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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 pictures with symbols that 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.
The linkages between leads were once simple crossings of lines. With the arrival of computerized drafting, the connection of two intersecting wires was shown by a crossing of wires with a "dot" or "blob" to indicate a connection. At the same time, the crossover was simplified to be the same crossing, but without a "dot". However, there was a danger of confusing the wires that were connected and not connected in this manner, if the dot was drawn too small or accidentally omitted (e.g. the "dot" could disappear after several passes through a copy machine). As such, the modern practice for representing a 4-way wire connection is to draw a straight wire and then to draw the other wires staggered along it with "dots" as connections (see Figure 3a), so as to form two separate T-junctions that brook no confusion and are clearly not a crossover.
For crossing wires that are insulated from one another, a small semicircle is commonly used to show one wire "jumping over" the other wire (similar to how jumper wires are used).
A common, hybrid style of drawing combines the T-junction crossovers with "dot" connections and the wire "jump" semi-circle symbols for insulated crossings. In this manner, a "dot" that is too small to see or that has accidentally disappeared can still be clearly differentiated from a "jump".
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.
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.
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.
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 find paths for tracks to connect various nodes. This results in the final layout artwork for the integrated circuit or printed circuit board.
A generalized design flow would be as follows:
- Schematic → Schematic capture → Rat's nest → Routing → Artwork → PCB development & etching → Component mounting → Testing
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- Circuit diagrams and component layouts
- Herzfeld, Noreen (2012). Computer Concepts and Applications. Minnesota: College of Saint Benedict/St. John's University. pp. 9-9.
- "It is good practice to never use a + connection with a dot. Why? The dot can disappear when the schematic is copied for the 12th time." -- "Notes on Reading Schematics"
- "We recommend against using a 4-way connection point ... To avoid confusion, use only three-way connections." -- "Design News Gadget Freak Submission Guidelines"
- "Wires connected at 'crossroads' should be staggered slightly to form two T-junctions" -- "The Electronics Club: Circuit Symbols"
- R. S. Khandpur (2005). Printed circuit boards: design, fabrication, assembly and testing. Tata McGraw-Hill. p. 10. ISBN 978-0-07-058814-1.
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- Circuits Tune A huge collection of electronics circuit diagram.