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This post was last edited by CokezzZ on 2021-7-13 08:39
The concept of electronic circuit schematics
A conductive loop consisting of metal wires and electrical and electronic components is called a circuit. When a power source is added to the input end of the circuit to generate a potential difference at the input end, the circuit can work when it is connected. The existence of current can be tested by some instruments, such as the deflection of a voltmeter or ammeter, the light of a light bulb, etc.; according to the nature of the current flowing through, it is generally divided into two types: a circuit through which direct current passes is called a "DC circuit", and a circuit through which alternating current passes is called an "AC circuit".
Electronic circuit diagrams are generally composed of circuit schematics, block diagrams and assembly (installation) diagrams. The circuit schematic is an important part of the electronic circuit diagram. It is composed of various symbols (graphics, text) and annotation characters representing actual electronic components. From the circuit schematic, we can see the specific parameters of each electronic component (such as model, nominal value) and the connection relationship between each component.
Reading drawings is a basic skill for those engaged in electronic technology. It can help people to quickly become familiar with the structure and working principle of the equipment, and understand the connection and installation of various components and instruments. Reading drawings is also a prerequisite for electronic production or maintenance. Reading drawings also helps us quickly become familiar with various new electronic instruments and equipment.
How to read electronic circuit schematics
To read electronic circuit schematics, one must understand certain basic knowledge of electronic technology. However, even if one has certain basic knowledge of electronic technology, one may still find it difficult to read circuit diagrams for the first time. However, from my many years of experience in teaching electronic technology, I think there are certain methods to follow for reading electronic circuit schematics. Below I would like to make a summary based on the light-controlled and voice-controlled delayed lighting corridor lamp circuits. The circuit is shown in the figure below.
Disassemble the circuit into blocks and divide it into several unit circuits
Some complex circuits can usually be divided into several parts according to the functions realized by the circuits. In this way, a complex circuit can be decomposed into several simple circuits for analysis, which simplifies the difficulty of analyzing the circuit. For example, the light-controlled and voice-controlled delayed lighting corridor lamp circuit can be decomposed into four parts: voice-controlled receiving amplifier circuit, monostable delay circuit, light-controlled circuit and power supply circuit. The dividing line of each part is shown in the figure above (Note: C2 belongs to the power supply circuit part). For example, the AM radio circuit can be decomposed into several unit circuits: input circuit, mixer, intermediate amplifier, preamplifier and power amplifier.
Master the structure and characteristics of typical unit circuits
Common typical unit circuits include amplifier circuits, oscillation circuits, filter circuits, etc. These unit circuits are usually composed of triodes or integrated circuits as core devices, and have a certain structural form. Some complex circuits are expanded on the basis of these typical unit circuits. For example, the amplifier circuit is usually a unit circuit with a triode or an integrated operational amplifier as the core, and its structural feature is that it has an input terminal and an output terminal; the oscillation circuit is usually a unit circuit with a triode or an integrated operational amplifier as the core, and its structural feature is that it has no external circuit input terminal, and a positive feedback network with a frequency selection function is connected between the input terminal and the output terminal of the triode or integrated operational amplifier; the filter circuit is usually based on an integrated operational amplifier, and its structural feature is that it contains capacitors or inductors, and a feedback element is connected between the output terminal and the input terminal.
In the figure above, the voice-controlled receiving amplifier circuit is a unit circuit with transistors VT1 and VT2 as the core, and the light-controlled circuit is a unit circuit with VT3 as the core. For example, in the trigger circuit, the basic RS trigger as a storage unit circuit is the basic logic unit of other complex triggers, such as the synchronous RS trigger, which is formed by adding two NAND gates on the basis of the basic RS trigger, the master-slave RS trigger is composed of two synchronous RS triggers, and the master-slave JK trigger is formed by adding two AND gates on the basis of the master-slave RS trigger. It can be seen that the synchronous RS trigger, the master-slave RS trigger, and the master-slave JK trigger are all formed by gradually expanding the basic RS trigger. The basic RS trigger is the basic logic unit of these complex triggers. Mastering it lays the foundation for our study of the following types of triggers.
Understand the characteristics of power supply circuits
Electronic circuits usually use DC regulated power supplies as power sources to provide energy to the circuits. DC regulated power supplies usually consist of four parts: voltage transformation, rectification, filtering, and voltage stabilization. Through the circuits of these four parts, AC power is converted into DC power. As shown in Figure 1, the AC 220V voltage is stepped down by C1 and R1, limited by the VDW diode, and rectified by VD1. The DC voltage is filtered by the C2 capacitor to provide the working voltage for the entire circuit. For example, some doorbell circuits, charging circuits, and switch circuits usually use 220V AC power to step down the voltage through a transformer, rectify through a rectifier bridge composed of four diodes, filter through a capacitor, and stabilize through a voltage regulator tube to convert DC power into AC power to provide a stable power supply for the circuit.
