Circuit Symbols: Key to Understanding Electrical and Electronic Diagrams

Circuit symbols are essential in electrical and electronic diagrams for representing complex circuits and components in a standardized and simplified manner. These symbols are crucial for engineers, electricians, and technicians to understand circuit functions without documentation. Universal recognition of these symbols ensures consistent interpretation across languages and regions, aiding education and training.

Circuit diagrams consist of various components like switches, capacitors, resistors, and batteries, connected by nets or trails. Each component has a unique symbol with specific characteristics. Understanding component operation is vital for effective circuit design and analysis. Knowledge of resistor, capacitor, and transistor properties enables engineers to predict component interactions, achieve desired outcomes, and troubleshoot issues.

How do the Circuit Symbols form the Circuit Diagram?

Electronic circuit symbols are concise drawings or pictograms that depict various components in a circuit’s schematic diagram. In such diagrams, electrical elements typically feature two or more terminals for connecting components. Fundamental electrical and electronic symbols include the ground electrode, battery, and resistor. These symbols facilitate the representation of even the most intricate circuits. With these essential symbols, anyone can sketch an electrical diagram. For instance, a basic circuit comprises a battery, a switch, and a bulb connected in a closed loop.

Although the same circuit can be presented in multiple formats, visual representations are more intuitive. However, as the number of components increases, the visual complexity also rises. When embarking on a project that involves building circuits or designing a PCB, comprehending electronic symbols becomes vital. The schematic layout is the initial step towards PCB design, and without understanding the symbols within, progressing in such a project can be challenging.

Representing a Circuit with Symbols v/s Words

Usually to define anything specific words are enough but the circuit diagram has its own advantages. Circuit diagrams become self explanatory by utilizing symbols format. Representing the circuit by words is an easy process. For example, the above shown circuit can be explained as “the battery is connected to the switch and bulb in the series combination”. This is very easy to understand, but when the diagrams become bigger and complex, you have to use circuit diagrams and symbols to represent them. It makes the person quickly analyze the circuit, that what is happening.In short, electronic symbols simplify our understanding of the circuit. It saves our time and makes it easy.

Advantages of the Circuit Symbols

A symbol's precise meaning is provided by the inclusion of a dot, line, letter, letter spacing, shading, and number. One has to be familiar with the basic structure of various symbols to comprehend circuits and their corresponding symbol meanings. Here are some main advantages of using symbols:

Reduces confusion in production - By using a defined set of electrical symbols, it is impossible to assign various symbols to similar components.

Lessens Rework - By developing and using a single, standardized set of electrical symbols, unnecessary and redundant schematic rework is eliminated.

Greater Electrical and Mechanical Communication - The use of standardized electrical symbols reduces instances of misunderstanding between members of the MCAD and ECAD teams. The use of connections between 2D symbols and 3D modeled parts puts everyone on the same page.

Saves Time - Standardized electrical symbols not only save time on rework and misunderstandings.

Physics Behind Designing of Symbols

Symbols are used to indicate an electric/electronic component. As said above it is very important to have knowledge about how a component operates, that is the starting point from where the design concept of different symbols came from. Each symbol has a capability to explain the basic physics behind the actual component. Most of the symbol designs are based on different national and international standards. Instances include IEC standards, JIC standards, ANSI standards, IEEE standards, etc. Although electrical symbols are standardized, they might differ from engineering discipline to engineering field depending on previous traditions.

Example explaining the physics of circuit: The majority of electrical circuit symbols include switches and batteries, the symbols for them is shown below:

Switch: The above switch symbol represents the closed and open circuit connection in a circuit and hence the symbol is universally adapted as per that.

Light Bulb: Bulb has some resistance which is heated up to produce light. And here in the source the symbol shows a cross mark representing light coming from a resistive source.

Battery: It is an energy source made of cells, having a chemical electrolyte inside which provides polarity. The cells in the battery symbol are represented by two uneven lines in parallel with positive and negative polarity mentioned on them.

Wire: It is just a connection conductor between two devices, which has very low resistance and hence for connection it is just represented by a straight line.

There are some other complicated symbols too which are not that easy to explain without having component knowledge.

