How to understand the circuit diagram

///How to understand the circuit diagram

First, how to look at the whole machine circuit diagram

1, according to the order from large to small, from coarse to fine, read various circuit diagrams

As mentioned above, the general circuit diagram mainly includes the whole machine or system block diagram, the block or system circuit schematic, the printed circuit board diagram and the block connection diagram. These circuit diagrams have their own uses and characteristics, but they are also intrinsically linked. When reading these circuit diagrams, you can read them in order from large to small, from coarse to fine. This order is in line with the general rules of people’s understanding of things, and practice has proved to be an effective way to make beginners take fewer detours.

The so-called “big” and “thick” refer to the general structure of the whole machine or system, as well as the main processing of the signal; the so-called “small” and “fine” refer to specific circuits, components and wiring, etc. To the above various circuit diagrams, the basic sequence should be the whole machine block diagram – → system block diagram – → plate schematic diagram – → system schematic diagram – → printed circuit board diagram – → whole machine wiring diagram, and finally the whole machine circuit principle Figure. First of all, you must learn to look at the block diagram of the whole machine. It is necessary to find out which parts (slabs or systems) the whole machine is composed of, how each part handles the signal, and what is the relationship between the parts. These problems have been clarified, and there is a general understanding of the whole machine. On this basis, we will read the block diagrams of each system and each section. It is necessary to understand the main components of each system and sector, the main role of each part and the processing of the signals, and to understand the relationship between the various parts. These problems are clarified, and they understand how the signals flow and how they are handled in the system. After mastering the block diagrams of each part, read the corresponding schematic diagram. The schematic is made up of the symbols and wires of the components. In the schematic diagram, draw out what specific circuits are made up of each part, what role each component has, how the detailed signal direction and the process being processed, and how to meet the design requirements. The schematic diagram of each part is clear, and the working principle of the whole machine is easy to understand. After that, read the printed circuit board diagram. This kind of diagram is not about working principle, but about how to turn the schematic into a utility machine. It mainly considers how the components are arranged better, how the wiring is more reasonable, so that the traces are not crossed, and the components do not interfere with each other. According to the drawing construction during assembly, find the suspect components in the repair, and test and judge to find out the cause of the malfunction. The wiring diagram of the whole machine is mainly used for the assembly and maintenance of the machine, and is drawn on the basis of the schematic diagram and the printed circuit board diagram. By reading the entire machine connection diagram, you can figure out the connection between the printed circuit boards, or between the printed circuit board and the display, buttons, input and output jacks, and so on. Therefore, the whole machine connection diagram can only be understood after understanding the schematic diagram and the printed circuit board diagram.

2, according to the basic circuit program can read the circuit block diagram (system, sector or machine)

There are several types of circuit diagrams mentioned above. The block diagrams are the basis of other types of circuit diagrams and the basis for reading circuit diagrams. The block diagram has a simplified block diagram of the whole machine, a detailed block diagram of the whole machine, a block diagram of the block, and a system block diagram. Sometimes, the reader’s data is incomplete, there may be no such block diagrams, or the block diagram type is incomplete. In order to read the map correctly and in depth, the reader should draw a reference block diagram.

According to the basic circuit program, a circuit block diagram can be drawn. According to the circuit diagram of the whole circuit schematic diagram, the block diagram of the whole machine can be drawn. According to the circuit program of the block circuit diagram, a block diagram of the block system can be drawn, and the system block diagram can be drawn according to the circuit program of the system circuit diagram. The block diagram of the circuit does not reflect the specific structure of the circuit. It mainly reflects the function of the circuit, reflects the transformation process of the signal, reflects the connection between the circuits of each level or the circuits of each system, and reflects the ins and outs of various signals. In fact, one of the important tasks of looking at circuit diagrams is to study and analyze the content, types, waveforms and their transformation laws of transmitted signals. The process of drawing a block diagram is a practical process of understanding the circuit, and is a practical stage of analyzing and researching the circuit, which can lay a theoretical and material foundation for the in-depth reading of the practical circuit diagram. The block diagrams shown reflect the reader’s reading results and levels.

