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Basic design methods and principle requirements for printed circuit boards

For an instrument with excellent performance, in addition to selecting high-quality components and reasonable circuits, the component layout of the printed circuit board and the correct structural design of the electrical connection direction are a key issue that determines whether the instrument can work reliably. And parameters of the circuit, due to component layout design and electrical connection direction will produce different results, the results may be very different. Therefore, it is necessary to combine the three aspects of how to correctly design the layout of the printed circuit board components, the correct selection of the wiring direction and the process structure of the overall instrument. A reasonable process structure can not only eliminate the noise interference caused by improper wiring, but also facilitates Installation, commissioning and maintenance in production.

Below we discuss the above issues. Since there is no strict “definition” and “pattern” for good “structure”, the following discussion is only for reference. The structure of each instrument must be based on specific requirements (electrical performance, structural installation of the whole machine and panel layout, etc.), adopt corresponding structural design plans, and compare and repeatedly modify several feasible design plans.

Selection of wiring structure for printed board power supply and ground bus – system structure: analog circuits and digital circuits have many similarities and differences in the design and wiring methods of component layout diagrams. In the analog circuit, due to the existence of the amplifier, the extremely small noise voltage generated by the wiring will cause serious distortion of the output signal. In the digital circuit, the TTL noise tolerance is 0.4V ~ 0.6V, and the CMOS noise tolerance is 0.3 Vcc. ~0.45 times, so the digital circuit has strong anti-interference ability.

Reasonable selection of a good power supply and ground bus is an important guarantee for the reliable operation of the instrument. Quite a number of interference sources are generated through the power supply and ground bus, among which the noise interference caused by the ground wire is the largest.

One basic principle requirements of printed circuit board design

1. The design of the printed circuit board starts from determining the size of the board. The size of the printed circuit board is limited by the size of the chassis shell, so it is appropriate to fit into the shell. Secondly, the printed circuit board and external components (mainly It is the connection method of potentiometer, socket or other printed circuit board). The printed circuit board and external components are generally connected by plastic wires or metal isolation wires. But sometimes it is also designed in the form of a socket. That is, installing a plug-in printed circuit board in the device leaves contact locations that act as sockets.

For larger components mounted on the printed circuit board, metal accessories should be added to fix them to improve the resistance to vibration and shock.

2.The basic method of wiring diagram design

First of all, it is necessary to have a complete understanding of the specifications, dimensions, areas, etc. of the selected components and various sockets; make a reasonable and careful consideration of the location of each component, mainly from the perspective of electromagnetic field compatibility and anti-interference. Consider the short line, less crossover, power supply, ground path and decoupling. After the position of each component is determined, it is the connection of each component, and the relevant pins are connected according to the circuit diagram. There are many ways to complete it. The design of the printed circuit diagram includes two methods: computer-aided design and manual design.

The most primitive is to arrange the layout by hand. This is more labor-intensive, and it often takes several repetitions to finally complete it. This is also possible when there is no other drawing equipment. This manual arrangement and layout method is also very helpful for those who are just learning the printing plate design. Computer-aided drawing, there are many kinds of drawing software, with different functions, but in general, it is more convenient to draw and modify, and it can be saved and printed.

Next, determine the required size of the printed circuit board, and preliminarily determine the position of each component according to the schematic diagram, and then make the layout more reasonable through continuous adjustment. The wiring arrangement between the components in the printed circuit board is as follows:

(1) Cross circuits are not allowed in printed circuits. For lines that may cross, two methods can be used to solve them: “drilling” and “winding”. That is, let a certain lead “drill” through the gap under other resistors, capacitors, and triode pins, or “wrap around” one end of a certain lead that may cross. In special cases, the circuit is very complicated, and it is also necessary to simplify the design. Wire jumpers are allowed to solve cross-circuit problems.

(2) Components such as resistors, Diodes, and tubular capacitors have two installation methods: “vertical” and “horizontal”. The vertical type means that the component body is installed and welded perpendicular to the circuit board, which has the advantage of saving space. The horizontal type means that the component body is parallel and close to the circuit board for installation and welding. The advantage is that the mechanical strength of the component installation is better. For these two different mounting components, the component hole spacing on the printed circuit board is different.

(3) The grounding points of the same-level circuit should be as close as possible, and the power supply filter capacitor of the current-level circuit should also be connected to the grounding point of this level. In particular, the grounding points of the base and emitter of the Transistor at this stage should not be too far apart, otherwise the copper foil between the two grounding points will cause interference and self-excitation due to too long. Stable and not easy to self-excite.