List of Circuit Diagram Symbols with Design Physics Behind it

Circuit diagrams are used to portray the symbols used in electronic circuits virtually. Every circuit uses standardized symbols to represent the various parts. To represent fundamental electrical devices, several electronic circuit symbols are employed. There is a circuit symbol for each electrical device or component used in a circuit, such as passive components (resistors, capacitors and inductors) , active components(diodes, transistors, amplifiers and transformers), measuring devices and logic gates. Here are the most used electronic component symbol with design physics behind them:

Passive Components:

Wires: These are used for connection of different components hence represented as straight lines. Two crossing wires may or may not have connection, if not connected to each other defined as crossover, if connection is there then the crossover has a dot.

Switches: Switch symbol represents the closed and open circuit connection in a circuit and hence the symbol is universally adapted as per that.

Resistor: Resistor is the obstacle in the path of current hence drawn as a ZIG-ZAG shaped. And a variable resistor on the other hand takes a desirable value in a specific range, which is shown as Resistor with an arrow in mid as per symbol.

Capacitor: Capacitors are used for electric energy storage in the form of charge. A capacitor has two plates inside for charge storage and hence the symbol is represented by two parallel bars separated by some distance. Variable capacitor also exists like resistors and has an arrow in the middle. Capacitors can be of polar/non-polar type, Polar one has positive and negative polarity and non-polar doesn’t have any polarity.

Inductors: Inductors are also an energy storage device in the form of their magnetic field. It is a non-polar device and has a coil like structure hence the symbol is represented as:

Antenna: Antenna is a most commonly used device in RF electronics, it is the first section of a receiver. Antenna emit/receive EM wave, the design may vary depending upon the applications but symbol is given as:

Power Symbols:

Battery: It is an energy source made of cells, having a chemical electrolyte inside which provides polarity. The cells in the battery symbol are represented by two uneven lines in parallel with positive and negative polarity mentioned on them.

Ground: The ground in an electric network serves as a common reference point for voltage levels, providing a return path for electric current. The symbol adopted as per the name and represented as:

VDD: VDD in an electric network represents the positive supply voltage for the circuits, it is often used in conjunction with a ground reference (GND). VDD and GND are the types of label which are connected to battery, positive(VDD) and negative(GND). The symbol of VDD is governed as:

Ammeter and Voltmeter: An ammeter measures current in an electric network and is connected in series with the circuit components. A voltmeter measures voltage across components and is connected in parallel. Both type of meters are traditionally analog in nature and has a moving coil of the symbol is adapted as:

Transformer: A transformer in an electric network transfers electrical energy between circuits through electromagnetic induction. It consists of primary and secondary windings around a core and hence the symbol represented as:

Active Analog/Digital Components:

Diodes: Diodes in an electric network allow current to flow in only one direction, acting as one-way valves. They are used for rectification, converting AC to DC, and protecting circuits by blocking reverse voltage. Diodes are polar device made up of a PN junction and the symbol is given as:

Bipolar Junction Transistor(BJT): A transistor in an electronic network acts as a switch or amplifier for electrical signals. It is a 3 terminal device and controls current flow between its collector and emitter terminals based on the voltage applied to its base terminal. These are of two types NPN(current leaving type) and PNP(current entering type), symbol represented as per the current entering/leaving:

MOSFET: These are types of modified transistors having the same operations like BJT. It is a 3 terminal device, controlling current flow between its drain and source terminals based on the voltage applied to its gate terminal. MOSFETS are known as metal oxide semiconductor transistors, metal oxide is a insulator material which makes the gate look like a capacitor and hence the symbol given as:

SCR: An SCR (Silicon Controlled Rectifier) in an electronic network is a type of thyristor used to control high power. It acts as a switch, allowing current flow when a gate signal is applied and remaining on until the current drops below a threshold. SCRs are used in power control and rectification. The behavior is quite similar to diode having gate terminal as enable pin hence the symbol given as:

DIAC: A DIAC (Diode for Alternating Current) in an electronic network is a bidirectional trigger device that conducts current only after its breakover voltage is exceeded. Due to it’s by directional capabilities symbol given as:

Amplifiers: Amplifiers in an electronic network increase the power, voltage, or current of a signal. They are essential in audio, radio, and communication systems, enabling weak signals to be strengthened for transmission. Amplifier may have different input and output terminal, a standard symbol given to this unit is:

