Understanding Circuit Schematics Basics
Every new electrical design starts as an idea. That idea is then defined, in words and diagrams, in a specification. Anyone can take an idea this far, but the next step requires a fundamental understanding of circuit schematics. Circuit schematics are the bridge between conceptual electrical design and physical realization of a printed circuit assembly, or PCA.
Schematics have two fundamental purposes. First, they exist to communicate the design intent. To someone skilled in the art of electrical design, the schematics should clearly convey the intent of the design. And second, they exist to direct and drive the PCB layout. To get a good start on understanding schematics you should understand the following four key things.
Component symbols in a schematic represent physical components that will be soldered to the printed circuit board (PCB) during the assembly process. Sometimes they also might represent structures to be laid out in the PCB, like vias or test points. Component symbols are often an industry-standard shape that indicates what type of electrical components they are, though sometimes they are nothing but a rectangle with pins. Component symbols always have one or more pins to which electrical connections can be made. One or more symbols may be used to define a single electrical component. In the case of a part defined by multiple symbols, reference designators are the key.
Reference Designators (REFDES)
Reference designators are unique identifying labels for each physical component, and they communicate a lot about the components to which they refer. Proper REFDES use tells the schematic reader the type of component and the number of symbols per component. They also serve as the link to the bill of material. By industry standard reference designators include a letter code, indicating the type of component, followed by a unique number.
In schematic and printed circuit board parlance, nets are the PCB wired electrical connections. Nets appear as lines connecting component symbol pins to other pins or nets. It is a best practice when drawing schematics to label important nets so that they can be clearly identified. If two nets are not drawn as connected but have the same label, they will be treated as physically connected by the schematic capture software so that when the design is exported to a PCB layout tool they will be the same net.
Outputs: Netlist and BOM
The most important output of a schematic is the netlist. The netlist is a database representation of every component and every net connection between them. This file, or set of files, is the primary input to the PCB layout software and is used by layout designers to guide placing and routing all of the circuits on a board. The other output is a bill of material or BOM. The BOM output is a spreadsheet or database that matches every REFDES with a physical component and a part number.
There is much more to schematics than just these four key things. Schematics can also be used for simulating circuits, for example. But understand these four things and you will be well on your way to understanding the most important fundamentals of schematics.