Components and Schematic Symbols

Schematic Diagrams, Power Sources, Grounds, and Meters

A schematic diagram uses standardized graphic symbols to represent electrical components and their connections. Rather than drawing realistic pictures of parts, schematics show which components are electrically connected and how a circuit is arranged. On paper or on a screen, conducting paths are drawn as lines. When intersecting wire lines on a schematic have a solid dot at their crossing point, that dot indicates an electrical junction where the wires physically connect. Conversely, wires that cross without a junction dot usually are not connected, though test-takers should always check the schematic's drawing convention to confirm how crossings are portrayed.

Every functioning circuit requires an electrical energy source. A direct-current (DC) cell is shown by one long and one short parallel line. A full battery symbol uses repeated pairs of these parallel lines. In this symbol, the longer parallel line represents the positive terminal and the shorter line represents the negative terminal. For systems running on alternating current, the schematic symbol for an alternating current voltage source is a circle containing a wavy sine wave line.

A circuit needs a complete path for current, but it does not necessarily need an earth-ground connection; a ground symbol may mark a voltage reference or a return connection. A ground symbol identifies a reference point, often treated as zero volts, and depending on the circuit, it may also indicate a current-return connection. An earth ground is represented schematically by a vertical line leading down to three progressively shorter horizontal parallel lines. In contrast, a chassis ground connects to the metal frame or enclosure of a device and uses its own distinct graphic symbol.

Technicians use specialized meters to monitor circuit parameters. An ammeter measures electric current, must be connected in series with the load, and is represented by a circle containing the letter A. A voltmeter measures electrical potential difference, must be connected in parallel across a component, and is represented by a circle containing the letter V.