OPAMP: An OPAMP (Operational Amplifier) in an electronic network is a versatile, high-gain voltage amplifier with differential inputs. It is used in signal processing, filtering, and mathematical operations like addition, subtraction, integration, and differentiation. It is also a multi pin device the symbol for which is adopted as:

Buffer: A buffer in an electronic network isolates different stages of a circuit and prevents loading effects, making it crucial for maintaining signal integrity in complex circuits. It can be thought of as an amplifier with a gain of 1, hence input signal appears on output without any loss. Symbol for buffer is taken as:

Logic gates:

Logic gates are fundamental building blocks in digital electronics, performing basic logical functions that are essential for circuit design and digital computing. They operate on binary inputs and produce binary outputs based on logical operations.

Types of Logic Gates:

● AND Gate: Outputs true (1) only if all inputs are true.

● OR Gate: Outputs true if at least one input is true.

● NOT Gate: Inverts the input; outputs true if the input is false.

● NAND Gate: Outputs false only if all inputs are true; otherwise, true.

● NOR Gate: Outputs true only if all inputs are false.

● XOR Gate: Outputs true if an odd number of inputs are true.

These gates are used in various combinations to perform complex logical operations and are integral to digital circuits, including computers and other electronic devices.

Here is a Document file, you can download and see to know more about other widely used schematic symbols.

LINK:

https://drive.google.com/file/d/1Ik6cuXN9-NXWe9lgpPLLt3YDHzpbGIUE/view?usp=sharing

How to Create a Symbol for Custom PCB Design

The transfer of a circuit design to a Printed Circuit Board or PCB is a multistep process. First the schematic of the circuit is created using a suitable PCB design software.  The schematic is nothing but the circuit diagram containing different symbols attached to each other with wires as described above. The same software is then used to translate this schematic into a PCB layout. A layout describes the placement and routing of the components on the real PCB. For designing a Custom symbol, a part symbol file must be generated before the schematic and layout design. A part symbol is simply a set of files containing all electronic and geometrical information about the component.

Here are the detailed step to make your own symbol in the PCB design editor:

I am using EastEDA, the procedure remains the same in most of the design software. Follow the below given instructions.

Step 1:  Login to EasyEDA and to open the symbol design window navigate to File -> New -> Symbol from the top menu. A new blank schematic drawing window will open up. Save it on your hard disk before moving on to the next step. It is a good practice to give the file a name that corresponds to the part you are creating the symbol for.

Step 2: Open the datasheet and see the pin info of the component you want to design in this window. Here is an example of a NE555 timer IC.

Step 3: EasyEDA offers a symbol wizard which has some predefined builtin symbols you can choose and work on, just choose the respective symbol and give name to all of the pins according to the one in the datasheet. This is an automatic symbol generator, if you want to do it manually follow the next steps.

Pin information can be edited from:

Step 4: To design the symbol manually, using the utilities in the drawing menu, draw the outline of the top view of the part. Although most electronic chips are rectangular in shape, you can essentially draw any shape. The actual shape and size of the part are important for the layout symbol only, which will be created later on.

Step 5:  Open up the pin assignment drawing on the component's data sheet. This drawing shows the component's pins around the periphery of the component with names beside each pin. Draw the first pin at the corner of the component outline you drew earlier using the pin drawing utility that is generally available on the left menu bar. Repeat this process for all component pins. A good practice is to try to replicate the pin assignments as given in the component's data sheet.

Step 6: Double click on the first pin. A dialog will open up with details about the pin. Enter its name as it shows on the component's data sheet, such as VCC. Repeat this for all pins.

Step 8: Modify all other pin details:

Step 9: Enter all the required details and save the file. Your PCB component symbol has been created.

Step 10: Browse through the component library and double click on the layout symbol you saved earlier. This will open up the symbol in the layout view.

Conclusion

Circuit symbols are an essential tool for understanding, designing, and troubleshooting electrical and electronic circuits. By using standardized symbols, engineers and technicians can ensure accurate communication, reduce errors, and streamline the design process. Understanding the importance and usage of circuit symbols empowers professionals to effectively work with and manage electrical and electronic systems.