(4) The general ground wire must be arranged in strict accordance with the principle of high frequency-medium frequency-low frequency first-level in the order of weak current to strong current. It must not be randomly connected over and over again. Follow this rule. In particular, the requirements for the grounding wire arrangement of the frequency conversion head, the regeneration head and the frequency modulation head are more strict.

High-frequency circuits such as FM heads often use a large-area surrounding ground wire to ensure a good shielding effect.

(5) The strong current lead (common ground wire, power amplifier power lead, etc.) should be as wide as possible to reduce the wiring resistance and its voltage drop, which can reduce the self-excitation caused by parasitic coupling.

(6) The trace with high impedance should be as short as possible, and the trace with low impedance can be longer, because the trace with high impedance is easy to whistle and absorb signals, causing circuit instability. The power line, the ground line, the base line of the non-feedback component, the emitter lead, etc. are all low-impedance lines. The base line of the emitter follower and the ground lines of the two channels of the tape recorder must be separated and formed into a separate channel. , until the end of the function is combined again, if two ground wires are connected, it is easy to generate crosstalk and reduce the degree of separation.

Two, the following points should be paid attention to in the design of the printed board

1. Wiring direction: From the welding surface, the arrangement of components should be as consistent as possible with the schematic diagram, and the wiring direction should preferably be consistent with the wiring direction of the circuit diagram. This is convenient for inspection, debugging and maintenance in production (Note: It refers to the premise of meeting the circuit performance and the requirements of the whole machine installation and panel layout).

2. The arrangement and distribution of each component should be reasonable and uniform, and strive to be neat, beautiful, and strict in structure.

3. Placement of resistors and diodes: There are two types: flat and vertical:

(1) Flat lay: when the number of circuit components is small and the size of the circuit board is large, it is generally better to use flat lay; for resistors below 1/4W, the distance between the two pads is generally When the resistance of 4/10 inches and 1/2W is placed flat, the distance between the two pads is generally 5/10 inches; when the Diode is placed flat, the 1N400X series rectifier is generally 3/10 inches; the 1N540X series rectifier is generally 3/10 inches. Take 4 to 5/10 of an inch.

(2) Vertical placement: When the number of circuit components is large and the size of the circuit board is not large, vertical placement is generally adopted, and the distance between the two pads is generally 1 to 2/10 inches.

4.Potentiometers: Principles of Placement of IC Blocks

(1) Potentiometer: It is used to adjust the output voltage in the voltage stabilizer, so the design potentiometer should increase the output voltage when the clockwise adjustment is full, and the output voltage decreases when the anti-clockwise adjuster adjusts; in the adjustable constant current charger The middle potentiometer is used to adjust the size of the charging current. When the potentiometer is designed, it should be adjusted clockwise when it is full, and the current will increase.

The potentiometer placement position should meet the requirements of the whole machine structure installation and panel layout, so it should be placed on the edge of the board as much as possible, with the rotating handle facing outward.

(2) IC seat: When designing the printed board diagram, in the case of using the IC seat, we must pay special attention to whether the positioning groove on the IC seat is placed in the correct position, and pay attention to whether the IC pins are correct. For example, the first pin can only be used. It is located on the lower right corner or upper left corner of the IC socket, and is close to the positioning groove (viewed from the soldering surface).

5.Incoming and outgoing terminal arrangement

(1) The distance between the two associated leads should not be too large, generally about 2 to 3/10 inches is more appropriate.

(2) The incoming and outgoing cable ends should be concentrated on 1 to 2 sides as much as possible, and should not be too discrete.

6. When designing the wiring diagram, pay attention to the order of the pins, and the spacing of the component pins should be reasonable.

7. On the premise of ensuring the performance of the circuit, the design should strive for reasonable wiring, use less external jumpers, and route the wiring according to certain smooth charging requirements, and strive to be intuitive, easy to install, height and maintenance.

8. When designing the wiring diagram, the wiring should be turned as little as possible, and the lines should be simple and clear.

9. The width of the wiring strip and the spacing between the lines should be moderate, and the spacing between the two pads of the capacitor should match the spacing of the capacitor lead pins as much as possible;

10. Design should be carried out in a certain sequential direction, such as left-to-right and top-down.

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