Schematic Diagrams In Electronics

What is a Schematic Diagram?

A schematic diagram, or schematic drawing, is a fundamental two-dimensional circuit representation showing the functionality and connectivity between different electrical components. It's composed of electronic symbols, wiring diagrams, designators, net names, and netlist. A PCB designer needs to get familiarized with the schematic symbols that represent the components on a schematic diagram. These symbols serve as a universal language understood by engineers, technicians, and electricians across different industries. They provide a concise and standardized way of communicating circuit designs.

How schematic diagram is different from a schematic block diagram?

A schematic block diagram also referred to as a block diagram or a functional diagram, is a simplified representation of a system or a process using blocks to represent its various components or stages. It provides an overview of the system's structure and the interconnections between its major functional units or subsystems.

In a schematic block diagram, each block represents a specific component, module, or function within the system. The blocks are typically labeled with descriptive text to indicate their purpose or functionality. The interconnections between the blocks are represented by lines or arrows, indicating the flow of signals, information, or energy between the different components or stages.

Schematic Component Symbols

Schematic symbols are typically designed to visually represent the physical characteristics, electrical properties, and functionality of components. They are usually simple, abstract, and intuitive, allowing quick recognition and identification.

For example, a resistor in a circuit diagram is represented by a zigzag line, which indicates its function of restricting current flow. A capacitor is depicted by two parallel lines separated by a gap, representing its ability to store electrical energy. Symbols exist for diodes, transistors, switches, connectors, IC (integrated circuits), and many other components used in electrical circuits and electronic circuits.

Reference Designators

Each component in a schematic diagram is assigned a component designator or reference designator (REFDES) to indicate its specific function within the circuit. It is a letter or combination of letters used to identify individual components within a circuit diagram uniquely. The designators serve as labels or identifiers for the components and are typically placed next to the corresponding symbols in the circuit diagram. It is important to follow a consistent and well-defined designator system within a circuit diagram to ensure clear communication and easy referencing of components.

Reference designators play a crucial role in the connection to the bill of materials (BOM). The BOM contains the part number for each component in your PCBA design and specifies where each part should be installed, based on its reference designator (REFDES).

Here are industry-standard formats for reference designators including a letter, indicating the type of component, followed by a number. For example R1, R2, R3.. etc

Are there any Standards for Schematic Symbols?

IEC 60617 and ANSI standard Y32 are two widely recognized and used standards for schematic diagram symbols and graphical representations in electrical and electronic circuit diagrams. These standards provide guidelines for the consistent depiction of components, connections, and other elements in circuit diagrams, ensuring clear communication and interoperability. 

1. IEC 60617, also known as the "Graphical Symbols for Diagrams" standard, is published by the International Electrotechnical Commission (IEC). It is based on the older standard, British Standard (BS 3939). This database includes over 1750 schematic symbols. It establishes a comprehensive set of symbols and graphical representations for various electrical, electronic, and related technologies.
2. ANSI standard Y32 is published by the American National Standards Institute (ANSI) and specifically focuses on graphical symbols originally used for aircraft applications. This standard is predominantly followed in the United States.

Both standards cover a wide range of components, including passive elements (resistors, capacitors, inductors), active devices (transistors, diodes), power sources, switches, connectors, logic gates, and more. They are periodically updated to incorporate new symbols, reflect technological advancements, and address industry needs. 

Symbol Attributes and Values

The schematic diagram should provide this additional information to ensure that appropriate components are selected. The resistor should have its resistance value expressed in ohms (Ω). The battery should state its potential difference (voltage) expressed in volts. Other components are described in different terms. For example, capacitors differ by their capacitance value expressed in farads (F), and inductors are differentiated by their inductance value expressed in Henrys (H).

Additional attributions can be given to the symbols like power ratings, values, tolerances, etc. This helps us in identifying the correct components for the circuit. Here are common attributes of a component are:

1. The symbol with its unique shape, pin names and pin numbers
2. Reference designator U1, R1, C1, L1, D1 etc.
3. Manufacturer part number (MPN)
4. Values such as resistance in ohms, capacitance, and inductance
5. Tolerances. For resistors: ±1%, ±5%
6. Component footprints or package case codes (resistors: 0402, 0805; for 8 pin IC: PDIP-8 or SOIC-8)
7. Maximum operating conditions. Like for polarized capacitors, maximum reverse voltage, max power wattage for resistors