Regardless of whether the whole board or system circuit is based on integrated circuits, one chip can perform one or several circuit system functions, but it often needs to be equipped with some discrete parts circuits. The discrete parts circuit diagram is a crystal transistor or crystal. The diode is the core. When the block diagram is drawn, the integrated circuit is centered, and several integrated circuit blocks are connected through the signal connection line to form a system, a block or a complete block diagram. Sometimes, the triode and diode circuits are drawn in a box form, and occasionally network circuits such as rlc are drawn in a box form. Because the block diagram can be simple and versatile, the content to be highlighted can be different, so the block diagram drawing is not unique.

Analyze the schematic diagram of the practical circuit (system, plate and machine) according to the principle of signal transformation

On the basis of reading the block diagram, it is necessary to further read the specific practical circuit schematic. In order to truly understand the circuit schematic, it must be combined with the basic principle of the whole machine to read, that is, to analyze the specific circuit to complete the signal processing process, why use this circuit to complete this function, instead of using other circuits .

According to the thickness and size of the circuit function, the utility circuit diagram can be divided into a unit circuit diagram, a system circuit diagram, a block circuit diagram, and a whole circuit diagram. Due to the increasing level of integration, a large number of unit circuits have entered the integrated chip, so the current analysis of the practical circuit diagram is mainly to analyze the system circuit diagram and the block circuit diagram. In fact, the reading system circuit and the block circuit are mainly to read the integrated circuit, that is, to read the type function of the integrated block, the signal processing process and the function of the pin, and to read the connection between the collector circuits, the integrated circuit. Contact with peripheral circuits or components, etc.

In order to smoothly read the circuit diagram of the disc player, the reader should have a certain basic knowledge, with basic knowledge of analog electronic circuits, digital electronic circuits, basic knowledge of microcomputers, optics and electrical engineering, etc., to master the basic working principle of the whole machine and Circuit program, signal processing method and transformation law; should also have rich practical experience, familiar with the name, model, specification and data of common and typical components, especially the basic situation of common integrated circuits. Without these basic conditions, reading the whole circuit diagram will encounter greater difficulties.

Second, look at the block circuit diagram method

Usually, a complete machine is composed of several printed circuit boards, and the printed circuit board can be large or small. The circuitry contained on each printed board is often drawn on a circuit diagram. Therefore, a complete machine circuit diagram often consists of a number of plate circuit diagrams. After understanding each board circuit diagram, you can understand the whole machine circuit diagram. Each board circuit diagram includes one or several circuit systems, and the complexity of the large-scale board circuit diagram. Almost the same as the ordinary small and medium screen TV circuit diagram. Although the circuit is more complicated, various slab circuits perform different functions, but they have regular rules and can be read in a certain way.

In order to read the circuit diagram with less detours, the correct reading method and steps should be used. Usually, the method of peripheral wrapping and externally inward is adopted. By adopting a combination of internal and external, front-to-back contact, and step-by-step breakthrough, all circuit diagrams can be read more smoothly. Look at the specific method steps of the figure, you can summarize three sentences, three steps: start the periphery, select the entrance; open the gap, contact before and after; difficult analysis, put it at the end.

1, intuitively start, choose the entrance

In other words, look at the most intuitive and readable components and circuits at the edge of the circuit as an entry point for reading the image. A number of circuits or integrated blocks can be found by these peripheral identities along the signal line inward (possibly in the opposite direction of the signal flow).

Open the gap, before and after the contact

There are some weak points in the inside of any kind of circuit diagram. The complexity and difficulty of each part of the circuit are always different, or the graphics and symbols of some components are different from the general components. These places are the internal weak links of the reading, and they are easy to read. They can be used as a picture. The internal breakthrough. You can choose these places as a breakthrough for reading the map. After opening the gap, you can quickly connect forward, backward, left, and right, and cooperate with the first method to read many circuits.

As long as the reader pays attention, there will always be many links that the reader is already familiar with inside the circuit diagram. Grasping it can quickly open the gap of the picture and develop in depth. The most intuitive and easy-to-read internal links are integrated circuits, especially those large-scale integrated circuits. They have more than 40 pins and more than 100. They are very eye-catching in the picture, and the plotters often put them. In the center or obvious position of the circuit diagram. Then, with these integrated circuits as the center, we can find many circuits. It is necessary to have an integrated circuit as an internal breakthrough. It is necessary to know the specific model of these integrated circuits, know the main functions of the integrated circuit, and be familiar with the names and uses of the main pin. Otherwise, it will bring a lot of inconvenience to the reading. Sometimes the reader is already familiar with most of the integrated circuits, unfamiliar with some of its individual integrated circuits, and can guess the function of the unknown integrated circuit based on the composition block diagram and the context.

