PCB Tech - Learn to read the circuit schematic diagram introductory knowledge accumulation

1. The meaning of electronic circuits

Circuit diagram is a graph that shows the circuit structure that people draw with agreed symbols for the needs of research and engineering. You can know the actual circuit situation through the circuit diagram. In this way, when we analyze the circuit, we don't have to think about the real thing over and over, but only need to hold a drawing. When designing the circuit, you can also do it on paper or computer calmly, and then perform the actual installation after confirming the perfection, through debugging and improvement, until it is successful. We can also apply advanced computer software to assist in circuit design, and even to conduct virtual circuit experiments, which greatly improves work efficiency.

Second, the classification of electronic circuit diagrams

Commonly encountered electronic circuit diagrams include schematic diagrams, block diagrams, assembly diagrams and printed layout diagrams.

1. Schematic

A schematic diagram is a circuit diagram used to embody the working principle of an electronic circuit, and is also called an "electrical schematic diagram". Because this kind of diagram directly embodies the structure and working principle of the electronic circuit, it is generally used in the design and analysis of the circuit. When analyzing the circuit, you can understand the actual working condition of the circuit by identifying the various circuit component symbols drawn on the drawing and the connection between them. The following figure shows the schematic diagram of a radio circuit.

2. Block diagram (block diagram)

A block diagram is a circuit diagram that uses blocks and connections to show the working principle and composition of the circuit. Fundamentally, this is also a schematic diagram. However, in this kind of drawing, there are almost no other symbols except boxes and lines. The main difference between it and the above schematic diagram is that all the components of the circuit and their connection methods are drawn in detail on the schematic diagram, while the block diagram simply divides the circuit installation function into several parts, and each part is depicted as A box, add a simple text description in the box, and use a line between the boxes (sometimes use a line with arrows) to illustrate the relationship between the boxes. Therefore, the block diagram can only be used to reflect the general working principle of the circuit, and the schematic diagram can be used as a basis for collecting components and making the circuit in addition to showing the working principle of the circuit in detail. The following figure shows the block diagram of the above-mentioned radio circuit.

3. Assembly drawing

It is a kind of drawing used for circuit assembly, and the symbols on the drawing are often the physical appearance drawings of the circuit components. We can complete the circuit assembly by simply connecting some circuit components as shown in the picture. This kind of circuit diagram is generally for beginners to use.

The assembly drawings are different according to the different assembly templates. In most cases where electronic products are used, the printed circuit boards described below are used, so printed board drawings are the main form of assembly drawings.

When learning electronic knowledge, in order to be able to get in touch with electronic technology earlier, we chose the screw hole plate as the basic installation template, so the installation drawing becomes another mode.

4. Printed board map

The full name of the printed board diagram is "printed circuit board diagram" or "printed circuit board diagram", which is actually the same type of circuit diagram as the assembly diagram, and is used for assembling the actual circuit.

The printed circuit board is covered with a layer of metal foil on an insulating board, and then the metal foil that is not needed by the circuit is corroded. The remaining part of the metal foil is used as the connection line between the circuit components, and then the components in the circuit The device is mounted on this insulating board, and the remaining metal foil on the board is used as a conductive connection between the components to complete the circuit connection. Because the metal covered on one or both sides of this circuit board is copper, the printed circuit board is also called "copper clad laminate." The component distribution of the printed board diagram is often quite different from the schematic diagram. This is mainly because, in the design of the printed circuit board, the main consideration is whether the distribution and connection of all components are reasonable. It is necessary to consider many factors such as component volume, heat dissipation, anti-interference, anti-coupling, etc., and the printed circuit board designed by combining these factors From the outside, it is difficult to be completely consistent with the schematic; in fact, it can better realize the function of the circuit.

With the development of science and technology, the production technology of printed circuit boards has been greatly developed; in addition to single-panel and double-panel, there are also multi-panel, which has been widely used in daily life, industrial production, national defense construction, aerospace industry, etc. Field.

In the four forms of circuit diagrams introduced above, the electrical schematic diagram is the most commonly used and the most important. If you can understand the schematic diagram, you can basically master the principle of the circuit. Draw block diagrams, design assembly diagrams, and printed board diagrams. It's easy. It is also very convenient to master the schematic diagram to repair and design electrical appliances. Therefore, the key is to master the schematic diagram.

