The production of adjustable voltage-stabilized power supply composed of LM317

The figure shows an adjustable voltage regulator integrated circuit composed of the most common LM317, which can be continuously adjusted from 1.25V to 30V. The output current can reach about 4A.

Working principle: The circuit is composed of T2, D5, VW1, R5, R6, C10 and relay K to form an adaptive switching circuit. When the output circuit is lower than 14V, VW1 is cut off due to insufficient breakdown voltage, T2 is also cut off, K is not attracted, and its contact K is in the normal position. The circuit inputs 14V AC. On the contrary, when the output voltage is higher than 14V, VW1 breaks down and conducts, T2 is turned on, relay K is attracted, and 28V AC is connected to the circuit to ensure that the input and output voltage difference will not be greater than 15V. At this time, the typical value of LM317 output current is 2.2A. Two LM317s are used in the circuit, and the output current can be above 4A. Since the parameters of the two LM317s cannot be the same, small resistance resistors R3 and R4 are connected in series at the output end of LM317 to divide the current equally.

The output voltage is adjusted by RP1 and RP2. The purpose of adding transistor T1 is to prevent the sliding end of potentiometer RP1 from having a bad contact, which would cause the LM317 common end to be open to the ground, resulting in a sudden change in output voltage and damage to the power supply and load.

To ensure accurate voltage regulation, the main current loop should be wide enough when designing the circuit board, and copper wires with a thickness of more than 1mm should be soldered and tinned to reduce ripple voltage. C6 and C8 should be as close to the input and output terminals of LM317 as possible, and non-inductive capacitors should be used first. R3 and R4 can be made of manganese wire.

During debugging, adjust RP1 and RP2 so that the relay is energized when the power output is around 14V. Otherwise, replace the voltage regulator diode and try again.