There are also some easy-to-read and easy-to-remember contents in the circuit diagram, which can also be used as a breakthrough inside the circuit. For example, the Chinese characters, foreign letters or abbreviations marked in the figure, some important and easy-to-read component graphic symbols, some adjustable resistors or potentiometers. In order to use these breakthroughs conveniently and smoothly, readers should be familiar with the physical meanings of various foreign letters and abbreviations, and have a certain foreign language foundation (usually English). Readers should be familiar with some professional terms and their abbreviations often used in audio-visual equipment. The reader should be familiar with the functions, parameters and indicators of these components. The reader’s knowledge is wider, which is conducive to reading the picture. The reader should consciously remember some useful common sense.

3, difficult analysis, put in the end

After the above two steps of reading, most of the contents of the circuit diagram can be understood. However, the remaining local circuits will not be understood or understood. The third step can be used to encircle the difficulties. This is the difficult part of the picture, and the last step in the picture. Readers can break through difficult circuits by various methods or means.

On the practical circuit diagram, the hard-to-understand places are often manifested in two places. One is the signal circulation process inside some integrated circuits. Since these contents are not understood, the external pin function cannot be understood. The other is a certain These peripheral discrete component circuits do not understand the purpose of setting up the circuit and do not know the signal processing process. For these difficult circuits, according to the block diagram of the whole machine, according to the functions and interconnections of each part of the circuit, through the logic analysis, the analysis function and the signal flow and other analysis methods, these difficult circuits can always be understood.

In fact, there are so many information available on the circuit diagram that you can read and use. With the reader’s synthesis, analysis, and research, you can understand the whole picture. The actual situation of each person is different. The method of drawing and judging may be slightly different, and the steps of viewing the pictures are not static. Electronic technology is developing very fast, and manufacturers often develop circuits with new circuits or new functions, and even circuit programs are rather strange. On the basis of the above basic ideas of the drawing, the drawing work can be flexibly completed.

Third, look at the methods and requirements of integrated circuit diagram

Generally, the whole machine uses integrated circuits (ic) of various models and functions, and is composed of several large-scale or ultra-large-scale integrated circuits as the core, and cooperates with other integrated circuits and discrete components to form a complete circuit. These core integrated circuits can contain the functionality of one or several system circuits. Looking at the whole circuit diagram, block circuit diagram and system circuit diagram are all looking at the circuit diagram with the integrated circuit as the core. How should I look at the integrated circuit diagram? What to see? What requirements should be met? The following is a brief description.

1, look at the method and content of the integrated circuit diagram

To look at the integrated circuit diagram, you can use the above three sentences, three steps; or you can focus on the integrated circuit, connect and expand outside the block based on the signal path in the chip, and then establish the connection between the integrated blocks. Finally, master the whole situation and details; you can also combine several methods, and view the picture according to the drawings.

When looking at the integrated circuit diagram, what should be mainly viewed? Can be summarized into the following four sentences: function type, signal flow, internal and external contact, pin function. Explained separately below.

The first step is to understand the type, type, and main functions of the integrated circuit used. This is the first step in reading an integrated circuit. For example, the types of integrated blocks used in the CD player mainly include rf amplifiers, servo signal processing and digital signal demodulation circuits, servo drives, system controllers (generally integrated in dsp integrated circuits), and the like. First of all, it is necessary to know the specific model of the integrated circuit, and to understand the type of the integrated circuit, especially to familiarize with its main functions. The main functions and circuit structures of different types of integrated circuits may be similar; the circuit structure and circuit program may be different, but the same functions can be accomplished. Familiarity with the function is the most important.

In order to quickly and correctly read the circuit diagram, the reader should consciously accumulate relevant information of some common integrated circuits, purposefully remember the specific models and types of some integrated circuits, and understand the general trends and latest developments of using integrated circuits. Some integrated circuits have different models, but the functions are the same, and can even be used directly by each other; some blocks have similar serial numbers, but the functions and pinouts are completely different; some integrated circuits that appear later can complete the functions of several early integrated blocks. After the readers have mastered the information, they will show great advantages when reading the integrated circuit diagram.

