Schematic Symbols

The simplest schematic symbol is the one representing a wire or electrical conductor: a straight, solid line. Sometimes, dashed lines are used to represent conductors, but usually, dashed lines are drawn to partition diagrams into constituent circuits, or to indicate that certain components interact with each other or operate in step with each other. Conductor lines are almost always drawn either horizontally across or vertically up and down the page. This keeps the diagram neat and easy to read.

When two conductor lines cross, they aren’t connected at the crossing point unless a heavy black dot is placed where the two lines meet. The dot should always be clearly visible wherever conductors are to be connected, no matter how many of them meet at the junction. A resistor is indicated by a zigzag. A variable resistor, or potentiometer, is indicated by a zigzag with an arrow through it, or by a zigzag with an arrow pointing at it. These symbols are shown below :

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Schematic symbols for a fixed resistor (A), a two-terminal variable resistor (B), and a three-terminal potentiometer (C).

An electrochemical cell (such as a common dime-store battery) is shown by two parallel lines, one longer than the other. The longer line represents the plus terminal. A true battery, which is a combination of two or more cells in series, is indicated by several parallel lines, alternately long and short. It’s not necessary to use more than four lines to represent a battery, although you’ll often see 6, 8, 10, or even 12 lines. Symbols for a cell and a battery are shown in figure :

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Schematic symbols for an electrochemical cell (A) and an electrochemical battery (B).

Meters are portrayed as circles. Sometimes the circle has an arrow inside it, and the meter type, such as mA (milliammeter) or V (voltmeter) is written alongside the circle, as shown in (following figure A). Sometimes the meter type is indicated inside the circle, and there is no arrow (following figure B). It doesn’t matter which way you draw them, as long as you’re consistent throughout a schematic diagram.

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Meter symbols can have the designator either outside the circle (A) or inside (B). In this case, both symbols represent a milliammeter (mA).

Some other common symbols include the incandescent lamp, the capacitor, the air-core coil, the iron-core coil, the chassis ground, the earth ground, the ac source, the set of terminals, and the black box (general component or device), a rectangle with the designator written inside. These are shown in Next topic