Three, the composition of the circuit diagram

The circuit diagram is mainly composed of four parts: component symbols, connections, nodes, and notes.

The component symbol indicates the component in the actual circuit, and its shape is not necessarily similar to the actual component, or even completely different. But it generally shows the characteristics of the component, and the number of pins is consistent with the actual component.

The wire means the wire in the actual circuit. Although it is a wire in the schematic diagram, it is often not a wire but a copper foil block of various shapes in a commonly used printed circuit board, just like many in the radio schematic diagram. The wiring in the printed circuit board diagram is not necessarily all linear, but may also be a copper film of a certain shape.

The node represents the connection relationship between several component pins or several wires. All component pins and wires connected to the node, regardless of the number, are conductive.

Notes are very important in circuit diagrams, and all text in circuit diagrams can be classified into the comment-type. If you look closely at the above diagrams, you will find that there are notes in various places in the circuit diagram, which are used to illustrate the model, name, etc. of the component.

If you don't know the function of the circuit, you can analyze the relationship between the input and output signals of the circuit first. For example, the law of signal change and the relationship between them, the phase problem is the same phase, or the opposite phase. Circuit and composition form, is it amplifying circuit, oscillating circuit, pulse circuit, or demodulating circuit. Electrical repair and circuit design staff must understand the function and working principle of the electrical appliance by analyzing the schematic diagram of the circuit, so that they can carry out their work handily. As a technician engaged in this work, you must first have excellent basic skills, be able to have a general understanding of circuit schematics with technical parameters, be able to divide functional modules, find out the direction of signal flow, and determine the role of components. To learn to repair electrical equipment and design circuits, one must master the principles of each unit circuit. Functional blocks are divided, and the components of the complete circuit can be grouped according to different functions, so that each functional block forms a specific functional component combination, such as basic amplifying circuit, switching circuit, waveform conversion circuit, etc.

To master several methods of analyzing commonly used circuits, and be familiar with the types of circuits and analysis steps that each method is suitable for.

1. The AC equivalent circuit analysis method first draws the AC equivalent circuit, and then analyzes the AC state of the circuit, that is: when the circuit has a signal input, whether the voltage and current of each link in the circuit change according to the law of the input signal, is amplified, Oscillation is still the same as limiting clipping, shaping, and discrimination.

2. DC equivalent circuit analysis method Draw the DC equivalent circuit diagram, analyze the DC system parameters of the circuit, find out the static operating point and bias nature of the transistor, and the coupling mode between stages. Analyze the status and role of relevant components in the circuit. For example: the working state of the triode, such as saturation, amplification, cut-off area, the diode is in conduction or cut-off, etc.

3. The frequency characteristic analysis method mainly depends on whether the frequency of the circuit itself is compatible with the frequency spectrum of the signal it processes. Roughly estimate its center frequency, upper and lower limit frequency and bandwidth, for example: various filtering, trapping, resonance, frequency selection and other circuits.

4. The time constant analysis method mainly analyzes the circuit and properties of R, L, C and diodes. The time constant is a parameter that reflects the speed of energy accumulation and consumption on the energy storage element. If the time constant is different, although its form and connection method are similar, it still plays a different role. Common ones include coupling circuit, differential circuit, integrator circuit, decoupling circuit, peak detector circuit and so on.

Finally, comparing the actual circuit with the basic principle, according to the role of the components in the circuit, step by step analysis according to the above method, it is not difficult to understand. Of course, you need to study persistently in order to truly understand it. After you have a certain theory, it will not be laborious to analyze the circuit diagram.

Definition of circuit diagram:

The circuit diagram is a diagram of the wiring trend of electronic components drawn by circuit component symbols. It describes the wiring and trend of each component in detail, the description of each pin, and some test data.

Schematic diagram is also called "electrical schematic diagram". This kind of diagram, because it directly embodies the structure and working principle of the electronic circuit, is generally used in the design and analysis of the circuit. When analyzing a circuit, you can understand the actual work of the circuit by identifying the various circuit component symbols drawn on the drawing and the connection between them. A schematic diagram is a circuit situation used to embody the working principle of an electronic circuit.

The PCB diagram is a mapping drawing of the circuit board, which depicts the wiring of the circuit board, the location of the components, etc. in detail.