(2) Signal flow

When reading a circuit diagram, it should not be satisfied to only grasp the type and basic functions of the integrated circuit, and should be familiar with the basic processing of the signal. Usually, the structure of the internal circuit of the integrated block is very complicated. The reader does not need to do a detailed analysis of it, but should be familiar with the signal processing of the internal circuit, or should understand the internal functional block diagram. The flow chart of the signal can be seen from the block diagram, and it can be seen which specific functions can be completed by the integrated block. The integrated circuit can be regarded as a component, and it is not necessary to pursue the structure and detailed working process of the component too much, but it should clarify the specific functions completed by each block in the integrated circuit, that is, familiar with the input and output signals, and familiar with the waveform amplitude of the signal. , the law of frequency changes, familiar with the connection between the various boxes, familiar with the ins and outs of the signal in the integrated block. At this point, it is a preliminary understanding of the integrated circuit diagram.

At present, some of the circuit diagram information of the whole machine is not comprehensive enough. Only the number of pins of the integrated block and the symbols of each pin are given. The internal block diagram of the integrated block is not given, and the DC operating point of the pin of the integrated block is not given. It will bring inconvenience to the in-depth reading of the map, the reader should look for some other information to make up for its shortcomings. Also, many internal component block diagrams are often labeled with foreign letters or abbreviations, and it is also difficult for beginners to come. This requires the reader to be familiar with the Chinese meaning of letters and abbreviations as soon as possible, otherwise it is difficult to read these circuit diagrams.

Due to the characteristics of the integrated process, the integrated circuit must be connected to the peripheral components through the pinout. In order for the integrated circuit to perform certain functions, it must be associated with external unit circuits or components, and must also be connected to the pre-stage and post-stage circuits through the pinouts. When reading a circuit diagram, the circuits inside and outside the integrated circuit must be connected, and they are used as a whole of the circuit system to perform certain functions. Can not contact the internal and external circuits, will not see the ins and outs of the signal, it is difficult to analyze the function of the circuit.

For the same integrated circuit, in the hands of different machine designers, different external circuits may be designed to match different component networks. Since the external discrete circuit can be varied, these discrete circuits are often difficult circuits for reading pictures. It takes some effort to break through these difficulties, otherwise it will not be able to fully and correctly read the whole circuit diagram.

(4) Pin function

On the integrated circuit diagram, each pin not only needs to be marked with a sequence number, but also its name is indicated by a simple letter symbol. These alphabetic symbols are often abbreviations in English and can represent the function of the pin. When looking at the integrated circuit diagram, we must pay great attention to the function of each pin. The lead pin is the link between the internal and external circuits. It is necessary to clarify the connection between each pin and the internal functional block diagram. It is the pinout of the corresponding internal frame; it is also necessary to explicitly take the function of the pin external component, and the external circuit is matched by the pin. The internal circuit works. Some pinouts are the input and output ports of an integrated circuit. These pinouts are important when looking at the circuit diagram. When reading the integrated circuit diagram, it is necessary to observe the individual block diagrams representing the internal functions one by one, and also identify the corresponding pinouts at the same time, and identify the external circuits or components. These tasks are often the main work of reading the integrated circuit diagram.

Generally, integrated circuit leads and external components have certain regularity, but the law is not static. In some models, some pins are not used, or the signal flow is different, and the reader should be flexible.

Read the lead to make “four clear”

Reading the pin is a regular and important task in reading the integrated circuit diagram. Familiar with the internal composition block diagram, can deepen the understanding of the function of the pin-out foot; familiar with the function of the pin-out foot can enhance the understanding of the block diagram. When reading the lead, the following “four clear” should be made.

(1) Symbol function

According to the original designer’s requirements, each pin has its own purpose and name. According to the design idea of ​​each pin, English letters or abbreviations are marked near each foot. Professionals or maintenance personnel can know the nature and function of the foot according to the English words marked on the drawings. However, it should be noted that different manufacturers may use different abbreviations for the same nature and function; in most countries, manufacturers use the same or similar abbreviations. If the reader is very familiar with these abbreviated word representations, it will bring great convenience to the viewing of integrated circuit diagrams. If you are not familiar with these words, it will cause many difficulties for viewing the pictures.