Classify circuits and study them by category
Electronic circuits can usually be divided into the following common categories: alarm circuits, doorbell circuits, oscillation circuits, power supply circuits, lighting and color light control circuits, switches and detection circuits, sensor application circuits, 555 timer application circuits, etc. Although each of the above categories of circuits uses different electronic components, the functions implemented by the circuits are basically the same, so the circuits can be analyzed starting from the functions implemented by the circuits. In addition, understanding the typical circuit structure and characteristics of some devices also brings convenience to our analysis of some complex circuits. For example, the typical circuit of the 555 timer mainly includes a monostable trigger, a multivibrator, and a bistable trigger composed of a 555 timer. These typical circuits can be used to form corresponding application circuits, such as a monostable trigger composed of 555 can form a touch switch circuit, a timer, etc., a multivibrator composed of 555 can form a clock pulse generator, etc., and a bistable trigger composed of 555 can form a logic level test circuit, etc. As shown in the figure above, the delay function of the corridor light is realized by the monostable trigger composed of a 555 timer.
Study a certain category of circuits from the basics to the depths
For example, for the doorbell circuit, we can first master the principle of the simple doorbell circuit, and then further study the principle of the simple tone-changing doorbell circuit and the dual-tone doorbell circuit, because the latter two types of doorbell circuits are formed by improving and expanding the simple doorbell circuit. As shown in the figure above, it is a light-controlled and sound-controlled delay switch circuit. We can start with the relatively simple light-controlled switch circuit, and then study the light-controlled delay switch circuit on this basis. Finally, it is relatively easy to study the sound-light dual-control delay switch circuit.
Inventory of Circuit Analysis Methods
DC Equivalent Circuit Analysis Method
When analyzing the circuit principle, it is necessary to understand the DC path and AC path in the circuit. The DC path refers to the static bias of each semiconductor transistor and integrated circuit when there is no input signal, that is, their static operating point. The AC circuit refers to the path of AC signal transmission, that is, the origin and development of the AC signal.
In actual circuits, AC circuits and DC circuits coexist in the same circuit. They are both interconnected and different from each other.
The DC equivalent analysis method is a method of separately analyzing the DC system of the circuit being analyzed. When performing DC equivalent analysis, the circuit's processing function for the input AC signal is completely ignored. Only the static DC current and voltage directly caused by the DC voltage of the power supply and their relationship are considered.
When performing DC equivalent analysis, the DC equivalent circuit diagram should be drawn first. The following principles should be followed when drawing the DC equivalent circuit diagram: capacitors are all treated as open circuits, inductors whose DC resistance can be ignored should be treated as short circuits, and inductors whose resistance components cannot be ignored can be equivalent to resistors. The voltage after step-down decoupling is taken as the supply voltage of the equivalent circuit; semiconductor diodes in the reverse bias state are treated as open circuits.
AC equivalent circuit analysis method
The AC equivalent circuit analysis method is a method of separating the AC system in the circuit from the circuit and analyzing it separately.
When performing AC equivalent analysis, the AC equivalent circuit diagram should be drawn first. The following principles should be followed when drawing the AC equivalent circuit diagram: the power supply should be regarded as a short circuit, the capacitors in the AC bypass should be regarded as short circuits, and the DC isolation couplers should be regarded as short circuits.
Time constant analysis
The time constant analysis method is mainly used to analyze the properties of the circuit composed of R, L, C and semiconductor diodes. The time constant is a parameter that reflects the speed of energy accumulation on the energy storage element. If the time constant is different, although the form and connection of the circuit are similar, the role played in the circuit is different. Common ones include coupling circuits, differential circuits, integration circuits, clamping circuits and peak detection circuits.
Frequency characteristics analysis method:
The frequency characteristic analysis method is mainly used to analyze whether the frequency of the circuit itself is compatible with the frequency of the signal it processes. In the analysis, its center frequency, upper and lower limit frequencies, and bandwidth should be simply calculated. Through this analysis, the properties of the circuit can be known, such as filtering, notching, resonance, frequency selection circuit, etc.
Summarize
The above are several electronic circuit schematic diagram recognition methods summarized by the editor based on years of study, accumulation, exploration and practice, and reference to relevant books and materials. The first three methods are mainly common methods for analyzing specific circuits, and the last two methods can be used as a reference for self-study of circuits or teaching. These methods have something in common, that is, they can be used alone or integrated.
Of course, there are many other ways to read electronic circuit schematics, such as following the flow and changes of signals, finding familiar components or circuits first, and turning the special into the general. We can choose according to the specific circuit and personal reading habits. In addition, I think that in order to better read electronic circuit schematics, you need to watch, read, analyze, and understand various circuit diagrams more often, and accumulate methods that are suitable for you. Of course, you can also read more books and materials on related topics. The more you see the diagrams, the more comfortable you will be in analyzing them. At the same time, you should also learn from experienced colleagues. These can continuously improve your reading skills and enable you to quickly and accurately read circuit schematics.
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