LM317T application circuit

When using LM317T to make an adjustable regulated power supply, poor contact of the potentiometer often causes the output voltage to rise and burn the load. If a transistor is added (as shown in the figure below), under normal circumstances, the base potential of T1 is 0 and T1 is turned off, which has no effect on the circuit; but when W1 has poor contact, the base potential of T1 rises. When it rises to At 0.7V, T1 is turned on, which reduces the voltage of the adjustment terminal of LM317T and the output voltage, thus protecting the load. If the transistor is removed and the center point of W1 is disconnected, the 3.8V small bead will be burned immediately, and the measured output voltage is as high as 21V. When T1 is added, the brightness of the small beads decreases. At this time, the output voltage of LM317T is only 2V, thus effectively protecting the load.