circuit driving an
LED. In electrical and electronic industry, a schematic diagram is often used to describe the design of equipment. Schematic diagrams are often used for the maintenance and repair of electronic and electromechanical systems. While schematics were traditionally drawn by hand, using standardized templates or pre-printed adhesive symbols, today
electronic design automation software (EDA or "electrical CAD") is often used. In electronic design automation, until the 1980s schematics were virtually the only formal representation for circuits. More recently, with the progress of computer technology, other representations were introduced and specialized
computer languages were developed, since with the explosive growth of the complexity of electronic circuits, traditional schematics are becoming less practical. For example,
hardware description languages are indispensable for modern
digital circuit design. Schematics for electronic circuits are prepared by designers using EDA (
electronic design automation) tools called
schematic capture tools or
schematic entry tools. These tools go beyond simple drawing of devices and connections. Usually they are integrated into the whole design flow and linked to other EDA tools for verification and simulation of the circuit under design.
Programmable logic controllers (PLC) can be programmed using
ladder diagrams. In electric power systems design, a schematic drawing called a
one-line diagram is frequently used to represent
substations, distribution systems or even whole electrical power grids. These diagrams simplify and compress the details that would be repeated on each phase of a
three-phase system, showing only one element instead of three. Electrical diagrams for switchgear often have common device functions designate by
standard function numbers. Another type of diagram used for power systems is a
three-line diagram. For analysis purposes of a power system, from the one-line diagram, if the system is balanced, an
equivalent per-phase (or
single-phase)
schematic diagram can be obtained. If all of the parameters are represented as impedances and voltage sources, the equivalent per-phase schematic diagram is called an
impedance diagram. If all of the parameters are represented as admittances and current sources, the equivalent per-phase schematic diagram is called an
admittance diagram. If the power system is unbalanced, but it is
linear (or can be approximated by a linear system), then Fortescue's theorem (
symmetrical components) can be applied. In this way, from the one-line diagram, three different per-phase schematic diagrams are obtained, known as
sequence diagrams:
positive sequence diagram,
negative sequence diagram, and
zero sequence diagram. Each of these diagrams can be represented as an impedance diagram or as an admittance diagram. ==Schematics in repair manuals==