300mA, and 600mA and one pass-through gear. It has output indication and over-current limit indication, which is easy to use and can meet the general experimental maintenance needs of electronic enthusiasts.

Since the maximum output current of LM317 is 1.5A, and the power consumption is large when the voltage difference between the input and output terminals is too large, the high-power transistor Q1 is used to expand the output current. RP is a wire-wound potentiometer that can accurately adjust the output voltage. Q2 is set to avoid the output voltage being higher than the set voltage when the RP contact is poor. In general, Q2 is cut off. Once the RP contact is open, Q2 is turned on by the bias voltage provided by RP, causing the adjustment terminal voltage to drop, thereby lowering the output voltage.

R1, R2, R3, Q3 and K2 form a current range detection circuit. When the voltage drop caused by the load current on the resistor R1, R2 or R3 reaches 0.3V, Q3 is turned on, Q4 is triggered to turn on, JK is energized, the output is cut off, LED2 is off, and LED1 becomes bright, indicating that it is in the overcurrent limit state. Pressing K1 can restore the normal output state. C6 at the G pole of the thyristor plays an anti-interference role, which can reduce the false triggering of the thyristor. In addition to indicating the working status, LED2 is also the load of the power supply when it is unloaded, so that the output voltage is not much different when it is loaded and unloaded. 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. It can be used after assembly without debugging. It should be noted that Q1 should be a high-power transistor and equipped with a heat sink. The rectifier bridge D1 should be greater than 3A. LED1 and LED2 use light-emitting diodes of different colors. The resistance values ​​of R1, R2, and R3 can be determined according to your needs. The switch K2 should have good contact, otherwise it will affect the use.