Design of nine-speed adjustable DC power supply circuit

　　The nine-speed adjustable DC power supply is mainly composed of a voltage-stabilized three-terminal adjustable integrated circuit (m31) as the core component. The LM317 adjustable terminal is connected with three-stage tube VT, adjustable potentiometer RP and resistors R3 R10, which can meet the nine different power supply voltages required for the output.

During production, transformer T can use the power transformer　　of a 12-inch black-and-white TV . It can also be wound by itself, using GEB22828 silicon steel sheet primary coil with 0.7MM enameled wire winding 158 turns, so that the secondary output voltage of the transformer is 25V AC voltage.

　　The voltage regulator IC uses LM317 voltage regulator integrated circuit. By turning S2, you can get a fixed nine-level DC voltage at the output terminals X3 and X4. If you adjust the potentiometer RP, you can choose any value between 0 and 24V. The values ​​of R3 to R10 in the figure are for reference only.

　　In order to prevent the transfer switch S2 from being temporarily disconnected or having poor contact during use, which would cause the output voltage to be too high and damage the load, a transistor VT is provided here.