0-±15V regulated power supply made with LM317 and UA741

This article introduces a tracking voltage-stabilized power supply that can be adjusted from 0. Since it uses LM317 and LM337 three-terminal adjustable integrated voltage regulators, it has perfect functions such as internal current limiting, short circuit and thermal protection. Its uniqueness: it can achieve "synchronous" adjustment of positive and negative voltages with only a single-connected potentiometer. It has the characteristics of simple circuit, convenient adjustment, excellent performance and low cost, and is suitable for radio enthusiasts to do experiments.

Working principle: The whole circuit is shown in the figure. The power input part is a common transformer step-down and bridge rectifier, and the capacitor filtering is increased to obtain a ±22V DC voltage that is symmetrical up and down. Two other groups of ±6.8V auxiliary voltages are also derived, which are connected to the V+ and V- terminals of op amp IC4 and op amp IC3 respectively to ensure that the operating voltage of IC3 and IC4 does not exceed the limit range. The voltage stabilization part is described in detail below:

1. The positive output circuit is composed of the voltage regulator IC1 and related components. The potentiometer is usually connected to the adjustment end of the voltage regulator IC1 and the other end is grounded. If the RP1 resistor is adjusted to 0, the output voltage Vout = 1.2V, that is, the internal reference voltage of the circuit is 1.2V, and a 10mA constant current is generated on the resistor R3. The output voltage can be changed by simply changing the resistance value of RP1. Here, the ground terminal of RP1 is connected to the output terminal of the op amp IC3, and the output voltage of IC3 is made to -1.2V to offset the reference voltage of IC1 +1.2V, so that the adjustment can be achieved from 0. It is also very simple to achieve the above purpose. Just connect the op amp IC3 into a differential amplifier and perform subtraction operations. As can be seen from the figure, the voltage at the non-inverting input terminal is V1, and the voltage at the inverting input terminal is V2. Since R4=R5=R6=R7, the output voltage of IC3 is VO=R5/R4×(V1-V2)=-1.2V. It can also be obtained that the output voltage of the regulator IC1 is +Vout=5mA×R3+10mA×PR1-1.2V.

2. The negative output circuit is composed of the voltage regulator IC2 and related components. The potentiometer RP3 originally set on the adjustment end of IC2 is omitted here, and it is connected to the output end of the op amp IC4 instead. The adjustment end is controlled by the output voltage of IC4, which can also achieve the purpose of adjusting the output voltage of the voltage regulator. Since the op amp IC4 is connected as an inverting amplifier with a gain of 1, and its inverting input end is connected to the output end of the voltage regulator of the positive output circuit, the negative output voltage regulator generates a regulated voltage with opposite polarity and equal amplitude, that is, -Vout=-R10/R9×(+Vout). Because R9=R10, -Vout＝＋Vout, that is, the negative output voltage tracks the positive output voltage.

3. The diodes D7 and D8 in the circuit are used to prevent the external load from increasing and discharging the capacitor, which would damage the output terminals of IC1 and IC3. In addition, the diodes D9 and D10 are used to prevent the positive saturation of IC3 output and the negative saturation of IC4 output for some reason, which would break down the adjustment terminals of IC1 and IC2, because the adjustment terminals of IC1 and IC2 are not allowed to flow with reverse current.

Component selection: All components in this machine are universal, without special specifications. IC1 and IC2 are three-terminal adjustable integrated voltage regulators, the positive output model is LM317, and the negative output model is LM337, both are TO-220 packages, available on the market, and a heat sink should be installed when the machine is installed. IC3 and IC4 are universal operational amplifiers, which can also be replaced by OP-07. All resistors are 1/4W metal film resistors, among which R4, R5, R6, R7, R9, and R10 are required to have a precision of 1%. RP1 should use a wire-wound potentiometer. If conditions permit, a multi-turn potentiometer is more ideal. The power transformer T can be replaced by a 14-inch black-and-white TV power transformer. If it is self-wound, EI type high-yield steel sheet should be selected, and the power is 35W-45W.

Assembly and use: As long as the welding is correct, the machine can be assembled without debugging. If the voltage upper limit is inaccurate, a 33k resistor can be connected in parallel to RP1 to correct it to 15V. In order to improve the assembly accuracy, three points of printed circuit board design precautions are proposed: 1. The left side is input and the right side is output; 2. The ground wire should be as large as possible, and the power supply three-limit lead wire should also be relatively thick; 3. The reference source 10mA wiring and the positive output and negative output high current wires should not share a multi-strand wire. When using, a voltmeter should be installed on the panel to indicate the voltage reading. Carefully rotate RP1 to achieve satisfactory results in the range of 0-±15V.