Looking at the circuit diagram, first look at the power supply section to understand the power supply under which the circuit works, AC or DC, single power supply or multiple power supplies and voltage levels. After you understand it, look at the subdivision circuits. First, distinguish between digital circuits and analog circuits. For analog circuits, look at signal acquisition and figure out the source of the signal. There are radio frequency, audio, various sensors, instrumentation or other circuits, etc. The analysis signal is AC, DC or pulse, voltage type or current type. Analyze the function of the subsequent circuit to find out whether it is demodulation, amplification, shaping or compensation. Finally, look at the output circuit, whether it is modulation or drive. The digital circuit mainly analyzes the logic function and role of the circuit.

To understand the circuit board, it is best to be able to understand its electrical schematic diagram (ie circuit diagram), master the marking method of electronic components and its working principle, and master the normal parameters and performance of some commonly used components. Knowledge of the role played in a normal circuit and so on, and then analyze the circuit board (called a printed circuit board), you can quickly understand its working principle and some situations that need to be mastered.

Molecular circuit module, then find the core component of the sub-circuit (of course, you must be familiar with this component) to find out the electrical connection between the sub-circuit modules, and finally the output and input or function of the entire circuit.

The circuit of the whole machine has a certain function and is composed of unit circuits. The unit circuits form a signal processing branch with a certain function, and these branch circuits form the whole machine circuit. First of all, you must find out what is the function of the circuit diagram you are looking at, which type of circuit is it, whether it is audio, video, digital, or hybrid circuit, and then use the corresponding unit circuit knowledge to interpret these circuits, and at the same time, from the AC signal level Analyze on the DC level. The DC part of the circuit is the basis for the normal operation of the circuit. The AC signal can only be processed after the DC circuit is normal. The circuit cannot work normally without a good DC state. It is also necessary to analyze from the frequency level and the gain level of the amplifier. When signals of different frequencies are processed by the circuit, due to the nonlinear components in the circuit, they will have different processing results for different frequencies. The amplifier will also respond to signals of different frequencies. With different amplification capabilities, the circuit will process the required frequency signals purposefully during the design, so as to achieve the needs of the machine function. Then there is the need to analyze the relationship between the unit circuits, and the relationship between the input and output of the unit circuits. How does the AC signal change after passing through these circuits? After understanding the working principle of each branch, the working principle of the whole machine can be analyzed. Sometimes there are signal cross-connections between each branch circuit. For example, the line reverse pulse of the line output circuit of the TV is used for color. The decoding circuit, the line output circuit and the color decoding circuit have signal interconnections. At this time, these branches can be understood as another unit circuit, and then they can be analyzed.

I think there is a sequence problem here: For high-frequency circuits, for example, you should first grasp the function of the circuit and the relationship between input and output. After you have an overall grasp, it is like catching the nose, because although the circuit is different, the device is different, but Their input and output relationship spectrum will not change. Then analyze the basic principles and methods to realize such functional transformation, and then analyze the specific part.

The circuit design is to start by analyzing the circuit schematic diagram, but you must first understand the pins and basic functions of the required chip. This is conducive to a better understanding of the working principle of the circuit, so that it can be applied to your own circuit. Cutting and expansion of the circuit. In the circuit analysis, first have a general understanding of the circuit schematic, and divide the various functional modules, such as the power module, the controller module, the memory module, the audio module, the GPRS module and so on. Analyze each module one by one, and finally look at it together to get a general understanding of the functions to be realized by the circuit. When designing a circuit, it is best to master the principles of common or commonly used unit circuits, such as power supply modules, voltage regulator modules, memory modules, etc., and commonly used chips such as 7805, 7812, etc.

When designinga circuit, divide the circuit you want to design into several modules, so that they are designed in different schematic diagrams, and finally integrated. When there is a signal input in the circuit, what is the voltage and current of each basic point, there must be a rough estimate. For circuits with amplifiers, R, L, and C, it depends on whether it is an oscillating circuit, an amplifying circuit, or a shaping circuit.

Analysis of the static operating point of transistors, analysis of working conditions, etc., capacitor filtering, inter-stage coupling, high-frequency and low-frequency circuits, etc. Generally, we use low-frequency circuits, and high-frequency circuits are generally used for communication.

After conducting self-analysis and self-design, you will have a good understanding and mastery of the basic principles of the circuit, and will accumulate design and debugging experience for yourself in the future design. Of course, true proficiency needs to be tempered!