A circuit diagram is also known as an electrical diagram, wiring diagram, elementary diagram, or electronic schematic. It is a simplified conventional symbolic representation of an electrical circuit. It shows the circuit components using easily understood symbols and the power and signal connections between the devices. The components’ arrangement and their interconnections on the diagram do 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 chart does not in any way represent the physical arrangement of components. Circuit diagrams are used for the design, construction, and maintenance of electrical and electronic equipment.
On a circuit diagram, each component’s symbols are labeled with a descriptor 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 in R1, L1,). It is commonly thought that the letters that precede the numbers were chosen in the early days of the electrical industry, even before the vacuum tube, so “Q” was the only one available for semiconductor devices in the mid-twentieth century.
Circuit diagram symbols have differed radically from country to country and have changed over time. At the moment, they 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 dates back to the days when that component was made from a long piece of wire wrapped so as not to produce inductance, which would have made it a coil. These wire-wound 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. Therefore, the internationally standardized symbol for a resistor is now simplified to an oblong, sometimes with the value in ohms written inside, instead of the zigzag character. The component’s value or type designation is often given on the diagram beside the part, but detailed specifications would go on the parts list.