How to understand circuit diagrams and component symbols

There are various diagrams in electronic devices. The one that can explain how they work is the electrical schematic, or circuit diagram for short.

A circuit diagram is like an article, various unit circuits are like sentences, and various components are the words that make up the sentences. So if you want to understand the circuit diagram, you have to start by knowing the words - components. For the purpose, category, and usage of components such as resistors, capacitors, inductors, and transistors, you can click on the links under the relevant articles in this article. This article only repeats the various symbols that often appear in circuit diagrams. I hope beginners will be familiar with them and remember them.

There are two types of circuit diagrams. One is to illustrate the working principle of analog electronic circuits. It uses various graphic symbols to represent resistors, capacitors, switches, transistors and other physical objects, and uses lines to connect components and unit circuits according to the working principle. This kind of diagram has long been called a circuit diagram.

The other is to explain the working principle of digital electronic circuits. It uses various graphic symbols to represent gates, triggers and various logic components, and connects them with lines according to logical relationships. It is used to illustrate the logical relationship between various logic units and the logical function of the whole machine. In order to distinguish it from the circuit diagram of analog circuits, this kind of diagram is called a logic circuit diagram, or logic diagram for short.

In addition to these two types of diagrams, block diagrams are also commonly used. They use a box to represent a part of the circuit, and they can clearly and concisely explain the relationship between the various parts of the circuit and the working principle of the whole machine.

Resistors and Potentiometers

The symbols are shown in Figure 1, where (a) represents a general fixed-resistance resistor, (b) represents a semi-adjustable or fine-tuning resistor, (c) represents a potentiometer, and (d) represents a potentiometer with a switch. The symbol for a resistor is "R", and the symbol for a potentiometer is "RP", that is, a "P" is added after R to indicate that it has an adjustable function.

In some circuits, there are certain requirements for the power of resistors, which can be represented by the symbols shown in (e), (f), (g), and (h) in Figure 1 respectively.

Symbols of several special resistors:

The first is the symbol of thermistor. The resistance value of thermistor changes with the external temperature. Some have negative temperature coefficient, represented by NTC; some have positive temperature coefficient, represented by PTC. Its symbol is shown in Figure (i), and temperature is represented by θ or t°. Its text symbol is "RT".

The second type is the photoresistor symbol, see Figure 1 (j), which has two diagonal arrows representing light. Its letter symbol is "RL".

The third symbol is the symbol of a varistor. The resistance of a varistor changes with the voltage applied across the resistor. The symbol is shown in Figure 1 (k), with the letter U representing voltage. Its text symbol is "RV". These three resistors are actually semiconductor devices, but we still treat them as resistors out of habit.

The fourth special resistor symbol represents the newly emerged fuse resistor, which has the functions of both resistor and fuse. When the temperature exceeds 500℃, the resistor layer will quickly peel off and melt, cutting off the circuit and protecting the circuit. Its resistance value is very small and it is currently widely used in color TVs. Its graphic symbol is shown in Figure 1 (1), and the text symbol is "RF".

Capacitor symbol

See Figure 2 for details, where (a) represents a capacitor with fixed capacity, (b) represents a polar capacitor, such as various electrolytic capacitors, (c) represents a variable capacitor with adjustable capacity. (d) represents a fine-tuning capacitor, and (e) represents a double-connected variable capacitor. The symbol for capacitor is C.

Symbols for inductors and transformers

The graphic symbols of the inductor in the circuit diagram are shown in Figure 3. Among them, (a) is the general symbol of the inductor, (b) is a coil with a magnetic core or an iron core, (c) is a coil with a gap in the iron core, (d) is an adjustable inductor with an adjustable magnetic core, and (e) is an inductor with multiple taps. The letter symbol of the inductor is "L".

The graphic symbols of transformers are shown in Figure 4. Among them, (a) is an air-core transformer, (b) is a core or iron-core transformer, (c) is an iron-core transformer with a shield between windings, (d) is a transformer with a center tap on the secondary, (e) is a transformer with variable coupling, (f) is an autotransformer, (g) is a transformer with an adjustable magnetic core, and the small dot in (h) is the mark of the transformer polarity.

Symbols for microphones, pickups and recording and playback heads

The symbols of the microphone are shown in Figure 5 (a), (b), (c), where (a) is the graphic symbol of a general microphone, (b) is the graphic symbol of a capacitive microphone, and (c) is the graphic symbol of a piezoelectric crystal microphone. The letter symbol of the microphone is "BM".

The pickup is commonly known as the phono head. Figure 5 (d) is the graphic symbol of a stereo phono head, and its letter symbol is "B". Figure 5 (e) is the graphic symbol of a monophonic recording and playback head. If it is a two-channel stereo, add a "2" to the symbol, see Figure (f).

Speaker, headphone symbol

Speakers and headphones are transducers that convert electrical signals into sound. The symbol of headphones is shown in Figure 5 (g). Its letter symbol is "BE". The symbol of speakers is shown in Figure 5 (h), and its letter symbol is "BL".

Symbols for wiring components

In electronic circuits, it is often necessary to connect, disconnect or switch the circuit, and wiring components are used at this time. There are two major types of wiring components: one is a switch; the other is a connector.

(1) Switch symbol

In an electromechanical switch, there is at least one moving contact and one stationary contact. When we turn, push or rotate the switch mechanism by hand, we can connect or disconnect the moving contact and the stationary contact to achieve the purpose of connecting or disconnecting the circuit. There are generally three types of combinations of moving contacts and stationary contacts: ① Moving Make (normally open) contact, the symbol is shown in Figure 6 (a); ② Moving Break (normally closed) contact, the symbol is Figure 6 (b); ③ Moving Change (conversion) contact, the symbol is shown in Figure 6 (c). The simplest switch has only one set of contacts, while a complex switch has several sets of contacts.

