Whatever we create, including a PC Board,is a picture or an idea that we convert to paper in the form of a diagram. You can’t proceed with manufacturing without a proper PCB layout or schematic.
An electrical schematic is a conceptual plan or drawing that helps understand the physical aspect of a PCB in terms of assembly.
The basic purpose of electrical schematicsis to have design communication.It should help manufacturers clearly understand it.Besides, the schematic directs you to makea PCB layout.
Schematics involve specific elements which are essential to understand, such as:
- REFDES or reference designates
- Symbols representing the components
To know schematics in detail, you have to understand the above elements.
REFDES or Reference Designators-Identifying Labels
Identifying labels or reference designators help understand components because they describeso much about them. If REFDS is precise, it helps a reader to identify the types of the components and symbols,depicting every component.
So, each electrical component or part has a standard symbol.However, onlyspecific schematics follow such standards.
In some cases, reference designators can fully describe the component type, like when there is a generic box with pins itrepresents a passive component.
At the same time, reference designators also connect to theBOM, and bill of materials. Every Printed Circuit Board component needs a part number that BOM shows. Besides, it also helps find the area or location where components should be installed.
The format of the reference designators consists of a code in the form of a letter that describes the component, and its type having a specific number.
Symbols Representing Components
The electrical schematic has specific symbols of the physical components of PCB assembly in terms of soldering. They also describe the circuit boards’ structures, such as test points and vias.
The symbols are specific shapes or they are diagrams to indicate the type of PCB components. There are symbols for all electrical components like capacitors, diodes, resistors, and transistors.
The symbol of each component consists of pins that help in creating the electrical connections. The pins of all schematic symbols have specific numbers,depicting the physical components.
Designers can assign either a single symbol or multiple symbols to represent a single component of a PCB. Whereas the components having several pins are assigned many symbols to create a readable electrical schematic.
When a component is having multiple symbols, each symbol representing the same component has a similar designator.
PCB has many electrical components, including resistors. The U.S designers use a zigzag line to describe a resistor. Whereas its symbol is a rectangle according tointernational standards.On the other hand, the letter R in schematics isa representation of the resistor.
CapacitorSymbols and Letter
Capacitors have two types, includingpolarized capacitors and non-polarized, and both have different symbols. The curved line is a symbol of a negative terminal of a polarized capacitor. Whereas the plus sign denotes a positive terminal of a polarized capacitor.Whereas the letter C defines a capacitor in schematics with REFDES.
Inductor Symbol and REFDES Reference
Inductors are also important passive components of a PCB, and their construction involves a series of curves.Manufacturers use a coil of wire to make inductors. Whereas the letter L defines a capacitor in schematics with REFDES.
Diode Symbol and REFDES Reference
Some other PCB electrical components within the PCB design include diodes that permit the current to travel in just one direction. The diode has different types, including
- Zener diodes
- LEDs or light-emitting diodes
- Schottky diodes
Zener diodesprevent the reverse flow of a current. However, the reverse flow can happen at a certain electrical power level.
There are also LEDs or light-emitting diodes which emit light when having a forward flow of a current.
The Schottky diode has simple features,but it switches quickly and the voltage drops in a lower forward direction.
Other than LED, the letter D or L defines a capacitor in schematics with REFDES.
Transistors Symbol and REFDES Reference
Transistors involve the flow of current at the primary terminal and they function like switches. The basic types of transistors include:
Bipolar Junction Transistors-BJTs: It is a current managing device that helps a current flow either into one base pin or out of it, changing to high currents passing through emitters and collectors.
Field-Effect Transistors-FETs: The FE transistor is another current-managing device that helps voltage turn on the current on the pin, via source pins and adrain. Various drawings indicate transistors and the letter Q and M define a resistor in schematics with REFDES and also represent the MOSFET. However, some designers use the letter T in the wrong way.
Variable Resistors’ Symbol andREFDES Reference
Variable resistors have two categories:
- Potentiometers-three terminals
- rheostats-two terminals
The function of the variable resistors is to customize resistance according to users. An arrow across the resistor represents the rheostats, whereas an arrow on the side of the resistor shows potentiometers.
There are also varistors or voltage-dependent resistors which resemble the variable resistors, but they are denoted by a line across them.The symbols of the resistors include R, VR, and RV, depending on the type of the resistor.
IntegratedCircuits, Symbol, and REFDES Reference
A single-package circuit developed with a semiconductor is an integrated circuit. You can also consider such circuits asamplifiers, processors, power regulators, and memory of a device. The shape of the integrated circuit is like a rectangle or a square that you can install on a PCB. It is a box with power inputs and outputs or pins and sometimes a board has many boxes.
The symbolof the integrated circuit in a schematic is the IC or the U.
Resonator, Crystals, and Oscillators, Symbol and REFDES Reference
PCB also has resonators, crystals, and oscillators, which give a consistent frequency output. These elements are different in terms of circuitry and characteristics but they have a commonfunction.
Some Other Symbols
There are also several other symbols to show interconnections on a schematic having labels to find their electrical nets.
Mostly there is no reference designator for non-component symbols, however, some have symbols like TP or test points, and X, and MH on REFDES.
Understanding Nets in Schematics
The wired connections on the printed circuit boards are what designers call nets. They are lines that connect symbol pins to the nets. You have to properly label the nets while making schematics to help manufacturers identify them. Specific symbols depict thenets to the same or different pages when you don’t draw them as connections, such as interpage and intrapage symbols.
Try not to overlap the nets to have good readability, however, you can’t ensure it every time. A dot or a circle on a schematic diagram shows two nets’ connection. If there is no DOT in a schematic, it means there is no connection between the nets. Whereas advanced drawings have wire hops to show that the nets don’t connect.
Outputs of Schematics-Netlists & BOM
The schematic diagram has two important outputs, including BOM, and netlists. The netlist is a file that designers use to understand routing and parts’ placement on a PCB. The format of thenetlists can vary in symbols. The name of the nets is presented in the netlist, showing thelocation of connectionsbetween the PCB parts. Netlist also has an output tool to give names to the nets which were without names.
Generally, there are several tables in a netlist, listing the PCB components and their names, net names, and some other essentials. Sometimes the netlists include additional information to help you understand the SPICE.
Bill of Materials-BOM
Bill of materials shows data in the form of a spreadsheet,matching the REFDES in schematicshaving a number and a physical component. Designers use different formats for BOM outputs, depending on the nature of schematics and your requirements in terms of output.
There can be many reference designators with independent partnumbers to help with manufacturing. Some detailed BOMs consist of specific part numbers of a company, quantities of components at different locations on a PCB, and the part numbers for different vendors. In other words, BOM has essential information to understand schematics and develop it through different steps, including SMT assembly.
You must know that the electrical schematic is a drawing that helps understand the physical aspect of a PCB in terms of assembly. The main aim of electrical schematics is to have design communication and help manufacturers construct the PCB. It should help you clearly understand parts and assembly. All electrical components have standard symbols. But, only specific schematics follow such standards. Different software help make electrical schematics that you can explore online.
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