DC stabilized power supply is an indispensable part of various electronic products. Its quality is directly related to the quality of the instrument and provides energy for the stable operation of the equipment. Therefore, mastering the installation and debugging methods of the regulated power supply plays a very important role in the regulated power supply. This article mainly introduces the circuit structure, assembly steps and debugging methods of the LM317 regulated power supply.
introduction
With the rapid development of integrated circuit technology, the voltage regulator circuit has also rapidly realized integration and become an important part of analog integrated circuits. At present, the most common application in the small power regulated power supply is the LM317 adjustable DC regulated power supply. It is one of the most widely used power supply integrated circuits. It not only has the simplest form of a fixed three-terminal voltage regulator circuit, but also has The output voltage is adjustable. In addition, it has the advantages of wide voltage regulation range, good voltage regulation performance, low noise and high ripple rejection ratio. The LM317 is a three-terminal adjustable positive voltage regulator with an output voltage range of 1.25V to 37V and a maximum load current of 1.5A. This regulator is very easy to use.
1.LM317 circuit diagram and working principle
The adjustable power supply circuit composed of LM317 is shown in Figure 1. The 220V mains is sent to the LM317 pin 3 (input) through the transformer step-down, diode bridge rectification, capacitor C1 filtering, and the second pin outputs a stable DC voltage. The first pin is the adjustment terminal, and the adjustment terminal and the output terminal are the 1.25V reference voltage. The output voltage is adjusted by two external resistors, that is, the output voltage U0 can be changed by adjusting the resistance of RW. The output voltage is calculated as U0=1.25&mes; (1+RW/R1).
C1: filter left and right, C2: suppress self-oscillation; C3: filter to reduce the ripple voltage of the output voltage (ie, the alternating voltage component in the output voltage); C4: filter to reduce the fluctuation in U0 . D5, D6 is mainly to protect the LM317, to prevent the capacitor C3, C4 from discharging to the inside of the integrated block when the input or output is short-circuited to damage the chip; R2, LED is the working indication circuit.
2. Assembly steps
(1) Installation
1Compared with LM317 adjustable DC stabilized power supply schematic diagram 2 and bill of materials, check whether the components are complete, the quality is qualified, and the unqualified components should be replaced in time;
2 Identify and detect diodes, capacitors, transformers, resistors, potentiometers, LM317 are consistent with the schematic and good or bad.
3 The marking direction of the components shall be in accordance with the requirements of the drawings, and the signs on the components can be seen after installation. If the orientation is not indicated on the assembly drawing, the marking should be outward. It is easy to read and read in order from left to right and bottom to top.
4 When installing polar components such as transformers, diodes, and capacitors, be careful not to misplace the polarity.
5 The installation height should meet the requirements of the regulations. Components (R, C, D) of the same specification should be installed at the same height.
6 The installation sequence is generally low first and then high, first light and then heavy, first small and then big, first inside and then outside, first easy and then difficult, first general components and special components. For example, resistors, diodes, and potentiometers should be installed first, then capacitors should be installed, then the regulator should be installed, and finally the transformer should be installed.
7 The distribution of components on the printed board should be as uniform as possible, neatly arranged, and not allowed to be diagonally arranged, three-dimensionally crossed and overlapped. The components and leads of the components must not touch each other. To ensure a safety clearance of about 1 mm, if necessary, sleeves should be placed.
8 installation of some special components, such as LM317 regulator heating components to maintain a certain distance from the circuit board and add heat sink, the heat dissipation area should generally not be less than 10mm2; the installation of larger components such as transformers should take fixed measures (Bundling, sticking, bracket fixing, etc.) to reduce vibration damping.
(2) Welding
1 The soldering iron should be grounded to prevent breakdown of components due to leakage during soldering. Therefore, it is recommended to use white light adjustable soldering iron, the general temperature adjustment is about 350 degrees, the welding time is less than 2 seconds;
2 When welding, keep the solder joints full and shiny, and the solder should not be too much.
3 When soldering, ensure that all inserted components do not move. The soldering time and the amount of tin used should be appropriate for each solder joint. For components with poor heat resistance, tools should be used to assist heat dissipation. Prevent cold welding and mis-welding, and avoid short circuit caused by dragging tin;
4 Post-weld treatment: Cut off the excess lead wires, inspect all solder joints, and repair the defects.
note:
1 Connect the sampling resistors R1 and RW correctly. When soldering the circuit, R1 should be placed as close as possible between the regulation terminal and the output terminal of the regulator. Otherwise, when a large current flows through the output terminal, an additional voltage drop will be generated on the circuit, making the output voltage unstable. . The ground point of the RW should be the same as the ground point at which the load current returns. Therefore, whether the connection of R1 and RW is correct will directly affect the voltage regulation performance.
2 should pay special attention to the polarity of the four rectifier diodes and capacitor C1 can not be reversed. A diode connection error can burn the integrated regulator or even burn the power transformer; if the polarity of capacitor C1 is reversed, the capacitor may burst.
3 The input stage and output stage of the transformer can not be connected incorrectly. The multimeter can be used to measure the resistance. The resistance is large for the input stage and the small resistance is the output stage; the red line of the general transformer is the input stage;
3. Debugging method
Check the circuit diagram carefully, check whether the position of the component is correct, whether the polarity is correct, whether there is leakage soldering, soldering, mis-welding, tinning or short circuit. After all the above points are checked correctly, the next step of inspection and debugging is carried out.
(1) After the radiator is installed reliably, it can be debugged. Before commissioning, check whether the input, output and grounding connections of the regulator are correct. After the connection between the input terminal and the output terminal and the ground is secure, the debugging can be performed after the inspection is correct.
(2) Turn on the power, and the power-on indicator LED is on;
(3) Test the voltage at the input terminal and the output terminal voltage of the LW317 regulator with a voltmeter. The input voltage should be higher than the output voltage. The input voltage is about 40V, which means that the LW317 works normally and the performance parameters can be tested. When RW is adjusted to 0Ω, the output voltage is 1.25V. When RW is adjusted to the maximum, the output voltage is about 37V.
(4) If the output voltage is 0 and the transformer has no abnormal heating, it means that the primary or secondary winding of the power transformer has been disconnected or not connected, or the power supply and bridge rectifier are not connected.
(5) Be sure to follow the safety operation rules when testing. When installing or replacing components, turn off the power supply and find that the ignition, smoke, smell, etc. are abnormal, and then turn off the power in time, and then find the cause.
4 Conclusion
LM317 is a fixed integrated voltage regulator chip. After the improvement of peripheral circuits, it can achieve a wide range of output voltage adjustment, which not only meets the needs of general low-power equipment for DC power supply, but also meets the needs of various comprehensive experiments in teaching. It is one of the ideal devices for the development of comprehensive machine circuits for various types of university science and engineering electronic technology and related majors.
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