The dot below indicates push-pull action; (d) indicates a rotary switch with 3-pole contacts that move and close simultaneously; (e) indicates a push-pull 1×6 band switch; (f) indicates the symbol of a rotary 1×6 band switch. The symbol for the switch is “S”, “SA” can be used for control switches and band switches, and “SB” can be used for push-button switches.

The graphic symbols of switches in the circuit diagram are shown in Figure 7. Among them, (a) represents a general manual switch; (b) represents a push button switch with a break contact; (c) represents a push-pull switch with a set of changeover contacts; the lever is drawn below the contact to represent the push-pull action; (d) represents a rotary switch with 3-pole simultaneous close contacts; (e) represents a push-pull 1×6 band switch; (f) represents the symbol of a rotary 1×6 band switch. The text symbol of the switch is "S", and "SA" can be used for control switches and band switches, and "SB" can be used for push button switches.

(2) Connector symbols

The graphic symbols of the connector are shown in Figure 8. Among them, (a) represents a plug and a socket (there are two ways of representation). The left side represents the socket and the right side represents the plug. (b) represents a plug that has been inserted into the socket. (c) represents a 2-pole plug socket, also known as a 2-core plug socket. (d) represents a 3-pole plug socket, which is the commonly used 3-core stereo headphone plug socket. (e) represents a 6-pole plug socket. For simplicity, it can also be represented by Figure (f), with the number 6 marked above the symbol to indicate 6 poles. The text symbol of the connector is X. In order to distinguish, "XP" can be used to represent the plug and "XS" can be used to represent the socket.

Relay symbol

Because the relay is composed of two parts, the coil and the contact group, the graphic symbol of the relay in the circuit diagram also includes two parts: a rectangular box represents the coil; a group of contact symbols represents the contact combination. When there are not many contacts and the circuit is relatively simple, the contact group is often drawn directly on one side of the coil frame. This method of drawing is called concentrated representation, as shown in Figure 9 (a). When there are many contacts and the circuits controlled by each pair of contacts are different, for convenience, a dispersed representation is often used. That is, the coil is drawn in the control circuit, and the contacts are drawn in each controlled circuit according to their respective working objects. This method of drawing is beneficial to simplifying and analyzing the circuit. However, this method of drawing must annotate the relay number and the contact number next to each pair of contacts, and stipulates that all contacts should be drawn according to the original state of the relay without power. Figure 9 (b) is a touch switch. When a person touches the metal sheet A, the 555 time base circuit outputs (terminal 3) a high potential, energizing the relay KR1, and the contacts close to light up the light and sound the bell. The 555 time base circuit is the control part, which uses 6V low voltage electricity. The light and bell are the controlled parts, which use 220V mains electricity.

The letter symbol of relay is "K". Sometimes, in order to distinguish, AC relay uses "KA", electromagnetic relay and reed relay can use "KR", and time relay can use "KT".

Battery and fuse symbols

The graphic symbol of the battery is shown in Figure 10. The long line represents the positive pole and the short line represents the negative pole. Sometimes the short line can be drawn thicker for emphasis. Figure 10 (b) represents a battery pack. Sometimes the battery pack can be simplified to a single battery, but the voltage or number of batteries should be noted next to it. Figure 10 (c) is the graphic symbol of the photocell. The text symbol of the battery is "GB". The graphic symbol of the fuse is shown in Figure 11, and its text symbol is "FU".

Diode and transistor symbols

The graphic symbols of semiconductor diodes in circuit diagrams are shown in Figure 12. (a) is the symbol of a diode. The direction indicated by the arrow is the direction of current flow, which means that when the upper end of this diode is connected to a positive voltage and the lower end is connected to a negative voltage, it can conduct. Figure (b) is the symbol of a voltage-stabilizing diode. Figure (c) is the symbol of a variable capacitance diode. The capacitor symbol next to it indicates that its junction capacitance changes with the voltage across the diode. Figure (d) is the symbol of a thermistor diode. Figure (e) is the symbol of a light-emitting diode, with two obliquely radiating arrows indicating that it can emit light. Figure (f) is the symbol of a magnetically sensitive diode, which can respond to an external magnetic field and is often made into a proximity switch for use in automatic control. The text symbol of a diode is "V", and sometimes it may be represented by "VD" to distinguish it from a triode.

triode

Since PNP and NPN transistors have different requirements for the polarity of the power supply when in use, they should be distinguished and represented in the graphic symbols of the transistors. The standard of graphic symbols stipulates that as long as it is a PNP transistor, whether it is made of germanium or silicon, it is represented by Figure 13 (a). Similarly, as long as it is an NPN transistor, whether it is made of germanium or silicon, it is represented by Figure 13 (b). Figure 13 (c) is the symbol of a photosensitive transistor. Figure 13 (d) represents a silicon NPN magnetic sensitive transistor.

Symbols of thyristor, unijunction transistor, and field effect transistor

Thyristor is the abbreviation of thyristor or silicon controlled rectifier. Commonly used ones are unidirectional thyristor, bidirectional thyristor and light-controlled thyristor. Their symbols are (a), (b) and (c) in Figure 14. The letter symbol of thyristor is "VS".

The symbol of a unijunction transistor is shown in Figure 15

Semiconductor devices controlled by electric fields are called field effect transistors. Their symbols are shown in Figure 16, where (a) represents an N-channel junction field effect transistor, (b) represents an N-channel enhancement-type insulated gate field effect transistor, and (c) represents a P-channel depletion-type insulated gate field effect transistor. Their text symbols are also "VT".