(2) Signal waveform

Many pinouts are input and output signal ports, some pinouts are switch or pulse signals, data streams or analog signal ports, and some pinouts are critical test pins. The reader should be familiar with the signal waveforms of some important pinouts, understand the shape, amplitude and frequency of the signal waveform of the pinout, which is of great significance for reading the circuit diagram and troubleshooting the whole machine. Usually, an oscilloscope can accurately read the input and output waveform shape, frequency and amplitude of the pin.

(3) Relevant data

Also be familiar with the relevant data for the lead. It is also important for reading circuit diagrams and troubleshooting faults. First, be familiar with the data about the signal waveform, and be clear about the range of the signal amplitude and the frequency value of the signal. Secondly, you should be familiar with the data of the current, voltage, and resistance of the pin, especially the static working voltage and dynamic working voltage of the pin. For those pins whose dynamic and static DC voltage values ​​are not the same, more attention should be paid.

(4) Clear flow

It is also necessary to pay attention to the flow direction of the pin signal. It is necessary to distinguish the name of the foot signal, and to distinguish whether the foot is a signal input terminal, a signal output terminal, or a bidirectional signal (i/o) port. If the signal flow is not clear, the circuit diagram will not be understood. Some pinouts are signal control terminals, and the control signal may be a switching voltage, a data stream, or a pwm signal.

Second, look at the basic method and main content of the unit circuit diagram

Each system circuit is composed of several unit circuits. In order to understand the whole circuit, the block circuit and the system circuit, you must read the unit circuit diagram of each system. The unit circuit is the circuit unit of the whole machine. As the level of integrated circuit technology increases, several or even dozens of unit circuits can be integrated into one integrated circuit. At present, on the whole circuit diagram, there are fewer and fewer discrete unit circuits, which makes the task of analyzing the unit circuit in the whole machine less and less, and it is easy. However, it is very familiar with the function principle of the various unit circuits of the whole machine. It is necessary for mastering the working principle of the whole machine and for repairing the whole machine.

When reading the unit circuit diagram, you should mainly read the following contents: principle, function, structure, type, transformation process, numerical waveform.

1, principle, function

Each circuit system has to perform a signal processing function, and each unit circuit only completes a certain part of the signal processing process. When reading the unit circuit, first understand the purpose and function of the unit circuit in the circuit system, and what tasks it mainly accomplishes. Then, according to the function and task requirements, analyze the working principle of the circuit. Sometimes the function and function of the unit circuit are difficult to determine immediately. The basic function of the unit circuit can be assumed based on its position in the system circuit, and its working principle is analyzed, then analyzed and verified, and finally determined.

The verification and verification work is mainly realized by oscilloscope detection. Analysis of the working principle of the circuit requires knowledge of the reader’s theoretical basis and reading experience. The principle of analysis mainly refers to clarifying the content and characteristics of the circuit signal, how the circuit generates new signals and how to implement signal transformation.

When analyzing circuit functions and principles, it is inevitably accompanied by the structure and type of the analysis unit circuit. For the integrated unit circuit, the specific form of the circuit can be omitted; for the discrete part circuit, the circuit structure must be analyzed in order to correctly analyze the circuit type and working principle. It should be noted that the unit circuits that perform the same function can use circuits of different structures and types; even circuits that implement the same function can use circuits with different working principles.

3, the transformation process

Analysis of the working principle of the circuit is inevitably accompanied by the transformation process of the analysis signal. Analysis of the transformation law of the signal is the key to analyzing the circuit principle. The signal analysis is clear, and the working principle must be solved. The analysis signal mainly analyzes the waveform, amplitude and frequency of the input signal and the output signal, analyzes their contents and characteristics, and analyzes the principle and function of the signal transformation.

4, the value, the waveform

When analyzing signals, be familiar with the laws of signal waveforms and familiarize yourself with their values. The amplitude and frequency of the signal are also important contents reflecting the characteristics and laws of the signal. By analyzing the waveform variation law of the input and output signals, and analyzing the signal amplitude and frequency variation law, the working principle of the circuit can be deeply grasped. In addition, for the discrete component circuit, the DC operating point value of the circuit is also the content of the analysis, which directly affects the working state and working principle of the circuit; for the integrated circuit, the dynamic and static operating voltage values of the corresponding lead-out pins are also We must pay great attention to it.

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