Circuit diagram

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The circuit diagram for a 4 bit TTL counter, a type of state machine

A circuit diagram (also known as an electrical diagram, wiring diagram, elementary diagram, or electronic schematic) is a simplified conventional pictorial representation of an electrical circuit. It shows the components of the circuit as simplified standard symbols, and the power and signal connections between the devices. 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.


[edit] 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,…). The letters that precede the numbers were chosen in the early days of the electrical industry, even before the vacuum tube (thermionic valve), so "Q" was the only one available for semiconductor devices in the mid-twentieth century[verification needed]. 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.

[edit] 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.

[edit] Standards

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

  • 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.

[edit] 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.

[edit] European and Australian codes

The following codes which vary slightly from the American codes are in common use in European and Australian standard electrical circuit diagrams. These codes are used for the "reference designators" printed on PCBs (which match the corresponding ones written on the corresponding schematic).

  • A: Assemblies
  • B: Transducers (photo cells, inductive proximity, thermocouple, flame detection)
  • C: Capacitors
  • D: Storage devices
  • E: Miscellaneous
  • F: Fuses
  • G: Generator, battery pack
  • H: Indicators, lamps (not for illumination), signalling devices
  • K: Relays, contactors
  • L: Inductors and filters
  • M: Motors
  • N: Analogue devices
  • P: Measuring/test equipment
  • Q: Circuit breakers, isolators, re-closers
  • R: Resistors, brake resistors
  • S: Switches, push buttons, emergency stops and limit switches
  • T: Transformers
  • U: Power converters, variable speed drives, soft starters, DC power supplies
  • V: Semiconductors
  • W: Wires, conductors, power, neutral and earthing busses
  • X: Terminal strips, terminations, joins
  • Y: Solenoids, electrical actuators
  • Z: Filters

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

[edit] 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.

[edit] Art Work

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 prepared 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 into an Art Work.

A generalized design flow would be as:

Schematic → Schematic Capture → Rat's Nest → Routing → Art Work → PCB Development & etching → Component Mounting → Testing

[edit] See also

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

[edit] External links

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