Circuit diagram

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Comparison of pictorial and schematic styles of circuit diagrams

The circuit diagram for a four-bit TTL counter, a type of state machine

A circuit diagram (also known as an electrical diagram, elementary diagram, or electronic schematic) is a simplified conventional graphical representation of an electrical circuit. A pictorial circuit diagram uses simple images of components, while a schematic diagram shows the components of the circuit as simplified standard symbols; both types show the connections between the devices, including power and signal connections. Arrangement of the components interconnections on the diagram does not correspond to their physical locations in the finished device.

Unlike a block diagram or layout diagram, a circuit diagram shows the actual wire connections being used. The diagram does not show the physical arrangement of components. A drawing meant to depict what the physical arrangement of the wires and the components they connect is called "artwork" or "layout" or the "physical design."

Circuit diagrams are used for the design (circuit design), construction (such as PCB layout), and maintenance of electrical and electronic equipment.

Legends

Common circuit diagram symbols (with US Resistor Symbol)

On a circuit diagram, the symbols for components are labelled with a descriptor (or reference designator) matching that on the list of parts. For example, C1 is the first capacitor, L1 is the first inductor, Q1 is the first transistor, and R1 is the first resistor (note that this is not written as a subscript, as in R₁, L₁,…). Often the value or type designation of the component is given on the diagram beside the part, but detailed specifications would go on the parts list.

Symbols

Circuit diagram symbols have differed from country to country and have changed over time, but are now to a large extent internationally standardized. Simple components often had symbols intended to represent some feature of the physical construction of the device. For example, the symbol for a resistor shown here dates back to the days when that component was made from a long piece of wire wrapped in such a manner as to not produce inductance, which would have made it a coil. These wirewound resistors are now used only in high-power applications, smaller resistors being cast from carbon composition (a mixture of carbon and filler) or fabricated as an insulating tube or chip coated with a metal film. The internationally standardized symbol for a resistor is therefore now simplified to an oblong, sometimes with the value in ohms written inside, instead of the zig-zag symbol. A less common symbol is simply a series of peaks on one side of the line representing the conductor, rather than back-and-forth as shown here.

Standards

There are several national and international standards for graphical symbols in circuit diagrams, in particular:

• IEC 60617 (also known as British Standard BS 3939)

• ANSI standard Y32 (also known as IEEE Std 315)

IEC 60617 originally consisted of 13 parts, from resistors and capacitors to logic symbols and even a generalised drawing standard of connections and bus line widths. It is now published as a subscription online database IEC 60617-DB [1].

Different symbols may be used depending on the discipline using the drawing; for example, lighting and power symbols used as part of architectural drawings may be different from symbols for devices used in electronics.

Linkages

Schematic wire junctions:
1. Old style: (a) connection, (b) no connection.
2. One CAD style: (a) connection, (b) no connection.
3. Alternative CAD Style: (a) connection, (b) no connection.

The linkages between leads were once simple crossings of lines; one wire insulated from and "jumping over" another was indicated by it making a little semicircle over the other line. With the arrival of computerized drafting, a connection of two intersecting wires was shown by a crossing with a dot or "blob", and a crossover of insulated wires by a simple crossing without a dot. However, there was a danger of confusing these two representations if the dot was drawn too small or omitted. Modern practice is to avoid using the "crossover with dot" symbol, and to draw the wires meeting at two points instead of one. It is also common to use a hybrid style, showing connections as a cross with a dot while insulated crossings use the semicircle.

Reference Designators

The components in a circuit diagram are identified by a unique name often called a Reference Designator. Follow this link to find a list of Reference Designators commonly used in the electronic industry.

Detailed rules for reference designations are provided in the International standard IEC 61346.

Organization of drawings

It is a usual although not universal convention that schematic drawings are organized on the page from left to right and top to bottom in the same sequence as the flow of the main signal or power path. For example, a schematic for a radio receiver might start with the antenna input at the left of the page and end with the loudspeaker at the right. Positive power supply connections for each stage would be shown towards the top of the page, with grounds, negative supplies, or other return paths towards the bottom. Schematic drawings intended for maintenance may have the principle signal paths highlighted to assist in understanding the signal flow through the circuit. More complex devices have multi-page schematics and must rely on cross-reference symbols to show the flow of signals between the different sheets of the drawing.

Detailed rules for the preparation of circuit diagrams (and other document kinds used in electrotechnology) are provided in the International standard IEC 61082-1.

Relay logic line diagrams (also called ladder logic diagrams) use another common standardized convention for organizing schematic drawings, with a vertical power supply "rail" on the left and another on the right, and components strung between them like the rungs of a ladder.

Artwork

A rat's nest

Once the schematic has been made, it is converted into a layout that can be fabricated onto a printed circuit board (PCB). The layout is usually started by the process of schematic capture. The result is what is known as a rat's nest. The rat's nest is a jumble of wires (lines) criss-crossing each other to their destination nodes. These wires are routed either manually or by the use of electronics design automation (EDA) tools. The EDA tools arrange and rearrange the placement of components and finds paths for tracks to connect various nodes. This results in the final layout artwork for the integrated circuit or printed circuit board.^[1]

A generalized design flow would be as:

Schematic → Schematic capture → Rat's nest → Routing → Artwork → PCB development & etching → Component mounting → Testing

See also

Electronics portal

• AutoTRAX EDA a multiplatform schematic capture, SPICE simulator and PCB designer.
• Kicad a GPL-ed EDA-Tool used for schematic circuit and PCB design.
• gEDA, a GNU EDA-Tool used for schematic circuit design.
• Schematic capture
• Schematic editor
• The schematic typically shows the pinout of each part.
• Electronic symbol

• OrCAD or Eagle (program) Software for electronic schematics and for the design of printed circuit boards.
• Edwinxp – Totally Integrated Schematic Capture, Simulation Software and PCB design.
• Multisim Electronic schematic capture and simulation software
• Wiring diagram

References

1. ^ R. S. Khandpur (2005). Printed circuit boards: design, fabrication, assembly and testing. Tata McGraw-Hill. p. 10. ISBN 9780070588141. http://books.google.com/books?id=m8sJBIMtETgC&pg=PA10&dq=circuit-diagram+artwork&as_brr=3&ei=L76QSsbyE5qIlQShoKymBw#v=onepage&q=circuit-diagram%20artwork&f=false. 

External links

Wikimedia Commons has media related to: Circuit diagrams

• List of Schematic Symbols
• Large Circuit Diagrams Database