This article introduces a tracking power supply that can be adjusted from 0. Due to the LM317 and LM337 three-terminal adjustable integrated regulator, the internal limited current, short circuit and thermal protection are perfect. Its unique features: It can realize the "synchronous" adjustment of positive and negative voltages with only one single potentiometer. It has the characteristics of simple circuit, convenient adjustment, excellent performance and low cost. It is suitable for radio enthusiasts to do experiments.
Working principle: The whole circuit is shown in the figure. The input part of the power supply is a common transformer buck and bridge rectification, and the capacitor filtering is increased to obtain a vertical and horizontal symmetrical ±22V DC voltage. Two sets of ±6.8V auxiliary voltages are also derived, which are connected to the V+ and V- terminals of the operational amplifier IC4 and the operational amplifier IC3 to ensure that the operating voltage of IC3 and IC4 does not exceed the limit range. The following details the voltage regulation section:
1. The positive output circuit consists of the regulator IC1 and related components, usually connected to the regulation terminal of the regulator IC1. The other end of the potentiometer is grounded. If the RP1 resistor is adjusted to a value of 0, then the output voltage Vout = 1.2V, that is, the internal reference voltage of the circuit is 1.2V, and a constant current of 10mA is generated on the resistor R3. The output voltage can be changed by changing the resistance of RP1. Here, the RP1 ground terminal is connected to the output of the op amp IC3, and the output voltage of IC3 is managed to be -1.2V to offset the reference voltage of IC1 +1.2V, so that the adjustment from 0 can be realized. The purpose is also very simple, as long as the op amp IC3 is connected to a differential amplifier, the subtraction can be performed. 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 The output voltage of the regulator IC1 can also be obtained as +Vout=5mA×R3+10mA×PR1-1.2V.
2. The negative output circuit is composed of the regulator IC2 and related components. The potentiometer RP3 originally set on the IC2 regulation terminal is omitted, and the output terminal of the IC4 output voltage is controlled by the output terminal of the IC4. The purpose of adjusting the output voltage of the regulator is achieved. Since the op amp IC4 is connected to an inverting amplifier with a gain of 1, the inverting input is connected to the output of the regulator of the positive output circuit, so the negative output regulator produces a regulated voltage of opposite magnitude. That is, -Vout=-R10/R9×(+Vout), since 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 the capacitance discharge, which causes the output terminals of IC1 and IC3 to be damaged. In addition, the diodes D9 and D10 prevent the IC3 output from being saturated and the IC4 output is negatively saturated for some reason. And IC2 regulation terminal breakdown, because the regulation terminals of IC1 and IC2 are not allowed to flow in reverse current.
Component selection: This machine is a general-purpose component, no special specifications. Both 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. They are available on the market and should be equipped with a heat sink. IC3 and IC4 are general-purpose op amps, which can also be used by OP-07. All resistors use 1/4W metal film resistors, of which R4, R5, R6, R7, R9, R10 are required to be 1% accurate. Wirewound potentiometers should be used for RP1. The conditional multi-turn potentiometer effect is ideal. The power transformer T can be replaced by a 14-inch black-and-white TV power transformer. For example, the self-winding should be EI-type high-grade steel sheet, and the power can be 35W-45W.
Assembly and use: As long as the welding of the machine is correct, it is generally not necessary to debug and install. If the upper voltage limit is not accurate, a 33k resistor can be connected in parallel with RP1 to correct 15V. In order to improve the assembly accuracy, the design considerations for the three-point printed board are as follows: 1. The input on the left side and the output on the right side; 2. The ground line should be as large as possible, and the power supply limit line should be relatively thicker; The reference source 10mA trace and the positive output, negative output high current line should not share a single strand. When in use, a voltmeter should be installed on the panel to indicate the voltage reading. If you carefully rotate RP1, you can achieve satisfactory results from 0-±15V.
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