Power supply circuit that can reduce LM317 ripple

The three-terminal voltage regulator integrated circuit LM317 is a three-terminal voltage regulator integrated circuit that is very familiar to electronics enthusiasts. It has the characteristics of variable output voltage, built-in protection function, small size, high cost performance, stable and reliable operation, etc., and is quite popular among electronics enthusiasts. Electronics enthusiasts often use it to make a voltage regulator with variable output voltage. The circuit mode used is shown in Figure 1. By adjusting the resistance value of the variable resistor R2, a variable output voltage U can be obtained from the output end of 317.

It can be seen from the circuit in Figure 1 that the output voltage of the LM317 power supply circuit [that is, the output voltage of the voltage-stabilized power supply] U. is the sum of two voltages. That is, the voltage between points A and B is the voltage applied to R2, UR2 = IR2XR2, and IR2 is actually the sum of two currents. One is the current IR1 flowing from R1 to R2, and its magnitude is UR1/R1. Because UR1 is a constant voltage of 1.25V and R1 is a fixed resistor, IR1 is a constant current. The other is the current ID flowing out of the adjustment end of 317. Due to different models (such as LM317T, LM317HVH, LM317LD, etc.) and different manufacturers, the values ​​of their IDs are different. Even if the LM317 is from the same manufacturer and the same batch, the current ID flowing out of its adjustment end is different.

The LM317 power supply circuit itself has an input ripple compression of more than 60dB. The following figure can be used to further reduce the ripple and obtain

Satisfactory DC voltage.

The function of capacitor C2 is to reduce the ripple. If the capacitance of C2 is around 10uF, the ripple compression ratio can be increased.

20dB, reducing the output ripple by about 1/10.

The purpose of D1 is to prevent reverse bias of internal components when the output is short-circuited.

The function of C3 is to discharge immediately when the output is short-circuited.